Bibliography - V Ramaswamy

1. Ganguly, D, Paul Ginoux, and V Ramaswamy, et al., in press: Retrieving the composition and concentration of aerosols over the Indo-Gangetic basin using CALIOP and AERONET data. Geophysical Research Letters. 3/09.
2. Li, F, Paul Ginoux, and V Ramaswamy, in press: Transport of South American dust to East Antarctica. Journal of Geophysical Research. 5/09.
   [ Abstract ]

   The transport of South American dust to East Antarctica is investigated by using the Geophysical Fluid Dynamics Laboratory (GFDL) Atmospheric Model in combination with satellite products. The mechanisms of dust transport to Antarctica are studied by analyzing the transport pathways and their relationship to main meteorological factors. The study shows that the transport to Antarctica is an intermittent process, occurring mainly through two pathways. A south-eastern corridor across southern Atlantic Ocean is the principal transport pathway. The dust transport is determined by the positions of the anomalous dipole of low and high sea level pressure systems over the southern Atlantic Ocean and the Antarctic Peninsula. South American dust goes through the south-eastern pathway when the dipole is located east of the Antarctic Peninsula, while a southward transport occurs when the two pressure anomaly systems are located on different side of the peninsula. Demonstrating these by following the journeys of specific dust plumes from the South American sources to East Antarctica, this study clarifies how climatic factors affect the amount of dust deposited in Antarctic ice cores.

3. Ming, Yi, and V Ramaswamy, March 2009: Nonlinear climate and hydrological responses to aerosol effects. Journal of Climate, 22(6), 1329-1339.
   [ Abstract PDF ]

   The equilibrium temperature and hydrological responses to the total aerosol effects (i.e., direct, semidirect, and indirect effects) are studied using a modified version of the Geophysical Fluid Dynamics Laboratory atmosphere general circulation model (AM2.1) coupled to a mixed layer ocean model. The treatment of aerosol–liquid cloud interactions and associated indirect effects is based upon a prognostic scheme of cloud droplet number concentration, with an explicit representation of cloud condensation nuclei activation involving sulfate, organic carbon, and sea salt aerosols. Increasing aerosols from preindustrial (1860) to presentday (1990) levels leads to a decrease of 1.9 K in the global annual mean surface temperature. The cooling is relatively strong over the Northern Hemisphere midlatitude land owing to the high aerosol burden there, while being amplified at high latitudes. When being subject to aerosols and radiatively active gases (i.e., wellmixed greenhouse gases and ozone) simultaneously, the model climate behaves nonlinearly; the simulated increase in surface temperature (0.55 K) is considerably less than the arithmetic sum of separate aerosol and gas effects (0.86 K). The thermal responses are accompanied by the nonlinear changes in cloud fields, which are amplified owing to the surface albedo feedback at high latitudes. The two effects completely offset each other in the Northern Hemisphere, while gas effect is dominant in the Southern Hemisphere. Both factors are crucial in shaping the regional responses. Interhemispheric asymmetry in aerosol-induced cooling yields a southward shift of the intertropical convergence zone, thus giving rise to a significant reduction in precipitation north of the equator, and an increase to the south. The simulations show that the change of precipitation in response to the simultaneous increases in aerosols and gases not only largely follows the same pattern as that for aerosols alone, but that it is also substantially strengthened in terms of magnitude south of 10°N. This is quite different from the damping expected from adding up individual responses, and further indicates the nonlinearity in the model's hydrological response.

4. Ganguly, D, Paul Ginoux, and V Ramaswamy, et al., in press: Inferring the composition and concentration of aerosols by combining AERONET and MPLNET data: comparison with measurements and GCM output. Journal of Geophysical Research. 9/08.
5. Huang, X, W Yang, N G Loeb, and V Ramaswamy, May 2008: Spectrally resolved fluxes derived from collocated AIRS and CERES measurements and their application in model evaluation: Clear sky over the tropical oceans. Journal of Geophysical Research, 113, D09110, doi:10.1029/2007JD009219.
   [ Abstract ]

   Spectrally resolved outgoing thermal-IR flux, the integrand of the outgoing longwave radiation (OLR), has a unique value in evaluating model simulations. Here we describe an algorithm for deriving such clear-sky outgoing spectral flux through the entire thermal-IR spectrum from the collocated Atmospheric Infrared Sounder (AIRS) and the Clouds and the Earth's Radiant Energy System (CERES) measurements over the tropical oceans. On the basis of the predefined scene types in the CERES Single Satellite Footprint (SSF) data set, spectrally dependent ADMs are developed and used to estimate the spectral flux each AIRS channel. A multivariate linear prediction scheme is then used to estimate spectral fluxes at frequencies not covered by the AIRS instrument. The whole algorithm is validated using synthetic spectra as well as the CERES OLR measurements. Using the GFDL AM2 model simulation as a case study, applications of the derived clear-sky outgoing spectral fluxes in model evaluation are illustrated. By comparing the observed spectral fluxes and simulated ones for the year of 2004, compensating errors in the simulated OLR from different absorption bands are revealed, along with the errors from frequencies within a given absorption band. Discrepancies between the simulated and observed spatial distributions and seasonal evolutions of the spectral fluxes are further discussed. The methodology described in this study can be applied to other surface types as well as cloudy-sky observations and also to corresponding model evaluations.

6. Huang, Yi, and V Ramaswamy, 2008: Observed and simulated seasonal co-variations of outgoing longwave radiation spectrum and surface temperature. Geophysical Research Letters, 35, L17803, doi:10.1029/2008GL034859.
   [ Abstract ]

   We analyze the seasonal variations of Outgoing Longwave Radiation (OLR) accompanying the variations in sea surface temperature (SST) from satellite observations and model simulations, focusing on the tropical oceans where the two quantities are strikingly anti-correlated. A spectral perspective of this “super-greenhouse effect” is provided, which demonstrates the roles of water vapor line and continuum absorptions at different altitudes and the influences due to clouds. A model-satellite comparison indicates that the GFDL General Circulation Model can fairly well represent the total-sky radiative response to SST in the water vapor infrared absorption band despite the significant bias in the mean state, but this comprises compensating water vapor- and cloud-related errors. The analysis also reveals that the GCM significantly underestimates the cloud induced radiative responses in the window region which arises from the model bias in the mean cloud forcing in convectively active regions. Thus, spectral decomposition proves essential to understand and assess the OLR-SST relationship and the impacts of water vapor and cloud upon this linkage.

7. Huang, Yi, and V Ramaswamy, in press: Evolution and trend of outgoing Longwave Radiation Spectrum. Journal of Climate. 10/08.
   [ Abstract ]

   We investigate the variability and change occurring in the Outgoing Longwave 9 Radiation (OLR) spectrum by using simulations performed with a Geophysical Fluid Dynamics Lab coupled atmosphere-ocean-land General Circulation Model. First, the variability in unforced climate (natural variability) is simulated. Then, the change of OLR spectrum due to forced changes in climate is analyzed for a continuous 25-year time series and for the difference between two time periods (1860s and 2000s). Spectrally resolved radiances have more pronounced and complex changes than broadband fluxes. In some spectral regions, the radiance change is dominated by just one controlling factor (e.g. the window region and CO2 band center radiances are controlled by surface and stratospheric temperatures, respectively) and well exceeds the natural variability. In some other spectral bands, the radiance change is influenced by multiple and often competing factors (e.g. the water vapor band radiance is influenced by both water vapor concentration and temperature) and, although still detectable against natural variability at certain frequencies, demands stringent requirements (spectral resolution no less than 1 cm-1; drift less than 0.1 K / decade) of observational platforms. The difference between clear-sky and all-sky radiances in the forced climate problem offers a measure of the change in the cloud radiative effect, with a substantive dependence on the temperature lapse rate change. These results demonstrate that accurate and continuous observations of the OLR spectrum provides an advantageous means for monitoring the changes in the climate system and a stringent means for validating climate models.

8. Levy II, Hiram, M Daniel Schwarzkopf, Larry Horowitz, V Ramaswamy, and Kirsten L Findell, March 2008: Strong sensitivity of late 21st Century climate to projected changes in short-lived air pollutants. Journal of Geophysical Research, 113, D06102, doi:10.1029/2007JD00917.
   [ Abstract PDF ]

   This study examines the impact of projected changes (A1B “marker” scenario) in emissions of four short-lived air pollutants (ozone, black carbon, organic carbon, and sulfate) on future climate. Through year 2030, simulated climate is only weakly dependent on the projected levels of short-lived air pollutants, primarily the result of a near cancellation of their global net radiative forcing. However, by year 2100, the projected decrease in sulfate aerosol (driven by a 65% reduction in global sulfur dioxide emissions) and the projected increase in black carbon aerosol (driven by a 100% increase in its global emissions) contribute a significant portion of the simulated A1B surface air warming relative to the year 2000: 0.2°C (Southern Hemisphere), 0.4°C globally, 0.6°C (Northern Hemisphere), 1.5–3°C (wintertime Arctic), and 1.5–2°C (∼40% of the total) in the summertime United States. These projected changes are also responsible for a significant decrease in central United States late summer root zone soil water and precipitation. By year 2100, changes in short-lived air pollutants produce a global average increase in radiative forcing of ∼1 W/m²; over east Asia it exceeds 5 W/m². However, the resulting regional patterns of surface temperature warming do not follow the regional patterns of changes in short-lived species emissions, tropospheric loadings, or radiative forcing (global pattern correlation coefficient of −0.172). Rather, the regional patterns of warming from short-lived species are similar to the patterns for well-mixed greenhouse gases (global pattern correlation coefficient of 0.8) with the strongest warming occurring over the summer continental United States, Mediterranean Sea, and southern Europe and over the winter Arctic.

9. Li, F, Paul Ginoux, and V Ramaswamy, May 2008: Distribution, transport, and deposition of mineral dust in the Southern Ocean and Antarctica: Contribution of major sources. Journal of Geophysical Research, 113, D10207, doi:10.1029/2007JD009190.
   [ Abstract ]

   A model-based investigation of the transport, distribution and deposition of mineral dust in the Southern Hemisphere (SH) is performed by using the GFDL Atmospheric Model (AM2). The study represents an attempt to quantify the contribution of the major sources by tagging dust based on its origin. We evaluate the contribution of each source to the emission, distribution, mass burden and deposition of dust in the Southern Ocean and Antarctica, and show that each source produces distinctive meridional transport, vertical distribution, and deposition patterns. The dust in SH originates primarily from Australia (120 Tg a⁻¹), Patagonia (38 Tg a⁻¹) and the inter-hemispheric transport from Northern Hemisphere (31 Tg a⁻¹). A small fraction of it (7 Tg a⁻¹) is transported and deposited in the Southern Ocean and Antarctica, where dust from South America, Australia, and Northern Hemisphere are essentially located in the boundary layer, mid-troposphere, and upper-troposphere, respectively. These three sources contribute to nearly all the dust burden in the Southern Ocean and Antarctica. South America and Australia are the main sources of the dust deposition, but they differ zonally, with each one dominating half of a hemisphere along 120°E–60°W: the half comprising the Atlantic and Indian oceans in the case of the South American dust and the Pacific half in the case of the Australian dust. Our study also indicates a potentially important role of Northern Hemisphere dust, as it appears to be a significant part of the dust burden but contributing little to the dust deposition in Antarctica.

10. Magi, B I., Paul Ginoux, Yi Ming, and V Ramaswamy, in press: Evaluation of tropical and extratropical Southern Hemisphere African aerosol properties simulated by a climate model. Journal of Geophysical Research. 9/08.
    [ Abstract ]

    We compare aerosol optical depth (AOD) and single scattering albedo (SSA) simulated by updated configurations of a version of the atmospheric model (AM2) component of the NOAA Geophysical Fluid Dynamics Laboratory (GFDL) general circulation model (GCM) over Southern Hemisphere Africa with AOD and SSA derived from research aircraft measurements and NASA Aerosol Robotic Network (AERONET) stations, and with regional AOD from the NASA Moderate Resolution Imaging Spectroradiometer (MODIS) satellite. The results of the comparisons suggest that AM2 simulates AOD seasonality reasonably well, but that compared to available observations, AM2 AOD is biased low by 30-40% in the tropics and 0-20% in the extratropics, while AM2 SSA is biased high by 3-8%. The AM2 SSA bias is higher during the biomass burning season, and the monthly variations in AM2 SSA are poorly correlated with AERONET. There are no significant improvements in the comparisons when an internally-mixed aerosol is used. Based on a comparison of aerosol mass in the models with available measurements from southern Africa, we suggest that the low bias in AOD and high bias in SSA are related to an underestimate of carbon aerosol mass in the inventories used by AM2. We estimate that the AM2 biases in AOD and SSA imply that top of the atmosphere radiative forcing over southern Africa is overestimated by ~8%, while surface radiative forcing is underestimated by ~20% throughout the year.

11. Randles, C A., and V Ramaswamy, November 2008: Absorbing aerosols over Asia: A Geophysical Fluid Dynamics Laboratory general circulation model sensitivity study of model response to aerosol optical depth and aerosol absorption. Journal of Geophysical Research, 113, D21203, doi:10.1029/2008JD010140.
    [ Abstract ]

    Forcing by absorbing atmospheric black carbon (BC) tends to heat the atmosphere, cool the surface, and reduce the surface latent and sensible heat fluxes. BC aerosol can have a large impact on regional climates and the hydrologic cycle. However, significant uncertainties remain concerning the increases in (1) the total amount of all aerosol species and (2) the amount of aerosol absorption that may have occurred over the 1950–1990 period. Focusing on south and east Asia, the sensitivity of a general circulation model's climate response (with prescribed sea surface temperatures and aerosol distributions) to such changes is investigated by considering a range of both aerosol absorption and aerosol extinction optical depth increases. We include direct and semidirect aerosol effects only. Precipitation changes are less sensitive to changes in aerosol absorption optical depth at lower aerosol loadings. At higher-extinction optical depths, low-level convergence and increases in vertical velocity overcome the stabilizing effects of absorbing aerosols and enhance the monsoonal circulation and precipitation in northwestern India. In contrast, the presence of increases in only scattering aerosols weakens the monsoonal circulation and inhibits precipitation here. Cloud amount changes can enhance or counteract surface solar flux reduction depending on the aerosol loading and absorption, with the changes also influencing the surface temperature and the surface energy balance. The results have implications for aerosol reduction strategies in the future that seek to mitigate air pollution concerns. At higher optical depths, if absorbing aerosol is present, reduction of scattering aerosol alone has a reduced effect on precipitation changes, implying that reductions in BC aerosols should be undertaken at the same time as reductions in sulfate aerosols.

12. Schwarzkopf, M D., and V Ramaswamy, February 2008: Evolution of stratospheric temperature in the 20^th Century. Geophysical Research Letters, 35, L03705, doi:10.1029/2007GL032489.
    [ Abstract PDF ]

    We employ a coupled atmosphere-ocean climate model to investigate the evolution of stratospheric temperatures over the twentieth century, forced by the known anthropogenic and natural forcing agents. In the global, annual-mean lower-to-middle stratosphere (∼20–30 km.), simulations produce a sustained, significant cooling by ∼1920, earlier than in any lower atmospheric region, largely resulting from carbon dioxide increases. After 1979, stratospheric ozone decreases reinforce the cooling. Arctic summer cooling attains significance almost as early as the global, annual-mean response. Antarctic responses become significant in summer after ∼1940 and in spring after ∼1990 (below ∼21 km.). The correspondence of simulated and observed stratospheric temperature trends after ∼1960 suggests that the model's stratospheric response is reasonably similar to that of the actual climate. We conclude that these model simulations are useful in explaining stratospheric temperature change over the entire 20th century, and potentially provide early indications of the effects of future atmospheric species changes.

13. Huang, Yi, and V Ramaswamy, 2007: Effect of the temperature dependence of gas absorption in climate feedback. Journal of Geophysical Research, 112, D07101, doi:10.1029/2006JD007398.
    [ Abstract ]

    In the context of climate feedback associated with temperature change, there exist two potential mechanisms that affect the outgoing longwave radiation (OLR) and the downward longwave radiation (DLR). One is the “Planck” effect that determines the blackbody thermal emission at a considered temperature. The other is the "absorptivity" effect, in which a temperature change causes a change in gas absorptivities and thus influences the longwave radiative transfer. By using the line-by-line computed radiative Jacobians, which quantify the sensitivity of the radiative fluxes to a perturbation in the atmospheric temperature, the absorptivity effect is separated from the Planck effect. The absorptivity effect is further partitioned into components, with each one having a distinct physical meaning. It is demonstrated that the absorptivity-induced changes in the longwave radiation are individually significant even though the net effect is largely one of cancellation. As a consequence, the Planck effect dominates the overall OLR and DLR sensitivities to temperature change. The absorptivity effect tends to counteract the Planck effect. This tendency is particularly significant for the DLR and is more prominent for a warmer climate, with the result being a reduction in the surface warming.

14. Huang, Yi, V Ramaswamy, and Brian J Soden, 2007: An investigation of the sensitivity of the clear-sky outgoing longwave radiation to atmospheric temperature and water vapor. Journal of Geophysical Research, 112, D05104, doi:10.1029/2005JD006906.
    [ Abstract ]

    The rate at which the outgoing longwave radiation (OLR) responds to perturbations in temperature and moisture plays a fundamental role in determining climate sensitivity. This study examines the clear-sky OLR sensitivities to temperature and water vapor, as quantified by its partial derivatives (radiative Jacobians). The Jacobians, as computed by the Geophysical Fluid Dynamics Laboratory (GFDL)'s line-by-line (LBL) radiative transfer model are used to verify the results from the parameterized GFDL GCM (general circulation model) radiation code. The results show that the (1) Jacobians of OLR due to incremental changes in temperature and water vapor are insensitive to different formulations of water vapor continuum absorption and (2) Jacobians of OLR are properly captured by the GCM longwave band approximation. Simulations with the GCM demonstrate that uncertainties in the formulation of continuum absorption have little impact on the climate model simulation of clear-sky OLR changes in response to prescribed sea surface temperature (SST) perturbation. The numerically computed Jacobians of OLR are used to reconstruct the tropical annual mean OLR from the variations of temperature and water vapor over the period 1980–1999. The reconstructed OLR anomaly time series agrees well with that computed explicitly by the GCM. On the basis of this result, it becomes possible to separate out the temperature and water vapor contributions to the OLR variation. The results show that the temperature contribution dominates the water vapor contribution in the lower and middle troposphere, while in the upper troposphere the two contributions largely offset each other.

15. Huang, Yi, V Ramaswamy, X Huang, Q Fu, and C Bardeen, 2007: A strict test in climate modeling with spectrally resolved radiances: GCM simulation versus AIRS observations. Geophysical Research Letters, 34, L24707, doi:10.1029/2007GL031409.
    [ Abstract ]

    The spectrally resolved infrared radiances observed by AIRS provide a strict and insightful test for general circulation models (GCMs). We compare the clear- and total- sky spectra simulated from the Geophysical Fluid Dynamics Laboratory GCM using a high resolution radiation code with the AIRS observations. After ensuring consistency in the sampling of the observed and simulated spectra and a proper representation of clouds in the radiance simulation, the observed and simulated global-mean radiances are shown to agree to within 2 K in the window region. Radiance discrepancies in the water vapor v2 (1300–1650 cm⁻¹) and carbon dioxide v2 (650–720 cm⁻¹) bands are consistent with the model biases in atmospheric temperature and water vapor. The existence of radiance biases of opposite signs in different spectral regions suggests that a seemingly good agreement of the model's broadband longwave flux with observations may be due to a fortuitous cancellation of spectral errors. Moreover, an examination of the diurnal difference spectrum indicates pronounced biases in the model-simulated diurnal hydrologic cycle over the tropical oceans, a feature seen to occur in other GCMs as well.

16. Ming, Yi, V Ramaswamy, Leo J Donner, V T J Phillips, Stephen A Klein, Paul Ginoux, and Larry Horowitz, February 2007: Modeling the interactions between aerosols and liquid water clouds with a self-consistent cloud scheme in a general circulation model. Journal of the Atmospheric Sciences, 64(4), doi:10.1175/JAS3874.1.
    [ Abstract ]

    To model aerosol-cloud interactions in general circulation models (GCMs), a prognostic cloud scheme of cloud liquid water and amount is expanded to include droplet number concentration (N_d) in a way that allows them to be calculated using the same large-scale and convective updraft velocity field. In the scheme, the evolution of droplets fully interacts with the model meteorology. An explicit treatment of cloud condensation nuclei (CCN) activation enables the scheme to take into account the contributions to N_d of multiple aerosol species (i.e., sulfate, organic, and sea-salt aerosols) and to consider kinetic limitations of the activation process. An implementation of the prognostic scheme in the Geophysical Fluid Dynamics Laboratory (GFDL) AM2 GCM yields a vertical distribution of N_d with a characteristic maximum in the lower troposphere; this feature differs from the profile that would be obtained if N_dis diagnosed from the sulfate mass concentration based on an often-used empirical relationship. Prognosticated N_d exhibits large variations with respect to the sulfate mass concentration. The mean values are generally consistent with the empirical relationship over ocean, but show negative biases over the Northern Hemisphere midlatitude land, perhaps owing to the neglect of subgrid variations of large-scale ascents and inadequate convective sources. The prognostic scheme leads to a substantial improvement in the agreement of model-predicted present-day liquid water path (LWP) and cloud forcing with satellite measurements compared to using the empirical relationship. The simulations with preindustrial and present-day aerosols show that the combined first and second indirect effects of anthropogenic sulfate and organic aerosols give rise to a steady-state global annual mean flux change of -1.8 W m^-2, consisting of -2.0 W m^-2 in shortwave and 0.2 W m^-2 in longwave. The ratios of the flux changes in the Northern Hemisphere (NH) to that in Southern Hemisphere (SH) and of the flux changes over ocean to that over land are 2.9 and 0.73, respectively. These estimates are consistent with the averages of values from previous studies stated in a recent review. The model response to higher Nd alters the cloud field; LWP and total cloud amount increase by 19% and 0.6%, respectively. Largely owing to high sulfate concentrations from fossil fuel burning, the NH midlatitude land and oceans experience strong radiative cooling. So does the tropical land, which is dominated by biomass burning-derived organic aerosol. The computed annual, zonal-mean flux changes are determined to be statistically significant, exceeding the model's natural variations in the NH low and midlatitudes and in the SH low latitudes. This study reaffirms the major role of sulfate in providing CCN for cloud formation.

17. Naik, V, D L Mauzerall, Larry Horowitz, M Daniel Schwarzkopf, V Ramaswamy, and M Oppenheimer, 2007: On the sensitivity of radiative forcing from biomass burning aerosols and ozone to emission location. Geophysical Research Letters, 34, L03818, doi:10.1029/2006GL028149.
    [ Abstract ]

    Biomass burning is a major source of air pollutants, some of which are also climate forcing agents. We investigate the sensitivity of direct radiative forcing due to tropospheric ozone and aerosols (carbonaceous and sulfate) to a marginal reduction in their (or their precursor) emissions from major biomass burning regions. We find that the largest negative global forcing is for 10% emission reductions in tropical regions, including Africa (−4.1 mWm⁻² from gas and −4.1 mWm⁻² from aerosols), and South America (−3.0 mWm⁻² from gas and −2.8 mWm⁻² from aerosols). We estimate that a unit reduction in the amount of biomass burned in India produces the largest negative ozone and aerosol forcing. Our analysis indicates that reducing biomass burning emissions causes negative global radiative forcing due to ozone and aerosols; however, regional differences need to be considered when evaluating controls on biomass burning to mitigate global climate change.

18. Solomon, S, D Qin, M Manning, V Ramaswamy, and Ronald J Stouffer, et al., 2007: Technical summary In Climate Change 2007: The Physical Science Basis, Cambridge, UK, Cambridge University Press, 19-92.
19. Collins, W D., V Ramaswamy, M Daniel Schwarzkopf, Y Sun, R W Portmann, Q Fu, S E B Casanova, J-L Dufresne, D W Fillmore, P M D Forster, V Y Galin, L K Gohar, W J Ingram, D P Kratz, M-P Lefebvre, P Marquet, V Oinas, V Tsushima, T Uchiyama, and W Y Zhong, July 2006: Radiative forcing by well-mixed greenhouse gases: Estimates from climate models in the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4). Journal of Geophysical Research, 111(D14), D14317, doi:10.1029/2005JD006713.
    [ Abstract ]

    The radiative effects from increased concentrations of well-mixed greenhouse gases (WMGHGs) represent the most significant and best understood anthropogenic forcing of the climate system. The most comprehensive tools for simulating past and future climates influenced by WMGHGs are fully coupled atmosphere-ocean general circulation models (AOGCMs). Because of the importance of WMGHGs as forcing agents it is essential that AOGCMs compute the radiative forcing by these gases as accurately as possible. We present the results of a radiative transfer model intercomparison between the forcings computed by the radiative parameterizations of AOGCMs and by benchmark line-by-line (LBL) codes. The comparison is focused on forcing by CO2, CH4, N2O, CFC-11, CFC-12, and the increased H2O expected in warmer climates. The models included in the intercomparison include several LBL codes and most of the global models submitted to the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4). In general, the LBL models are in excellent agreement with each other. However, in many cases, there are substantial discrepancies among the AOGCMs and between the AOGCMs and LBL codes. In some cases this is because the AOGCMs neglect particular absorbers, in particular the near-infrared effects of CH4 and N2O, while in others it is due to the methods for modeling the radiative processes. The biases in the AOGCM forcings are generally largest at the surface level. We quantify these differences and discuss the implications for interpreting variations in forcing and response across the multimodel ensemble of AOGCM simulations assembled for the IPCC AR4.

20. Delworth, Thomas L., Anthony J Broccoli, Anthony Rosati, Ronald J Stouffer, Ventakramani Balaji, J A Beesley, W F Cooke, Keith W Dixon, John Dunne, Krista A Dunne, J W Durachta, Kirsten L Findell, Paul Ginoux, Anand Gnanadesikan, C Tony Gordon, Stephen Griffies, Rich Gudgel, Matthew J Harrison, Isaac Held, Richard S Hemler, Larry Horowitz, Stephen A Klein, Thomas R Knutson, P J Kushner, A R Langenhorst, H C Lee, Shian-Jiann Lin, Jian Lu, S Malyshev, P C D Milly, V Ramaswamy, J L Russell, M Daniel Schwarzkopf, Elena Shevliakova, Joseph J Sirutis, Michael J Spelman, William F Stern, Michael Winton, Andrew T Wittenberg, Bruce Wyman, Fanrong Zeng, and Rong Zhang, 2006: GFDL's CM2 Global Coupled Climate Models. Part I: Formulation and Simulation Characteristics. Journal of Climate, 19(5), doi:10.1175/JCLI3629.1.
    [ Abstract ]

    The formulation and simulation characteristics of two new global coupled climate models developed at NOAA's Geophysical Fluid Dynamics Laboratory (GFDL) are described. The models were designed to simulate atmospheric and oceanic climate and variability from the diurnal time scale through multicentury climate change, given our computational constraints. In particular, an important goal was to use the same model for both experimental seasonal to interannual forecasting and the study of multicentury global climate change, and this goal has been achieved. Two versions of the coupled model are described, called CM2.0 and CM2.1. The versions differ primarily in the dynamical core used in the atmospheric component, along with the cloud tuning and some details of the land and ocean components. For both coupled models, the resolution of the land and atmospheric components is 2° latitude × 2.5° longitude; the atmospheric model has 24 vertical levels. The ocean resolution is 1° in latitude and longitude, with meridional resolution equatorward of 30° becoming progressively finer, such that the meridional resolution is 1/3° at the equator. There are 50 vertical levels in the ocean, with 22 evenly spaced levels within the top 220 m. The ocean component has poles over North America and Eurasia to avoid polar filtering. Neither coupled model employs flux adjustments. The control simulations have stable, realistic climates when integrated over multiple centuries. Both models have simulations of ENSO that are substantially improved relative to previous GFDL coupled models. The CM2.0 model has been further evaluated as an ENSO forecast model and has good skill (CM2.1 has not been evaluated as an ENSO forecast model). Generally reduced temperature and salinity biases exist in CM2.1 relative to CM2.0. These reductions are associated with 1) improved simulations of surface wind stress in CM2.1 and associated changes in oceanic gyre circulations; 2) changes in cloud tuning and the land model, both of which act to increase the net surface shortwave radiation in CM2.1, thereby reducing an overall cold bias present in CM2.0; and 3) a reduction of ocean lateral viscosity in the extratropics in CM2.1, which reduces sea ice biases in the North Atlantic. Both models have been used to conduct a suite of climate change simulations for the 2007 Intergovernmental Panel on Climate Change (IPCC) assessment report and are able to simulate the main features of the observed warming of the twentieth century. The climate sensitivities of the CM2.0 and CM2.1 models are 2.9 and 3.4 K, respectively. These sensitivities are defined by coupling the atmospheric components of CM2.0 and CM2.1 to a slab ocean model and allowing the model to come into equilibrium with a doubling of atmospheric CO2. The output from a suite of integrations conducted with these models is freely available online (see http://nomads.gfdl.noaa.gov/). Manuscript received 8 December 2004, in final form 18 March 2005

21. Erlick, C, V Ramaswamy, and L M Russell, 2006: Differing regional responses to a perturbation in solar cloud absorption in the SKYHI general circulation model. Journal of Geophysical Research, 111, D06204, doi:10.1029/2005JD006491.
    [ Abstract ]

    In this study we perform an idealized experiment to investigate the effect of solar absorption in clouds on climate using a general circulation model with prescribed sea surface temperatures, focusing on the manner of regional changes during the northern summer season. The response arising from this type of perturbation is akin to “semidirect” effects of absorbing aerosols, namely, dissipation of clouds owing to a high aerosol absorption in the cloud layers. In the experiment, we apply a similar perturbation to all low-cloud layers, reducing their single-scattering albedo to a value of 0.99, which enables us to isolate the effect of such solar absorption from other aerosol related influences. We find that in both midlatitude and equatorial regions, the reduction in low-cloud single-scattering albedo causes a reduction in low-cloud amount and a warming of the surface. However, the dynamical response of the system varies from one continental region to another. In the midlatitude regions of the United States and Europe/east Asia, the diabatic heating perturbation leads to the dissipation of low clouds, an increase in shortwave flux to the surface, an increase in horizontal heat advection, and an increase in atmospheric stability. In the tropical region of North Africa, the diabatic heating perturbation translates into an increase in convection, a decrease in stability, an increase in middle- and high-level clouds, and a reduction in shortwave flux to the surface. In agreement with previous studies, these results demonstrate the distinctive response of the tropical versus midlatitude regions to a similar solar perturbation.

22. Ginoux, Paul, Larry Horowitz, V Ramaswamy, I V Geogdzhayev, B Holben, G Stenchikov, and X Tie, 2006: Evaluation of aerosol distribution and optical depth in the Geophysical Fluid Dynamics Laboratory coupled model CM2.1 for present climate. Journal of Geophysical Research, 111, D22210, doi:10.1029/2005JD006707.
    [ Abstract ]

    This study evaluates the strengths and weaknesses of aerosol distributions and optical depths that are used to force the GFDL coupled climate model CM2.1. The concentrations of sulfate, organic carbon, black carbon, and dust are simulated using the MOZART model (Horowitz, 2006), while sea-salt concentrations are obtained from a previous study by Haywood et al. (1999). These aerosol distributions and precalculated relative-humidity-dependent specific extinction are utilized in the CM2.1 radiative scheme to calculate the aerosol optical depth. Our evaluation of the mean values (1996–2000) of simulated aerosols is based on comparisons with long-term mean climatological data from ground-based and remote sensing observations as well as previous modeling studies. Overall, the predicted concentrations of aerosol are within a factor 2 of the observed values and have a tendency to be overestimated. Comparison with satellite data shows an agreement within 10% of global mean optical depth. This agreement masks regional differences of opposite signs in the optical depth. Essentially, the excessive optical depth from sulfate aerosols compensates for the underestimated contribution from organic and sea-salt aerosols. The largest discrepancies are over the northeastern United States (predicted optical depths are too high) and over biomass burning regions and southern oceans (predicted optical depths are too low). This analysis indicates that the aerosol properties are very sensitive to humidity, and major improvements could be achieved by properly taking into account their hygroscopic growth together with corresponding modifications of their optical properties.

23. Huang, X, V Ramaswamy, and M Daniel Schwarzkopf, 2006: Quantification of the source of errors in AM2 simulated tropical clear-sky outgoing longwave radiation. Journal of Geophysical Research, 111, D14107, doi:10.1029/2005JD006576.
    [ Abstract ]

    The global and tropical means of clear-sky outgoing longwave radiation (hereinafter OLRc) simulated by the new GFDL atmospheric general circulation model, AM2, tend to be systematically lower than ERBE observations by about 4 W m-2, even though the AM2 total-sky radiation budget is tuned to be consistent with these observations. Here we quantify the source of errors in AM2-simulated OLRc over the tropical oceans by comparing the synthetic outgoing IR spectra at the top of the atmosphere on the basis of AM2 simulations to observed IRIS spectra. After the sampling disparity between IRIS and AM2 is reduced, AM2 still shows considerable negative bias in the simulated monthly mean OLRc over the tropical oceans. Together with other evidence, this suggests that the influence of spatial sampling disparity, although present, does not account for the majority of the bias. Decomposition of OLRc shows that the negative bias comes mainly from the H2O bands and can be explained by a too humid layer around 6–9 km in the model. Meanwhile, a positive bias exists in channels sensitive to near-surface humidity and temperature, which implies that the boundary layer in the model might be too dry. These facts suggest that the negative bias in the simulated OLRc can be attributed to model deficiencies, especially the large-scale water vapor transport. We also find that AM2-simulated OLRc has ~1 W m-2 positive bias originating from the stratosphere; this positive bias should exist in simulated total-sky OLR as well.

24. Knutson, Thomas R., Thomas L Delworth, Keith W Dixon, Isaac Held, Jian Lu, V Ramaswamy, M Daniel Schwarzkopf, G Stenchikov, and Ronald J Stouffer, 2006: Assessment of Twentieth-Century regional surface temperature trends using the GFDL CM2 coupled models. Journal of Climate, 19(9), doi:10.1175/JCLI3709.1.
    [ Abstract ]

    Historical climate simulations of the period 1861–2000 using two new Geophysical Fluid Dynamics Laboratory (GFDL) global climate models (CM2.0 and CM2.1) are compared with observed surface temperatures. All-forcing runs include the effects of changes in well-mixed greenhouse gases, ozone, sulfates, black and organic carbon, volcanic aerosols, solar flux, and land cover. Indirect effects of tropospheric aerosols on clouds and precipitation processes are not included. Ensembles of size 3 (CM2.0) and 5 (CM2.1) with all forcings are analyzed, along with smaller ensembles of natural-only and anthropogenic-only forcing, and multicentury control runs with no external forcing. Observed warming trends on the global scale and in many regions are simulated more realistically in the all-forcing and anthropogenic-only forcing runs than in experiments using natural-only forcing or no external forcing. In the all-forcing and anthropogenic-only forcing runs, the model shows some tendency for too much twentieth-century warming in lower latitudes and too little warming in higher latitudes. Differences in Arctic Oscillation behavior between models and observations contribute substantially to an underprediction of the observed warming over northern Asia. In the all-forcing and natural-only forcing runs, a temporary global cooling in the models during the 1880s not evident in the observed temperature records is volcanically forced. El Niño interactions complicate comparisons of observed and simulated temperature records for the El Chichón and Mt. Pinatubo eruptions during the early 1980s and early 1990s. The simulations support previous findings that twentieth-century global warming has resulted from a combination of natural and anthropogenic forcing, with anthropogenic forcing being the dominant cause of the pronounced late-twentieth-century warming. The regional results provide evidence for an emergent anthropogenic warming signal over many, if not most, regions of the globe. The warming signal has emerged rather monotonically in the Indian Ocean/western Pacific warm pool during the past half-century. The tropical and subtropical North Atlantic and the tropical eastern Pacific are examples of regions where the anthropogenic warming signal now appears to be emerging from a background of more substantial multidecadal variability.

25. Ming, Yi, V Ramaswamy, Leo J Donner, and V T J Phillips, 2006: A new parameterization of cloud droplet activation applicable to general circulation models. Journal of the Atmospheric Sciences, 63(4), doi:10.1175/JAS3686.11348-1356.
    [ Abstract ]

    A new parameterization is proposed to link the droplet number concentration to the size distribution and chemical composition of aerosol and updraft velocity. Except for an empirical assumption of droplet growth, the parameterization is formulated almost entirely on first principles to allow for satisfactory performance under a variety of conditions. For a series of updraft velocity ranging from 0.03 to 10.0 m s−1, the droplet number concentrations predicted with the parameterization are in good agreement with the detailed parcel model simulations with an average error of −4 ± 26% (one standard deviation). The accuracy is comparable to or better than some existing parameterizations. The parameterization is able to account for the effects of droplet surface tension and mass accommodation coefficient on activation without adjusting the empirical parameter. These desirable attributes make the parameterization suitable for being used in the prognostic determination of the cloud droplet number concentration in general circulation models (GCMs).

26. Ramaswamy, V, M Daniel Schwarzkopf, W Randel, B D Santer, Brian J Soden, and G Stenchikov, 2006: Anthropogenic and natural influences in the evolution of lower stratospheric cooling. Science, 311(5764), doi:10.1126/science.1122587.
    [ Abstract ]

    Observations reveal that the substantial cooling of the global lower stratosphere over 1979–2003 occurred in two pronounced steplike transitions. These arose in the aftermath of two major volcanic eruptions, with each cooling transition being followed by a period of relatively steady temperatures. Climate model simulations indicate that the space-time structure of the observed cooling is largely attributable to the combined effect of changes in both anthropogenic factors (ozone depletion and increases in well-mixed greenhouse gases) and natural factors (solar irradiance variation and volcanic aerosols). The anthropogenic factors drove the overall cooling during the period, and the natural ones modulated the evolution of the cooling.

27. Ramaswamy, V, J W Hurrell, G A Meehl, A Phillips, B D Santer, M Daniel Schwarzkopf, D J Seidel, S C Sherwood, and P W Thorne, 2006: Why do temperatures vary vertically (from the surface to the stratosphere) and what do we understand about why they might vary and change over time? In Temperature Trends in the Lower Atmosphere: Steps for Understanding and Reconciling Differences, Karl, T R, S J Hassol, C D Miller, W L Murray, eds., Washington, DC, A Report by the Climate Change Science Program/Subcommittee on Global Change Research, 15-28.
    [ PDF ]
28. Ramaswamy, V, S Ramachandran, G Stenchikov, and A Robock, 2006: A model study of the effect of Pinatubo volcanic aerosols on the stratospheric temperature In Frontiers of Climate Modeling, Kiehl, J T, V. Ramanathan, eds., UK, Cambridge University Press, 152-178.
29. Stenchikov, G, K P Hamilton, Ronald J Stouffer, A Robock, V Ramaswamy, B D Santer, and H F Graf, et al., 2006: Arctic Oscillation response to volcanic eruptions in the IPCC AR4 climate models. Journal of Geophysical Research, 111, D07107, doi:10.1029/2005JD006286.
    [ Abstract ]

    Stratospheric sulfate aerosol particles from strong volcanic eruptions produce significant transient cooling of the troposphere and warming of the lower stratosphere. The radiative impact of volcanic aerosols also produces a response that generally includes an anomalously positive phase of the Arctic Oscillation (AO) that is most pronounced in the boreal winter. The main atmospheric thermal and dynamical effects of eruptions typical of the past century persist for about two years after each eruption. In this paper we evaluate the volcanic responses in simulations produced by seven of the climate models included in the model intercomparison conducted as part of the preparation of the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4). We consider global effects as well as the regional circulation effects in the extratropical Northern Hemisphere focusing on the AO responses forced by volcanic eruptions. Specifically we analyze results from the IPCC historical runs that simulate the evolution of the circulation over the last part of the 19th century and the entire 20th century using a realistic time series of atmospheric composition (greenhouse gases and aerosols). In particular, composite anomalies over the two boreal winters following each of the nine largest low-latitude eruptions during the period 1860–1999 are computed for various tropospheric and stratospheric fields. These are compared when possible with observational data. The seven IPCC models we analyzed use similar assumptions about the amount of volcanic aerosols formed in the lower stratosphere following the volcanic eruptions that have occurred since 1860. All models produce tropospheric cooling and stratospheric warming as in observations. However, they display a considerable range of dynamic responses to volcanic aerosols. Nevertheless, some general conclusions can be drawn. The IPCC models tend to simulate a positive phase of the Arctic Oscillation in response to volcanic forcing similar to that typically observed. However, the associated dynamic perturbations and winter surface warming over Northern Europe and Asia in the post-volcano winters is much weaker in the models than in observations. The AR4 models also underestimate the variability and long-term trend of the AO. This deficiency affects high-latitude model predictions and may have a similar origin. This analysis allows us to better evaluate volcanic impacts in up-to-date climate models and to better quantify the model Arctic Oscillation sensitivity to external forcing. This potentially could lead to improving model climate predictions in the extratropical latitudes of the Northern Hemisphere.

30. Wigley, T M., V Ramaswamy, J R Christy, John R Lanzante, C Mears, B D Santer, and C K Folland, 2006: Executive Summary In Temperature Trends in the Lower Atmosphere: Steps for Understanding and Reconciling Differences, Karl, T R, S J Hassol, C D Miller, W L Murray, eds., Washington, DC, A Report by the Climate Change Science Program and the Subcommittee on Global Change Research, 1-14.
    [ PDF ]
31. Delworth, Thomas L., V Ramaswamy, and G Stenchikov, 2005: The impact of aerosols on simulated ocean temperature and heat content in the 20th century. Geophysical Research Letters, 32, L24709, doi:10.1029/2005GL024457.
    [ Abstract ]

    Observational analyses have documented increases in global ocean temperature, heat content, and sea level in the 20th century. Previous studies argued that the observed ocean warming is a response to increasing greenhouse gases. We use a new climate model to decompose simulated ocean temperature changes into components attributable to subsets of anthropogenic and natural influences. The model simulates a positive trend in global ocean volume mean temperature from the mid 1950s to 2000, consistent with observational estimates. We show that for the period 1861–2000 aerosols have delayed the onset of ocean warming by several decades and reduced the magnitude of the transient warming by approximately two-thirds when compared to the response that arises solely from increasing greenhouse gases. The simulated cooling signature from large volcanic eruptions in the late 19th and early 20th centuries is clearly visible in the subsurface ocean well into the middle part of the 20th century.

32. Freidenreich, Stuart, and V Ramaswamy, 2005: Refinement of the Geophysical Fluid Dynamics Laboratory solar benchmark computations and an improved parameterization for climate models. Journal of Geophysical Research, 110, D17105, doi:10.1029/2004JD005471.
    [ Abstract PDF ]

    A recent intercomparison study of solar radiative transfer models has revealed a notable difference (5%) in the total spectrum column absorptance, for a specified clear-sky atmospheric profile, between two principal line-by-line benchmark results (namely, the Geophysical Fluid Dynamics Laboratory (GFDL) and the Atmospheric and Environmental Research, Inc. models). We resolve this discrepancy by performing a series of “benchmark” computations which show that the water vapor continuum formulation, spectral line information, and spectral distribution of the solar irradiance at the top of the atmosphere are key factors. Accounting for these considerations reduces the difference between the two benchmarks to less than 1%. The analysis establishes a high level of confidence in the use of benchmark calculations for developing and testing solar radiation parameterizations in weather and climate models. The magnitude of the change in absorption in the newer GFDL benchmark computations, associated with the use of a more recent spectral line catalog and inclusion of the water vapor continuum, has also necessitated revising the solar parameterization used in the operational GFDL general circulation model (GCM). When compared with the newer reference computation, the older parameterization shows an underestimate of the clear-sky heating rate throughout the atmosphere, with the error in the atmospheric solar absorbed flux being about 20 W m-2 for a midlatitude summer atmosphere and overhead Sun. In contrast, the new parameterization improves the representation of the solar absorption and reduces the bias to about 5 W m-2. Another important feature of the new parameterization is a nearly 50% reduction in the number of pseudomonochromatic columnar calculations compared to the older formulation, with only relatively small increases in the biases in absorption for cloudy layers. This yields a reduction of about 10% in the GCM computational time. The effect of the new parameterization on the simulated temperature in the new operational GFDL climate GCM is also examined. There is an increased solar heating; this yields temperature increases exceeding 1 K in the lower stratosphere.

33. Halthore, R N., D Crisp, S E Schwartz, G P Anderson, A Berk, B Bonnel, O Boucher, F-L Chang, M-D Chou, E E Clothiaux, P Dubuisson, B Fomin, Y Fouquart, Stuart Freidenreich, C Gautier, S Kato, I Laszlo, Z Li, J H Mather, A Plana-Fattori, V Ramaswamy, P Ricchiazzi, Y Shiren, A Trishchenko, and W Wiscombe, 2005: Intercomparison of shortwave radiative transfer codes and measurements. Journal of Geophysical Research, 110, D11206, doi:10.1029/2004JD005293.
    [ Abstract ]

    Computation of components of shortwave (SW) or solar irradiance in the surface-atmospheric system forms the basis of intercomparison between 16 radiative transfer models of varying spectral resolution ranging from line-by-line models to broadband and general circulation models. In order of increasing complexity the components are: direct solar irradiance at the surface, diffuse irradiance at the surface, diffuse upward flux at the surface, and diffuse upward flux at the top of the atmosphere. These components allow computation of the atmospheric absorptance. Four cases are considered from pure molecular atmospheres to atmospheres with aerosols and atmosphere with a simple uniform cloud. The molecular and aerosol cases allow comparison of aerosol forcing calculation among models. A cloud-free case with measured atmospheric and aerosol properties and measured shortwave radiation components provides an absolute basis for evaluating the models. For the aerosol-free and cloud-free dry atmospheres, models agree to within 1% (root mean square deviation as a percentage of mean) in broadband direct solar irradiance at surface; the agreement is relatively poor at 5% for a humid atmosphere. A comparison of atmospheric absorptance, computed from components of SW radiation, shows that agreement among models is understandably much worse at 3% and 10% for dry and humid atmospheres, respectively. Inclusion of aerosols generally makes the agreement among models worse than when no aerosols are present, with some exceptions. Modeled diffuse surface irradiance is higher than measurements for all models for the same model inputs. Inclusion of an optically thick low-cloud in a tropical atmosphere, a stringent test for multiple scattering calculations, produces, in general, better agreement among models for a low solar zenith angle (SZA = 30°) than for a high SZA (75°). All models show about a 30% increase in broadband absorptance for 30° SZA relative to the clear-sky case and almost no enhancement in absorptance for a higher SZA of 75°, possibly due to water vapor line saturation in the atmosphere above the cloud.

34. Ming, Yi, V Ramaswamy, Paul Ginoux, and Larry Horowitz, 2005: Direct radiative forcing of anthropogenic organic aerosol. Journal of Geophysical Research, 110, D20208, doi:10.1029/2004JD005573.
    [ Abstract ]

    This study simulates the direct radiative forcing of organic aerosol using the GFDL AM2 GCM. The aerosol climatology is provided by the MOZART chemical transport model (CTM). The approach to calculating aerosol optical properties explicitly considers relative humidity–dependent hygroscopic growth by employing a functional group–based thermodynamic model, and makes use of the size distribution derived from AERONET measurements. The preindustrial (PI) and present-day (PD) global burdens of organic carbon are 0.17 and 1.36 Tg OC, respectively. The annual global mean total-sky and clear-sky top-of-the atmosphere (TOA) forcings (PI to PD) are estimated as −0.34 and −0.71 W m⁻², respectively. Geographically the radiative cooling largely lies over the source regions, namely part of South America, Central Africa, Europe and South and East Asia. The annual global mean total-sky and clear-sky surface forcings are −0.63 and −0.98 W m⁻², respectively. A series of sensitivity analyses shows that the treatments of hygroscopic growth and optical properties of organic aerosol are intertwined in the determination of the global organic aerosol forcing. For example, complete deprivation of water uptake by hydrophilic organic particles reduces the standard (total-sky) and clear-sky TOA forcing estimates by 18% and 20%, respectively, while the uptake by a highly soluble organic compound (malonic acid) enhances them by 18% and 32%, respectively. Treating particles as non-absorbing enhances aerosol reflection and increases the total-sky and clear-sky TOA forcing by 47% and 18%, respectively, while neglecting the scattering brought about by the water associated with particles reduces them by 24% and 7%, respectively.

35. Ming, Yi, V Ramaswamy, Paul Ginoux, Larry Horowitz, and L M Russell, 2005: Geophysical Fluid Dynamics Laboratory general circulation model investigation of the indirect radiative effects of anthropogenic sulfate aerosol. Journal of Geophysical Research, 110, D22206, doi:10.1029/2005JD006161.
    [ Abstract ]

    The Geophysical Fluid Dynamics Laboratory (GFDL) atmosphere general circulation model, with its new cloud scheme, is employed to study the indirect radiative effect of anthropogenic sulfate aerosol during the industrial period. The preindustrial and present-day monthly mean aerosol climatologies are generated from running the Model for Ozone And Related chemical Tracers (MOZART) chemistry-transport model. The respective global annual mean sulfate burdens are 0.22 and 0.81 Tg S. Cloud droplet number concentrations are related to sulfate mass concentrations using an empirical relationship (Boucher and Lohmann, 1995). A distinction is made between "forcing" and flux change at the top of the atmosphere in this study. The simulations, performed with prescribed sea surface temperature, show that the first indirect "forcing" ("Twomey" effect) amounts to an annual mean of -1.5 W m^-2, concentrated largely over the oceans in the Northern Hemisphere (NH). The annual mean flux change owing to the response of the model to the first indirect effect is -1.4 W m^-2, similar to the annual mean forcing. However, the model's response causes a rearrangement of cloud distribution as well as changes in longwave flux (smaller than solar flux changes). There is thus a differing geographical nature of the radiation field than for the forcing even though the global means are similar. The second indirect effect, which is necessarily an estimate made in terms of the model's response, amounts to -0.9 W m^-2, but the statistical significance of the simulated geographical distribution of this effect is relatively low owing to the model's natural variability. Both the first and second effects are approximately linearly additive, giving rise to a combined annual mean flux change of -2.3 W m^-2, with the NH responsible for 77% of the total flux change. Statistically significant model responses are obtained for the zonal mean total indirect effect in the entire NH and in the Southern Hemisphere low latitudes and midlatitudes (north of 45°S). The area of significance extends more than for the first and second effects considered separately. A comparison with a number of previous studies based on the same sulfate-droplet relationship shows that, after distinguishing between forcing and flux change, the global mean change in watts per square meter for the total effect computed in this study is comparable to existing studies in spite of the differences in cloud schemes.

36. Naik, V, D L Mauzerall, Larry Horowitz, M Daniel Schwarzkopf, V Ramaswamy, and M Oppenheimer, 2005: Net radiative forcing due to changes in regional emissions of tropospheric ozone precursors. Journal of Geophysical Research, 110, D24306, doi:10.1029/2005JD005908.
    [ Abstract ]

    The global distribution of tropospheric ozone (O₃) depends on the emission of precursors, chemistry, and transport. For small perturbations to emissions, the global radiative forcing resulting from changes in O₃ can be expressed as a sum of forcings from emission changes in different regions. Tropospheric O₃ is considered in present climate policies only through the inclusion of indirect effect of CH₄ on radiative forcing through its impact on O₃ concentrations. The short-lived O₃ precursors (NO_x , CO, and NMHCs) are not directly included in the Kyoto Protocol or any similar climate mitigation agreement. In this study, we quantify the global radiative forcing resulting from a marginal reduction (10%) in anthropogenic emissions of NO_x alone from nine geographic regions and a combined marginal reduction in NO_x , CO, and NMHCs emissions from three regions. We simulate, using the global chemistry transport model MOZART-2, the change in the distribution of global O₃ resulting from these emission reductions. In addition to the short-term reduction in O₃, these emission reductions also increase CH₄concentrations (by decreasing OH); this increase in CH₄ in turn counteracts part of the initial reduction in O₃ concentrations. We calculate the global radiative forcing resulting from the regional emission reductions, accounting for changes in both O₃ and CH₄. Our results show that changes in O₃ production and resulting distribution depend strongly on the geographical location of the reduction in precursor emissions. We find that the global O₃ distribution and radiative forcing are most sensitive to changes in precursor emissions from tropical regions and least sensitive to changes from midlatitude and high-latitude regions. Changes in CH₄ and O₃ concentrations resulting from NO_x emission reductions alone produce offsetting changes in radiative forcing, leaving a small positive residual forcing (warming) for all regions. In contrast, for combined reductions of anthropogenic emissions of NO_x , CO, and NMHCs, changes in O₃ and CH₄ concentrations result in a net negative radiative forcing (cooling). Thus we conclude that simultaneous reductions of CO, NMHCs, and NO_x lead to a net reduction in radiative forcing due to resulting changes in tropospheric O₃ and CH₄ while reductions in NO_x emissions alone do not.

37. Santer, B D., T M L Wigley, C Mears, F J Wentz, Stephen A Klein, D J Seidel, K E Taylor, P W Thorne, M F Wehner, P J Gleckler, J S Boyle, W D Collins, Keith W Dixon, C Doutriaux, M Free, Q Fu, J E Hansen, G S Jones, R Ruedy, T R Karl, John R Lanzante, G A Meehl, V Ramaswamy, G Russell, and G A Schmidt, 2005: Amplification of surface temperature trends and variability in the tropical atmosphere. Science, 309(5740), doi:10.1126/science.1114867.
    [ Abstract ]

    The month-to-month variability of tropical temperatures is larger in the troposphere than at Earth's surface. This amplification behavior is similar in a range of observations and climate model simulations and is consistent with basic theory. On multidecadal time scales, tropospheric amplification of surface warming is a robust feature of model simulations, but it occurs in only one observational data set. Other observations show weak, or even negative, amplification. These results suggest either that different physical mechanisms control amplification processes on monthly and decadal time scales, and models fail to capture such behavior; or (more plausibly) that residual errors in several observational data sets used here affect their representation of long-term trends.

38. Seidel, D J., J K Angell, A Robock, B Hicks, K Labitzke, John R Lanzante, J Logan, Jerry D Mahlman, V Ramaswamy, W Randel, E Rasmusson, R Ross, and S F Singer, 2005: Jim Angell's contributions to meteorology. Bulletin of the American Meteorological Society, 86(3), doi:10.1175/BAMS-86-3-403.
39. Soden, Brian J., D L Jackson, V Ramaswamy, M Daniel Schwarzkopf, and X Huang, 2005: The radiative signature of upper tropospheric moistening. Science, 310(5749), doi:10.1126/science.1115602.
    [ Abstract ]

    Climate models predict that the concentration of water vapor in the upper troposphere could double by the end of the century as a result of increases in greenhouse gases. Such moistening plays a key role in amplifying the rate at which the climate warms in response to anthropogenic activities, but has been difficult to detect because of deficiencies in conventional observing systems. We use satellite measurements to highlight a distinct radiative signature of upper tropospheric moistening over the period 1982 to 2004. The observed moistening is accurately captured by climate model simulations and lends further credence to model projections of future global warming.

40. Thorne, P W., T R Karl, H Coleman, C K Folland, B Murray, D E Parker, V Ramaswamy, W Rossow, A A Scaife, and S F B Tett, 2005: Vertical profiles of temperature trends. Bulletin of the American Meteorological Society, 86(8), 1471-1476.
    [ PDF ]
41. Ramaswamy, V, 2004: A report of the Gordon Research Conference on. SPARC Newsletter, 22, 18-20.
    [ PDF ]
42. Randel, W, V Ramaswamy, D J Karoly, D J Seidel, and S Yoden, 2004: The SPARC Workshop on understanding seasonal temperature trends in the stratosphere. SPARC Newsletter, 22, 24-28.
    [ PDF ]
43. Randles, C A., L M Russell, and V Ramaswamy, 2004: Hygroscopic and optical properties of organic sea salt aerosol and consequences for climate forcing. Geophysical Research Letters, 31, L16108, doi:10.1029/2004GL020628.
    [ Abstract ]

    Scattering of incoming solar radiation by sea salt aerosol is strongly dependent on relative humidity (RH) since hygroscopic particles take up water at high RH. Organic compounds may constitute up to 50% of marine aerosol mass in internal mixtures. We used a detailed thermodynamic and optical model to calculate hygroscopic growth and extinction of sea salt aerosol internally mixed with a soluble organic compound. Increasing organic content from 10 to 50% suppresses growth at high RH compared to a pure NaCl particle by 4 to 20%. For a mildly absorbing organic, the scattering increase with RH is reduced by up to 32% for these mixtures, consistent with observations. Internal mixtures of 90% NaCl and 10% non-absorbing organics cause 3% less cooling than 100% NaCl particles in the visible spectrum over the clear-sky oceans. For a mildly absorbing organic compound, 10% organic content reduces radiative cooling substantially compared to 100% NaCl aerosol.

44. Stenchikov, G, K P Hamilton, A Robock, V Ramaswamy, and M Daniel Schwarzkopf, 2004: Arctic oscillation response to the 1991 Pinatubo eruption in the SKYHI general circulation model with a realistic quasi-biennial oscillation. Journal of Geophysical Research, 109(D3), D03112, doi:10.1029/2003JD003699.
    [ Abstract ]

    Stratospheric aerosol clouds from large tropical volcanic eruptions can be expected to alter the atmospheric radiative balance for a period of up to several years. Observations following several previous major eruptions suggest that one effect of the radiative perturbations is to cause anomalies in the Northern Hemisphere extratropical winter tropospheric circulation that can be broadly characterized as positive excursions of the Arctic Oscillation (AO). We report on a modeling investigation of the radiative and dynamical mechanisms that may account for the observed AO anomalies following eruptions. We focus on the best observed and strongest 20th century eruption, that of Mt. Pinatubo on 15 June 1991. The impact of the Pinatubo eruption on the climate has been the focus of a number of earlier modeling studies, but all of these previous studies used models with no quasi-biennial oscillation (QBO) in the tropical stratosphere. The QBO is a very prominent feature of interannual variability of tropical stratospheric circulation and could have a profound effect on the global atmospheric response to volcanic radiative forcing. Thus a complete study of the atmospheric variability following volcanic eruptions should include a realistic representation of the tropical QBO. Here we address, for the first time, this important issue. We employed a version of the SKYHI troposphere-stratosphere-mesosphere model that effectively assimilates observed zonal mean winds in the tropical stratosphere to simulate a very realistic QBO. We performed an ensemble of 24 simulations for the period 1 June 1991 to 31 May 1993. These simulations included a realistic prescription of the stratospheric aerosol layer based on satellite observations. These integrations are compared to control integrations with no volcanic aerosol. The model produced a reasonably realistic representation of the positive AO response in boreal winter that is usually observed after major eruptions. Detailed analysis shows that the aerosol perturbations to the tropospheric winter circulation are affected significantly by the phase of the QBO, with a westerly QBO phase in the lower stratosphere resulting in an enhancement of the aerosol effect on the AO. Improved quantification of the QBO effect on climate sensitivity helps to better understand mechanisms of the stratospheric contribution to natural and externally forced climate variability.

45. Barker, H W., G L Stephens, P T Partain, J W Bergman, B Bonnel, K Campana, E E Clothiaux, S Clough, S Cusack, J Delamere, J Edwards, K F Evans, Y Fouquart, Stuart Freidenreich, V Y Galin, Y Hou, S Kato, J-L Li, E Mlawer, J-J Morcrette, W O'Hirok, P Räisänen, V Ramaswamy, B Ritter, E Rozanov, M Schlesinger, K Shibata, P Sporyshev, Z Sun, M Wendisch, N Wood, and S Yang, 2003: Assessing 1D atmospheric solar radiative transfer models: Interpretation and handling of unresolved clouds. Journal of Climate, 16(16), 2676-2699.
    [ Abstract PDF ]

    The primary purpose of this study is to assess the performance of 1D solar radiative transfer codes that are used currently both for research and in weather and climate models. Emphasis is on interpretation and handling of unresolved clouds. Answers are sought to the following questions: (i) How well do 1D solar codes interpret and handle columns of information pertaining to partly cloudy atmospheres? (ii) Regardless of the adequacy of their assumptions about unresolved clouds, do 1D solar codes perform as intended? One clear-sky and two plane-parallel, homogeneous (PPH) overcast cloud cases serve to elucidate 1D model differences due to varying treatments of gaseous transmittances, cloud optical properties, and basic radiative transfer. The remaining four cases involve 3D distributions of cloud water and water vapor as simulated by cloud-resolving models. Results for 25 1D codes, which included two line-by-line (LBL) models (clear and overcast only) and four 3D Monte Carlo (MC) photon transport algorithms, were submitted by 22 groups. Benchmark, domain-averaged irradiance profiles were computed by the MC codes. For the clear and overcast cases, all MC estimates of top-of-atmosphere albedo, atmospheric absorptance, and surface absorptance agree with one of the LBL codes to within ±2%. Most 1D codes underestimate atmospheric absorptance by typically 15-25 W m-2 at overhead sun for the standard tropical atmosphere regardless of clouds. Depending on assumptions about unresolved clouds, the 1D codes were partitioned into four genres: (i) horizontal variability, (ii) exact overlap of PPH clouds, (iii) maximum/random overlap of PPH clouds, and (iv) random overlap of PPH clouds. A single MC code was used to establish conditional benchmarks applicable to each genre, and all MC codes were used to establish the full 3D benchmarks. There is a tendency for 1D codes to cluster near their respective conditional benchmarks, though intragenre variances typically exceed those for the clear and overcast cases. The majority of 1D codes fall into the extreme category of maximum/random overlap of PPH clouds and thus generally disagree with full 3D benchmark values. Given the fairly limited scope of these tests and the inability of any one code to perform extremely well for all cases begs the question that a paradigm shift is due for modeling 1D solar fluxes for cloudy atmospheres.

46. Erlick, C, and V Ramaswamy, 2003: Note on the definition of clear sky in calculations of shortwave cloud forcing. Journal of Geophysical Research, 108(D5), 4156, doi:10.1029/2002JD002990.
    [ Abstract ]

    An important item to distinguish in estimations of cloud forcing is the characteristics of the "clear sky." In this study we investigate the influence of the composition of the clear sky in calculations of shortwave cloud forcing based on two case studies from the Monterey Area Ship Track Experiment (MAST). The forcing is calculated with respect to a clear sky devoid of aerosol particles and with respect to a clear sky containing the aerosol particles present in and below the cloud layer at below-cloud ambient humidity. It is found that in the case of a continentally influenced stratocumulus cloud containing a large concentration of dust and/or soot aerosols, the definition of clear sky makes an 8-10% difference in the upwelling solar irradiance and cloud forcing ratio.

47. Erlick, C, and V Ramaswamy, 2003: Sensitivity of the atmospheric lapse rate to solar cloud absorption in a radiative-convective model. Journal of Geophysical Research, 108(D16), 4522, doi:10.1029/2002JD002966.
    [ Abstract ]

    Previous radiative-convective model studies of the radiative forcing due to absorbing aerosols such as soot and dust have revealed a strong dependence on the vertical distribution of the absorbers. In this study, we extend this concept to absorption in cloud layers, using a one-dimensional radiative-convective model employing high, middle, and low cloud representations to investigate the response of the surface temperature and atmospheric lapse rate to increases in visible cloud absorption. The visible single-scattering albedo (ssa) of the clouds is prescribed, ranging from 1.0 to 0.6, where 0.99 is the minimum that would be expected from the presence of absorbing aerosols within the cloud drops on the basis of recent Monterey Area Ship Track (MAST) Experiment case studies. Simulations are performed with respect to both a constant cloud optical depth and an increasing cloud optical depth and as a function of cloud height. We find that increases in solar cloud absorption tend to warm the troposphere and surface and stabilize the atmosphere, while increases in cloud optical depth cool the troposphere and surface and slightly stabilize the atmosphere between the low cloud top and surface because of the increase in surface cooling. In the absence of considerations involving microphysical or cloud-climate feedbacks, we find that two conditions are required to yield an inversion from a solar cloud absorption perturbation: (1) The solar absorption perturbation must be included throughout the tropospheric clouds column, distributing the solar heating to higher altitudes, and (2) the ssa of the clouds must be 0.6, which is an unrealistically low value. The implication is that there is very little possibility of significant stabilization of the global mean atmosphere due to perturbation of cloud properties given current ssa values.

48. Garrett, T J., L M Russell, V Ramaswamy, S F Maria, and B Huebert, 2003: Microphysical and radiative evolution of aerosol plumes over the tropical North Atlantic Ocean. Journal of Geophysical Research, 108(D1), 4022, doi:10.1029/2002JD002228.
    [ Abstract PDF ]

    Over the tropical North Atlantic Ocean in the summer, plumes of aerosol extend from Saharan Africa to the Caribbean. The microphysical and radiative evolution of such plumes is studied using a Lagrangian column model and measurements obtained near the west coast of Africa (during the second Aerosol Characterization Experiment [ACE-2]) and the Caribbean Sea (during the Passing Efficiency of the Low Turbulence Inlet [PELTI] experiment). Mass and scattering in the plumes can be separated into two layers that overlay one another over much of the Atlantic Ocean. Mineral dust dominates in the lower free troposphere, and sea-salt aerosol dominates in the boundary layer. Carbonaceous, sulfate, and nitrate (CSN) aerosols are a minor component of mass but contribute significantly to total column optical depth. Combined, CSN aerosols and sea-salt contribute to more than half of total aerosol clear-sky shortwave forcing associated with such plumes. Satellite and model data suggest that the reduction of plume forcing between the African coastline and the Caribbean is less than ~20%. The reduction is due principally to settling of large dust particles and atmospheric subsidence; however, the reduction of forcing remains small because (a) boundary layer trade winds provide a steady source of sea-salt, (b) dust particles are initially elevated 2.5–5.5 km from the surface and therefore have long settling distances before removal, and (c) small CSN and dust particles in the free troposphere have high specific extinction and lack significant removal processes. Measurements and climatology suggest that the CSN aerosols in the free troposphere are anthropogenic pollution from Europe.

49. Shine, K P., M S Bourqui, P M D Forster, S H E Hare, U Langematz, P Braesicke, V Grewe, M Ponater, C Schnadt, C A Smith, J D Haigh, John Austin, N Butchart, D Shindell, W Randel, T Nagashima, R W Portmann, S Solomon, D J Seidel, John R Lanzante, Stephen A Klein, V Ramaswamy, and M Daniel Schwarzkopf, 2003: A comparison of model-simulated trends in stratospheric temperatures. Quarterly Journal of the Royal Meteorological Society, 129(590), 1565-1588.
    [ Abstract PDF ]

    Estimates of annual-mean stratospheric temperature trends over the past twenty years, from a wide variety of models, are compared both with each other and with the observed cooling seen in trend analyses using radiosonde and satellite observations. The modelled temperature trends are driven by changes in ozone (either imposed from observations or calculated by the model), carbon dioxide and other relatively well-mixed greenhouse gases, and stratospheric water vapour. The comparison shows that whilst models generally simulate similar patterns in the vertical profile of annual-and global-mean temperature trends, there is a significant divergence in the size of the modelled trends, even when similar trace gas perturbations are imposed. Coupled-chemistry models are in as good agreement as models using imposed observed ozone trends, despite the extra degree of freedom that the coupled models possess. The modelled annual- and global-mean cooling of the upper stratosphere (near 1 hPa) is dominated by ozone and carbon dioxide changes, and is in reasonable agreement with observations. At about 5 hPa, the mean cooling from the models is systematically greater than that seen in the satellite data; however, for some models, depending on the size of the temperature trend due to stratospheric water vapour changes, the uncertainty estimates of the model and observations just overlap. Near 10 hPa there is good agreement with observations. In the lower stratosphere (20-70 hPa), ozone appears to be the dominant contributor to the observed cooling, although it does not, on its own, seem to explain the entire cooling. Annual- and zonal-mean temperature trends at 100 hPa and 50 hPa are also examined. At 100 hPa, the modelled cooling due to ozone depletion alone is in reasonable agreement with the observed cooling at all latitudes. At 50 hPa, however, the observed cooling at midlatitudes of the northern hemisphere significantly exceeds the modelled cooling due to ozone depletion alone. There is an indication of a similar effect in high northern latitudes, but the greater variability in both models and observations precludes a firm conclusion. The discrepancies between modelled and observed temperature trends in the lower stratosphere are reduced if the cooling effects of increased stratospheric water vapour concentration are included, and could be largely removed if certain assumptions were made regarding the size and distribution of the water vapour increase. However, given the uncertainties in the geographical extent of water vapour changes in the lower stratosphere, and the time period over which such changes have been sustained, other reasons for the discrepancy between modelled and observed temperature trends cannot be ruled out.

50. Allan, R P., V Ramaswamy, and A Slingo, 2002: Diagnostic analysis of atmospheric moisture and clear-sky radiative feedback in the Hadley Centre and Geophysical Fluid Dynamics Laboratory (GFDL) climate models. Journal of Geophysical Research, 107(D17), doi:10.1029/2001JD001131.
    [ Abstract PDF Supplemental ]

    The interannual variability of the hydrological cycle is diagnosed from the Hadley Centre and Geophysical Fluid Dynamics Laboratory (GFDL) climate models, both of which are forced by observed sea surface temperatures. The models produce a similar sensitivity of clear-sky outgoing longwave radiation to surface temperature of ~2 W m-2 K-1 , indicating a consistent and positive clear-sky radiative feedback. However, differences between changes in the temperature lapse-rate and the height dependence of moisture fluctuations suggest that contrasting mechanisms bring about this result. The GFDL model appears to give a weaker water vapor feedback (i.e., changes in specific humidity). This is counteracted by a smaller upper tropospheric temperature response to surface warming, which implies a compensating positive lapse-rate feedback.

51. Allan, R P., A Slingo, and V Ramaswamy, 2002: Analysis of moisture variability in the European Centre for Medium-Range Weather Forecasts 15-year reanalysis over the tropical oceans. Journal of Geophysical Research, 107(D15), doi:10.1029/2001JD001132.
    [ Abstract PDF ]

    We compare European Centre for Medium-Range Weather Forecasts 15-year reanalysis (ERA-15) moisture over the tropical oceans with satellite observations and the U.S. National Centers for Environmental Prediction (NCEP) National Center for Atmospheric Research 40-year reanalysis. When systematic differences in moisture between the observational and reanalysis data sets are removed, the NCEP data show excellent agreement with the observations while the ERA-15 variability exhibits remarkable differences. By forcing agreement between ERA-15 column water vapor and the observations, where available, by scaling the entire moisture column accordingly, the height-dependent moisture variability remains unchanged for all but the 550–850 hPa layer, where the moisture variability reduces signifcantly. Thus the excess variation of column moisture in ERA-15 appears to originate in this layer. The moisture variability provided by ERA-15 is not deemed of sufficient quality for use in the validation of climate models.

52. Cooke, W F., V Ramaswamy, and P S Kasibhatla, 2002: A general circulation model study of the global carbonaceous aerosol distribution. Journal of Geophysical Research, 107(D16), doi:10.1029/2001JD001274.
    [ Abstract PDF ]

    Atmospheric distributions of carbonaceous aerosols are simulated using the Geophysical Fluid Dynamics Laboratory SKYHI general circulation model (GCM) (latitude-longitude resolution of ~3° x 3.6°). A number of systematic analyses are conducted to investigate the seasonal and interannual variability of the concentrations at specific locations and to investigate the sensitivity of the distributions to various physical parameters. Comparisons are made with several observational data sets. At four specific sites (Mace Head, Mauna Loa, Sable Island, and Bondville) the monthly mean measurements of surface concentrations of black carbon made over several years reveal that the model simulation registers successes as well as failures. Comparisons are also made with averages of measurements made over varying time periods, segregated by geography and rural/remote locations. Generally, the mean measured remote surface concentrations exceed those simulated. Notwithstanding the large variability in measurements and model simulations, the simulations of both black and organic carbon tend to be within about a factor of 2 at a majority of the sites. There are major challenges in conducting comparisons with measurements due to inadequate sampling at some sites, the generally short length of the observational record, and different methods used for estimating the black and organic carbon amounts. The interannual variability in the model and in the few such measurements available points to the need for doing multiyear modeling and to the necessity of comparing with long-term measurements. There are very few altitude profile measurements; notwithstanding the large uncertainties, the present comparisons suggest an overestimation by the model in the free troposphere. The global column burdens of black and organic carbon in the present standard model integration are lower than in previous studies and thus could be regarded as approximately bracketing a lower end of the simulated anthropogenic burden due to these classes of aerosols, based on the current understanding of the carbonaceous aerosol cycle. Of the physical factors examined, the intensity and frequency of precipitation events are critical in governing the column burdens. Biases in the frequency of precipitation are likely the single biggest cause of discrepancies between simulation and observations. This parameter is available from very few sites and thus lacks a comprehensive global data set, unlike, say, monthly mean precipitation. Several multiyear GCM integrations have been performed to evaluate the sensitivity of the global mean black carbon distribution to the principal aerosol parameters, with due regard to variability and statistical significance. The most sensitive parameters, in order of importance, turn out to be the wet deposition, transformation from hydrophobic to hydrophilic state, and the partitioning of the emitted aerosol between the hydrophobic and hydrophilic varieties. From the sensitivity tests, it is estimated that the variations of the global mean column burden and lifetime of black carbon are within about a factor of 2 about their respective standard values. The studies also show that the column burdens over remote regions appear to be most sensitive to changes in each parameter, reiterating the importance of measurements in these locations for a proper evaluation of model simulation of these aerosols.

53. Ramaswamy, V, 2002: Infrared radiation In Encyclopedia of Global Environmental Change, Vol. I, Chichester, UK:, John Wiley & Sons, Ltd, 470-475.
    [ Abstract ]

    Infrared radiation is one of the two critical components in the heat balance of the planet taken as a whole, being distinct from the other component (viz., solar radiation). It is comprised of radiative energy emitted by the Earth's surface and the atmosphere. In contrast to solar radiation, which is a source of heat from the Sun into the climate system, the infrared radiation represents a loss of heat from Earth. The absorption and emission of infrared radiation by the Earth's surface and atmosphere constitute the basis for the greenhouse effect, and plays a critical role in governing the climate of the planet.

54. Ramaswamy, V, M E Gelman, M Daniel Schwarzkopf, and J-J R Lin, 2002: An update of stratospheric temperature trends. SPARC Newsletter, 18, 7-9.
    [ PDF ]
55. Ramaswamy, V, and M Daniel Schwarzkopf, 2002: Effects of ozone and well-mixed gases on annual-mean stratospheric temperature trends. Geophysical Research Letters, 29(22), 2064, doi:10.1029/2002GL015141.
    [ Abstract PDF ]

    The effects of changes in ozone and well-mixed greenhouse gases upon the annual-mean stratospheric temperatures are investigated using a general circulation model and compared with the observed (1979–2000) trends. In the global-mean lower stratosphere (50–100 hPa), ozone changes exert the most important influence upon the cooling trend. In the upper stratosphere, where both ozone and greenhouse gas changes influence the temperature trends, the amount of cooling is sensitive to the background ozone climatology. Taking into account the uncertainties in the observed temperature trend estimates and the dynamical variability of the model, the simulated results are in reasonable quantitative agreement with the vertical profile of the observed global-and-annual-mean stratospheric cooling, and with the observed lower stratospheric zonal-and-annual-mean cooling. This affirms the major role of these species in the temperature trend of the stratosphere over the past two decades.

56. Schwarzkopf, M D., and V Ramaswamy, 2002: Effects of changes in well-mixed gases and ozone on stratospheric seasonal temperatures. Geophysical Research Letters, 29(24), doi:10.1029/2002GL015759.
    [ Abstract PDF ]

    Monthly and seasonal stratospheric zonal-mean temperature trends arising from recent changes in stratospheric ozone and well-mixed greenhouse gases (WMGGs) are simulated using a general circulation model and compared with observed (1979–2000) trends. The combined effect of these gases yields statistically significant cooling trends over the entire globally averaged stratosphere in all months. In the Arctic (60°N–90°N), statistically significant trends occur only in summer and extend through the entire stratosphere. In the Antarctic (90°S–65°S), the simulations reproduce the observed seasonal pattern of the lower stratosphere temperature trend. Seasonal trends at 50 hPa are consistent with observed trends at all latitudes, considering model dynamical variability and observational uncertainty. The lack of robustness in simulated and observed Arctic winter trends indicates the futility of attributing these trends to trace gas concentration changes. Such attribution arguments may be made with greater confidence regarding middle and high latitude Northern Hemisphere summer temperature trends.

57. Stenchikov, G, A Robock, V Ramaswamy, M Daniel Schwarzkopf, K P Hamilton, and S Ramachandran, 2002: Arctic oscillation response to the 1991 Mount Pinatubo eruption: effects of volcanic aerosols and ozone depletion. Journal of Geophysical Research, 107(D24), 4803, doi:10.1029/2002JD002090.
    [ Abstract ]

    Observations show that strong equatorial volcanic eruptions have been followed by a pronounced positive phase of the Arctic Oscillation (AO) for one or two Northern Hemisphere winters. It has been previously assumed that this effect is forced by strengthening of the equator-to-pole temperature gradient in the lower stratosphere, caused by aerosol radiative heating in the tropics. To understand atmospheric processes that cause the AO response, we studied the impact of the 1991 Mount Pinatubo eruption, which produced the largest global volcanic aerosol cloud in the twentieth century. A series of control and perturbation experiments were conducted with the GFDL SKYHI general circulation model to examine the evolution of the circulation in the 2 years following the Pinatubo eruption. In one set of perturbation experiments, the full radiative effects of the observed Pinatubo aerosol cloud were included, while in another only the effects of the aerosols in reducing the solar flux in the troposphere were included, and the aerosol heating effects in the stratosphere were suppressed. A third set of perturbation experiments imposed the stratospheric ozone losses observed in the post-Pinatubo period. We conducted ensembles of four to eight realizations for each case. Forced by aerosols, SKYHI produces a statistically significant positive phase of the AO in winter, as observed. Ozone depletion causes a positive phase of the AO in late winter and early spring by cooling the lower stratosphere in high latitudes, strengthening the polar night jet, and delaying the final warming. A positive phase of the AO was also produced in the experiment with only the tropospheric effect of aerosols, showing that aerosol heating in the lower tropical stratosphere is not necessary to force positive AO response, as was previously assumed. Aerosol-induced tropospheric cooling in the subtropics decreases the meridional temperature gradient in the winter troposphere between 30°N and 60°N. The corresponding reduction of mean zonal energy and amplitudes of planetary waves in the troposphere decreases wave activity flux into the lower stratosphere. The resulting strengthening of the polar vortex forces a positive phase of the AO. We suggest that this mechanism can also contribute to the observed long-term AO trend being caused by greenhouse gas increases because they also weaken the tropospheric meridional temperature gradient due to polar amplification of warming.

58. Erlick, C, L M Russell, and V Ramaswamy, 2001: A microphysics-based investigation of the radiative effects of aerosol-cloud interactions for two MAST Experiment case studies. Journal of Geophysical Research, 106(D1), 1249-1269.
    [ Abstract PDF ]

    We use a size- and composition-resolved externally mixed aerosol microphysical model and a delta-Eddington exponential-sum-fit radiation algorithm to examine the interactions between aerosol particles and cloud drops, and their influence on solar radiation. Both the aerosol model and the radiation code are designed to explicitly handle external and internal aerosol particle and cloud drop mixtures. Using observations from the Monterey Area Ship Track (MAST) Experiment, we model changes in aerosol and cloud drop size distributions for a clean marine cloud and ship track and a continentally influenced marine cloud and ship track. Linking these results to the radiation algorithm with a Mie-scattering subroutine, we investigate the corresponding changes in cloud albedo, cloud absorption, and transmission. The differences in 0.3-3.0 µm albedo and transmission between the clouds and ship tracks as a result of the changes in drop size distribution and composition are found to be substantial, and the composition of the cloud drops is found to be important particularly in the continentally influenced case. Both the clouds and ship tracks enhance atmospheric absorption with respect to a clear sky, with a cloud forcing ration ranging from 1.15 to 1.37, where the clear sky is defined to be cloud- and aerosol-free. Sensitivity studies are performed with respect to the updraft velocity, the updraft area fraction, dilution of the ship remissions, and the composition of supermicron continental particles. The radiation results are also compared with Meteorological Research Flight (MRF) C-130 in situ aircraft measurements and with parameterizations of the "Twomey effect."

59. Ramaswamy, V, 2001: Stratospheric temperature changes: Observations and model simulations In Long Term Changes and Trends in the Atmosphere, Vol. II, New Dehli, India, New Age International Limited, 3-32.
    [ Abstract ]

    Observations of stratospheric temperatures over the past three decades indicate a general cooling of the global lower stratosphere, and a pronounced cooling of the upper and middle stratosphere. This paper examines the long-term trends as inferred from a variety of available observations. Model simulations of temperature response due to changes in concentrations of radiatively active species are also analyzed. A comparative evaluation of the model simulations with observations reveals the extent to which the global-mean and zonal-mean lower stratosphere temperature trends can be attributed to trace gas changes.

60. Ramaswamy, V, and M Daniel Schwarzkopf, et al., 2001: Radiative forcing of climate change In Climate Change 2001: The Scientific Basis. Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change, Cambridge, UK, Cambridge University Press, 350-416.
61. Ramaswamy, V, et al., 2001: Stratospheric temperature trends: observations and model simulations. Reviews of Geophysics, 39(1), 71-122.
    [ Abstract PDF ]

    Trends and variations in global stratospheric temperatures are an integral part of the changes occurring in the Earth's climate system. Data sets for analyzing long-term (a decade and more) changes in the stratospheric temperatures consist of radiosonde, satellite, lidar, and rocketsonde measurements; meteorological analyses based on radiosonde and/or satellite data; and products based on assimilating observations using a general circulation model. Each of these contain varying degrees of uncertainties that influence the interpretation and significance of trends. We review the long-term trends from approximately the mid-1960s to the mid-1990s period. The stratosphere has, in general, undergone considerable cooling over the past 3 decades. At northern midlatitudes the lower stratosphere (~16-21 km) cooling over the 1979-1994 period is strikingly coherent among the various data sets with regard to magnitude and statistical significance. A substantial cooling occurs in the polar lower stratosphere during winter-spring; however, there is a large dynamical variability in the northern polar region. The vertical profile of the annual-mean stratospheric temperature change in the northern midlatitudes over the 1979-1994 period is robust among the different data sets, with ~0.75 K/decade cooling in the ~20- to 35-km region and increasing cooling above (e.g., ~2.5 K/decade at 50 km). Model investigations into the cause or causes of the observed temperature trends are also reviewed. Simulations based on the known changes in species' concentrations indicate that the depletion of lower stratospheric ozone is the major radiative factor in accounting for the 1979-1990 cooling trend in the global, annual-mean lower stratosphere (~0.5 to 0.6 K/decade), with a substantially lesser contribution by the well-mixed greenhouse gases. Ozone loss is also an important causal factor in the latitude-month pattern of the lower stratospheric cooling trend. Uncertainties arise due to incomplete knowledge of the vertical profile of ozone loss near the tropopause. In the middle and upper stratosphere, both well-mixed greenhouse gases and ozone changes contribute in an important manner to the cooling, but model simulations underestimate the observed decadal-scale trend. While there is a lack of reliable information on water vapor changes over the 1980s decade, satellite measurements in the early to middle 1990s indicate increases in water vapor that could be a significant contributor to the cooling of the global lower stratosphere.

62. Stocker, T F., Thomas L Delworth, Stephen Griffies, Isaac Held, V Ramaswamy, and Brian J Soden, et al., 2001: Physical climate processes and feedbacks In Climate Change 2001: The Scientific Basis. Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change, Cambridge, UK, Cambridge University Press, 418-470.
63. Ramachandran, S, V Ramaswamy, G Stenchikov, and A Robock, 2000: Radiative impact of the Mount Pinatubo volcanic eruption: Lower stratospheric response. Journal of Geophysical Research, 105(D19), 24,409-24,429.
    [ Abstract PDF ]

    Volcanic aerosols in the stratosphere produce significant transitory solar and infrared radiative perturbations, which warm the stratosphere, cool the surface and affect stratospheric circulation. In this study, using the Geophysical Fluid Dynamics Laboratory SKYHI general circulation model (GCM) with a high vertical resolution and a recently improved radiative transfer code, we investigate the aerosol radiative forcing and the stratospheric temperature response for the June 15, 1991 Mount Pinatubo eruption, the most well observed and largest volcanic eruption of the 20th Century. The investigation is carried out using an updated, comprehensive monthly and zonal-mean Pinatubo aerosol spectral optical properties data set. While the near-infrared solar spectral effects contribute substantially to the total stratospheric heating due to aerosols, over the entire global domain the longwave component exceeds the solar in causing a warming of the lower stratosphere (30- 100 hPa). In contrast, the magnitude of the solar perturbation (increased reflection) in the overall surface-atmosphere radiative heat balance exceeds that due to the longwave (infrared trapping effect). The troposphere affects the stratospheric radiative forcing, mainly because of the dependence of the reflected solar and upward longwave radiation on cloudiness, and this adds to the uncertainty in the calculation of the stratospheric temperature response. A four-member ensemble of 2-year GCM integrations (June 1991 to May 1993) were performed using fixed sea surface temperatures and a cloud prediction scheme, one set with and another without the volcanic aerosols. The temperature of the tropical lower stratosphere increases by a statistically significant 3 K, which is almost 1 K less than in previous investigations that employed coarser vertical resolution in the stratosphere, but is still larger than observed. In the low latitudes the evolution of the simulated temperature response mimics that observed only through about the first year. Thereafter, despite a significant aerosol optical depth perturbation in the tropical atmosphere, there is a lack of a signature in the temperature response that can be unambiguously attributed to the Pinatubo aerosols, suggesting other forced or unforced variations (e.g., ozone changes, quasi-biennial oscillation) occurring in the actual atmosphere which are unaccounted for in the model. In the high latitudes the large interannual variability prohibits a clear quantitative comparison between simulated and observed temperature changes and renders the aerosol-induced thermal signals statistically insignificant. In the global mean the evolution of the simulated lower stratospheric temperature response is in excellent agreement with the observation for the entire 2-year period, in contrast to the model-observation comparison at the low latitudes. This arises because in the global mean the stratospheric response is not sensitive to dynamical adjustments within the atmosphere caused by internal variations, and depends principally on the external radiative forcing caused by the aerosols.

64. Andronache, C, Leo J Donner, Charles J Seman, V Ramaswamy, and Richard S Hemler, 1999: Atmospheric sulfur and deep convective clouds in tropical Pacific: A model study. Journal of Geophysical Research, 104(D4), 4005-4024.
    [ Abstract PDF ]

    A high-resolution limited area nonhydrostatic model was used to simulate sulfate-cloud interactions during the convective activity in a case study from the Tropical Ocean Global Atmosphere Coupled Ocean Atmosphere Response Experiment, December 20-25, 1992. The model includes a new detailed sulfate-cloud microphysics scheme designed to estimate the effects of sulfate on cloud microphysics and radiative properties and the effects of deep convection on the transport and redistribution of aerosol. The data for SO2 and SO4(2-) species were taken from the Pacific Exploratory Mission West B observations during February-March 1994. Results show that a change in sulfate loading from the minimum to the maximum observed value scenarios (i.e., from about 0.01 to 1 µg m-3) causes a significant decrease of the effective radius of cloud droplets (changes up to 2 µm on average) and an increase of the diagnostic number concentration of cloud droplets (typical changes about 5-20 cm-3). The change in the average net shortwave (SW) radiation flux above the clouds was estimated to be on average -1.5 W m-2, with significant spatial and temporal variations. The horizontal average of the changes in the net SW radiation fluxes above clouds has a diurnal cycle, reaching typical values approximately -3 W m-2. The changes in the average net longwave radiation flux above the clouds were negligible, but they showed significant variations, typically between -10 W m-2 and 10 W m-2 near the surface. These variations were associated mainly with the changes in the distribution of cloud water, which showed typical relative changes of cloud water path of about 10-20%. Other notable changes induced by the increase of aerosol were the variations in air temperature of the order of 1°C. The case study presented here suggests that characteristics of convective clouds in tropical areas are sensitive to atmospheric sulfate loading, particularly during enhanced sulfate episodes.

65. Andronache, C, Leo J Donner, V Ramaswamy, Charles J Seman, and Richard S Hemler, 1999: Possible impact of atmospheric sulfur increase on tropical convective systems: A TOGA COARE Case In Proceedings of a Conference on the TOGA Coupled Ocean-Atmosphere Response Experiment (COARE) - COARE-98, WCRP-107, WMO/TD-No. 940, Geneva, Switzerland, WMO, 243-244.
66. Chanin, M-L, and V Ramaswamy, 1999: Trends in stratospheric temperatures In Scientific Assessment of Ozone Depletion: 1998, World Meteorological Organization, Global Ozone Research and Monitoring Project - Report No. 44, Gen, NOAA/NASA/UNEP/WMO, European Commission, 5.1-5.59.
67. Freidenreich, Stuart, and V Ramaswamy, 1999: A new multiple-band solar radiative parameterization for general circulation models. Journal of Geophysical Research, 104(D24), 31,389-31,409.
    [ Abstract PDF ]

    An extensive set of line-by-line plus doubling-adding reference computations for both clear and overcast skies has been utilized to develop, calibrate, and verify the accuracy of a new multiple-band solar parameterization, suitable for use in atmospheric general circulation models. In developing this parameterization the emphasis is placed on reproducing accurately the reference absorbed flux in clear and overcast atmospheres. In addition, a significantly improved representation of the reference stratospheric heating profile, in comparison with that derived from older, broadband solar parameterizations, has been attained primarily because of an improved parameterization of CO₂ heating. The exponential-sum-fit technique is used to develop the paramterization of water vapor transmission in the main absorbing bands. An absorptivity approach is used to represent the heating contributions by CO₂ and O₂, and a spectral averaging of the continuum-like properties is used to represent the O₃ heating. There are a total of 72 pseudomonochromatic intervals needed to do the radiative transfer problem in the vertically inhomogeneous atmosphere. The delta-Eddington method is used to solve for the reflection and transmission of the homogeneous layers, while the "adding" method is used to combine the layers. The single-scattering properties of the homogeneous layers can account for all types of scattering and absorbing constituents (molecules, drops, ice particles, and aerosols), given their respective single-scattering properties and mass concentrations. With respect to the reference computational results the clear-sky heating rates are generally accurate to within 10%, and the atmospheric absorbed flux is accurate to within 2%. An analysis is made of the factors contributing to the error in the parameterized cloud absorption in the near infrared. Derivation of the representative drop coalbedo for a band using the mean reflection for an infinitely thick cloud (thick-averaging technique) generally results in a better agreement with the reference cloud absorbed flux than that derived using the mean drop coalbedo (thin-averaging technique), except for high, optically thin water clouds. Further, partitioning the 2500 < v < 8200 cm^-1 spectral region into several more bands than two (the minimum required) results in an improved representation of the cloud absorbed flux, with a modest increase in the shortwave radiation computational time. The cloud absorbed flux is accurate to within 10%, and the cloudy layer heating rates are accurate to within 15%, for water clouds, while larger errors can occur for ice clouds. The atmospheric absorbed, downward surface, and upward top-of-the-atmosphere fluxes are generally accurate to within 10%. Corrections appear in: Journal of Geophysical Research, 105(D6), 7371.

68. Fung, K K., and V Ramaswamy, 1999: On shortwave radiation absorption in overcast atmospheres. Journal of Geophysical Research, 104(D18), 22,233-22,241.
    [ Abstract PDF ]

    Using a numerical model of solar radiative transfer that is calibrated against benchmark computations, it is shown that atmospheric water vapor, together with the microphysical characteristics of water drops (liquid water path and effective radius), plays an important role in the total solar spectrum reflection and absorption in overcast skies. For any specific cloud type, the water vapor column above the cloud and the presence of saturated water vapor inside the cloud contribute significantly to atmospheric absorption. These factors also affect the relationship between the net shortwave fluxes at the top and bottom of overcast atmospheres, in particular, inhibiting a general universal linkage between these two quantities. Thus neglect of details concerning the vertical location, extent, and microphysical aspects of clouds can lead to biases in the inference of surface irradiance using top-of-the-atmosphere measurements.

69. Haywood, J M., V Ramaswamy, and Brian J Soden, 1999: Tropospheric aerosol climate forcing in clear-sky satellite observations over the oceans. Science, 283(5406), 1299-1303.
    [ Abstract PDF ]

    Tropospheric aerosols affect the radiative forcing of Earth's climate, but their variable concentrations complicate an understanding of their global influence. Model-based estimates of aerosol distributions helped reveal spatial patterns indicative of the presence of tropospheric aerosols in the satellite-observed clear-sky solar radiation budget over the world's oceans. The results show that, although geographical signatures due to both natural and anthropogenic aerosols are manifest in the satellite observations, the naturally occurring sea-salt is the leading aerosol contributor to the global-mean clear-sky radiation balance over oceans.

70. Schwarzkopf, M D., and V Ramaswamy, 1999: Radiative effects of CH₄, N₂O, halocarbons and the foreign-broadened H₂O continuum: A GCM experiment. Journal of Geophysical Research, 104(D8), 9467-9488.
    [ Abstract PDF ]

    The simplified exchange approximation (SEA) method for calculation of infrared radiative transfer, used for general circulation model (GCM) climate simulations at the Geophysical Fluid Dynamics Laboratory (GFDL) and other institutions, has been updated to permit inclusion of the effects of methane (CH₄), nitrous oxide (N₂O), halocarbons, and water-vapor-air molecular broadening (foreign broadening). The effects of CH₄ and N₂O are incorporated by interpolation of line-by-line (LBL) transmissivity calculations evaluated at standard species concentrations; halocarbon effects are calculated from transmissivities computed using recently measured frequency-dependent absorption coefficients. The effects of foreign broadening are included by adoption of the "CKD" formalisim for the water vapor continuum [Clough et al., 1989]. For a standard midlatitude summer profile, the change in the net infrared flux at the model tropopause due to the inclusion of present-day concentrations of CH₄ and N₂O is evaluated to within ~5% of corresponding LBL results; the change in net flux at the tropopause upon inclusion of 1 ppbv of CFC-11, CFC-12, CFC-113, and HCFC-22 is within ~10% of the LBL results. Tropospheric heating rate changes resulting from the introduction of trace species (CH₄, N₂O, and halocarbons) are calculated to within ~0.03 K/d of the LBL results. Introduction of the CKD water vapor continuum causes LBL-computed heating rates to decrease by up to ~0.4 K/d in the upper troposphere and to increase by up to ~0.25 K/d in the midtroposphere; the SEA method gives changes within ~0.05 K/d of the LBL values. The revised SEA formulation has been incorporated into the GFDL "SKYHI" GCM. Two simulations (using fixed sea surface temperatures and prescribed clouds) have been performed to determine the changes to the model climate from that of a control calculation upon inclusion of (1) the trace species and (2) the foreign-broadened water vapor continuum. When the trace species are added, statistically significant warming (~1 K) occurs in the annual-mean tropical upper troposphere, while cooling (~1.5 K) is noted in the upper stratosphere and stratopause region. The changes are generally similar to annual-mean equilibrium calculations made using a radiative-convective model assuming fixed dynamical heating. The effects of the CKD water vapor continuum include cooling (~1 K) in the annual-mean troposphere above ~6 km, with significant warming in the lower troposophere. When effects of both trace gases and the CKD continuum are included, the annual-mean temperature increases below ~5 km and cools between 5 and 10 km, indicating that continuum effects dominate in determining temperature changes in the lower and middle troposphere. Above, trace gas effects dominate, resulting in warming in the tropical upper troposphere and cooling in most of the middle atmosphere. Clear-sky outgoing longwave irradiances have been computed for observed European Centre for Medium-Range Weather Forecasting atmospheric profiles using three versions of the SEA formulation, including the effects of (1) water vapor, carbon dioxide, and ozone; (2) the above species plus present-day concentrations of the new trace species; (3) all of the above species plus the CKD H₂O continuum. Results for all three cases are within ~10 W/m² of corresponding Earth Radiation Budget Experiment clear-sky irradiance measurements. The combined effect of trace gases and the CKD continuum result in a decrease of ~8 W/m² in the computed irradiances.

71. Andronache, C, Leo J Donner, V Ramaswamy, Charles J Seman, and Richard S Hemler, 1998: The effects of atmospheric sulfur on the radiative properties of convective clouds: a limited area modeling study. Geophysical Research Letters, 25(9), 1423-1426.
    [ Abstract PDF ]

    Convective clouds in tropical areas can be sensitive to the atmospheric sulfate loading, particularly during enhanced sulfate episodes. This assertion is supported by simulations with a high resolution limited area non-hydrostatic model (LAN) employing a detailed sulfate-cloud microphysics scheme, applied to estimate the effects of sulfate on convective clouds in a case study from the Tropical Ocean Global Atmosphere Coupled Ocean Atmosphere Response Experiment (TOGA COARE). Results show that a change in sulfate loading for scenarios using the minimum to the maximum observed values produces a change in the average net flux of shortwave radiation above clouds. This time-average change was estimated between -1.1 and -0.3 Wm -2 over the integration domain.

72. Boucher, O, S E Schwartz, T P Ackerman, T L Anderson, and V Ramaswamy, et al., 1998: Intercomparison of models representing direct shortwave radiative forcing by sulfate aerosols. Journal of Geophysical Research, 103(D14), 16979-16998.
    [ Abstract PDF ]

    The importance of aerosols as agents of climate change has recently been highlighted. However, the magnitude of aerosol forcing by scattering of shortwave radiation (direct forcing) is still very uncertain even for the relatively well characterized sulfate aerosol. A potential source of uncertainty is in the model representation of aerosol optical properties and aerosol influences on radiative transfer in the atmosphere. Although radiative transfer methods and codes have been compared in the past, these comparisons have not focused on aerosol forcing (change in net radiative flux at the top of the atmosphere). Here we report results of a project involving 12 groups using 15 models to examine radiative forcing by sulfate aerosol for a wide range of values of particle radius, aerosol optical depth, surface albedo, and solar zenith angle. Among the models that were employed were high and low spectral resolution models incorporating a variety of radiative transfer approximations as well as a line-by-line model. The normalized forcings (forcing per sulfate column burden) obtained with the several radiative transfer models were examined, and the discrepancies were characterized. All models simulate forcings of comparable amplitude and exhibit a similar dependence on input parameters. As expected for a non-light-absorbing aerosol, forcings were negative (cooling influence) except at high surface albedo combined with small solar zenith angle. The relative standard deviation of the zenith-angle-averaged normalized broadband forcing for 15 models was 8% for particle radius near the maximum in this forcing (~0.2 µm) and at low surface albedo. Somewhat greater model-to-model discrepancies were exhibited at specific solar zenith angles. Still greater discrepancies were exhibited at small particle radii, and much greater discrepancies were exhibited at high surface albedos, at which the forcing changes sign; in these situations, however, the normalized forcing is quite small. Discrepancies among the models arise from inaccuracies in Mie calculations, differing treatment of the angular scattering phase function, differing wavelength and angular resolution, and differing treatment of multiple scattering. These results imply the need for standardized radiative transfer methods tailored to the direct aerosol forcing problem. However, the relatively small spread in these results suggests that the uncertainty in forcing arising from the treatment of radiative forcing of a well-characterized aerosol at well-specified surface albedo is smaller than some of the other sources of uncertainty in estimates of direct forcing by anthropogenic sulfate aerosols and anthropogenic aerosols generally.

73. Harshvardhan, W R., V Ramaswamy, Stuart Freidenreich, and M Batey, 1998: Spectral characteristics of solar near-infrared absorption in cloudy atmospheres. Journal of Geophysical Research, 103(D22), 28,793-28,799.
    [ Abstract PDF ]

    Theoretical and experimentally derived estimates of the atmospheric absorption of solar energy in the presence of clouds have been reported to be at variance for quite a long time. A detailed set of near-monochromatic computations of the reflectance, transmittance, and absorptance of a standard midlatitude atmosphere with embedded water clouds is used to identify spectral features in the solar near-infrared that can be utilized to study this discrepancy. The results are framed in terms of the cloud radiative forcing both at the surface and at the top of the atmosphere, and it is shown that water vapor windows are the most sensitive to variations in cloud optical properties and cloud placement in the vertical. The ratio of the cloud radiative forcing at the surface to that at the top of the atmosphere, R, varies from near zero in the band centers at small wavenumbers for high clouds to ~1 in the band centers at larger wavenumbers for low clouds and to values in excess of 2 in the water vapor windows at small wavenumbers. The possibility of using measurements from space with the future Moderate Resolution Imaging Spectroradiometer (MODIS) and simultaneous surface measurements is discussed. It is also shown that horizontal inhomogeneities in the cloud layers do not alter appreciably the estimates of the R factor, but areal mean cloud absorption is lower for an inhomogeneous cloud having the same mean liquid water as the corresponding homogeneous cloud.

74. Haywood, J M., and V Ramaswamy, 1998: Global sensitivity studies of the direct radiative forcing due to anthropogenic sulfate and black carbon aerosols. Journal of Geophysical Research, 103(D6), 6043-6058.
    [ Abstract PDF ]

    The direct radiative forcing (DRF) of sulfate and black carbon (BC) aerosols is investigated using a new multispectral radiation code within the R30 Geophysical Fluid Dynamics Laboratory general circulation model (GCM). Two independent sulfate climatologies from chemical transport models are applied to the GCM; each climatology has a different atmospheric burden, vertical profile, and seasonal cycle. The DRF is calculated to be approximately -0.6 and -0.8 W m-2 for the different sulfate climatologies. Additional sensitivity studies show that the vertical profile of the sulfate aerosol is important in determining the DRF; sulfate residing near the surface gives the strongest DRF due to the effects of relative humidity. Calculations of the DRF due to BC reveal that the DRF remains uncertain to approximately a factor of 3 due to uncertainties in the total atmospheric burden, the vertical profile of the BC, and the assumed size distribution. Because of the uncertainties in the total global mass of BC, the normalized DRF (the DRF per unit column mass of aerosol in watts per milligram (W mg-1)) due to BC is estimated; the range is +1.1 to +1.9 W mg-1 due to uncertainties in the vertical profile. These values correspond to a DRF of approximately +0.4 W m-2 with a factor of 3 uncertainty when the uncertainty in the total global mass of BC is included. In contrast to sulfate aerosol, the contribution to the global DRF from cloudy regions is very significant, being estimated as approximately 60%. The vertical profile of the BC is, once again, important in determining the DRF, but the sensitivity is reversed from that of sulfate; BC near the surface gives the weakest DRF due to the shielding effects of overlying clouds. Although the uncertainty in the estimates of the DRF due to BC remains high, these results indicate that the DRF due to absorption by BC aerosol may contribute a significant positive radiative forcing and may consequently be important in determining climatic changes in the Earth-atmosphere system.

75. Haywood, J M., V Ramaswamy, and Leo J Donner, 1998: Reply. Geophysical Research Letters, 25(7), 1041.
    [ PDF ]
76. Haywood, J M., M Daniel Schwarzkopf, and V Ramaswamy, 1998: Estimates of radiative forcing due to modeled increases in tropospheric ozone. Journal of Geophysical Research, 103(D14), 16,999-17,007.
    [ Abstract PDF ]

    The GFDL R30 general circulation model (GCM) and a fixed dynamical heating model (FDHM) are used to assess the instantaneous and adjusted radiative forcing due to changes in troposopheric ozone caused by anthropogenic activity. Ozone perturbations from the GFDL global chemical transport model are applied to the GCM, and the instantaneous solar and terrestrial radiative forcings are calculated excluding and including clouds. The FDHM is used to calculate the adjusted radiative forcing at the tropopause. The net global annual mean adjusted radiative forcing, including clouds, ranges from +0.29 to +0.35 W m-2 with ~80% of this forcing being in the terrestrial spectrum. If stratospheric adjustment is ignored, the forcing increases by ~10%, and if clouds are ignored, the radiative forcing increases by a further 20-30%. These results are in reasonable agreement with earlier studies and suggest that changes in tropospheric ozone due to anthropogenic emissions exert a global mean radiative forcing that is of similar magnitude but of opposite sign to the direct forcing of sulfate aerosols.

77. Ramaswamy, V, and Stuart Freidenreich, 1998: A high-spectral resolution study of the near-infrared solar flux disposition in clear and overcast atmospheres. Journal of Geophysical Research, 103(D18), 23,255-23,273.
    [ Abstract PDF ]

    The sensitivity of the near-infrared spectral atmospheric and surface fluxes to the vertical location of clouds is investigated, including a study of factors (drop-size distribution, drop optical depth, solar zenith angle, cloud geometrical thickness, atmospheric profiles) which govern this dependence. Because of the effects of the above-cloud, in-cloud and below-cloud water vapor the atmospheric absorbed flux in each spectral band depends critically on the cloud location, with a high cloud resulting in lesser absorption and greater reflection than a low one having the same drop optical depth. The difference between a high and a low cloud forcing of atmospheric absorption increases with drop optical depth. For any optical depth, clouds with larger drops cause a greater forcing of the spectral atmospheric absorption than those with smaller ones, so high clouds can even cause an increase rather than a decrease of the atmospheric absorption relative to clear skies. In contrast, the spectral and total surface fluxes are relatively insensitive to cloud vertical location. Instead, they are determined by the drop characteristics, notably drop optical depth. This near-invariance characteristic is attributable to the fact that most of the insolation reaching the surface is in the weak water vapor spectral absorption regions; here drops dominate the radiative interactions and thus there is little dependence on cloud height. In addition, the overlap of the drop spectral features with the moderate-to-strong vapor absorption bands ensures that insolation in these regimes fails to reach the surface no matter where the cloud is located; instead, these bands contribute the most to atmospheric absorption. The near-invariant behavior of the spectral and total surface flux holds separately for a wide variety of conditions studied. As a consequence, the difference in reflection, between two columns containing clouds with the same optical depth but located at different altitudes, is approximately balanced in magnitude by the difference in the atmospheric absorption; this holds for every spectral interval whether it be a weak, moderate, or strong vapor/drop absorption band. It also follows that the net fluxes at the top and surface of overcast atmospheres do not have a general, unambiguous relationship; this is in sharp contrast to a linear relation between them in clear skies. However, under certain overcast conditions (e.g., specific vertical location of clouds and solar zenith angle), a simple linear relationship is plausible.

78. Chen, C-T, and V Ramaswamy, 1997: Climate sensitivity to solar and greenhouse radiative forcings In IRS '96: Current Problems in Atmospheric Radiation, Proceedings of the International Radiation Symposium, Fairbanks, Alaska, 19-24 August 1996. Hampton,, Deepak Publishing, 279-281.
    [ Abstract ]

    Using a version of the GFDL Climate GCM, the climate sensitivity to solar and greenhouse radiative forcings is investigated. We consider both global and spatially confined perturbations, of the kinds that are now recognized to play an important role in global climate change (IPCC, 1995). We analyze the annual, global-mean and the zonal-mean surface temperature responses, and their dependence on the imposed forcings.

79. Gelman, M E., V Ramaswamy, and Abraham H Oort, et al., 1997: Stratospheric temperature trends derived from SPARC datasets In Stratospheric Processes and Their Role in Climate (SPARC), Proceedings of the First SPARC General Assembly, WMO/TD-No. 814, WCRP-99, Geneva, Switzerland, World Meteorological Organization, 153-156.
    [ Abstract ]

    The SPARC Stratospheric Temperature Trends Assessment Group has collected 12 datasets of monthly mean, zonal mean stratospheric temperatures, for the purpose of deriving a best estimate of global stratospheric temperature trends. All but one of the datasets cover the years 1979 to 1994, and some extend further back in time. The pressure-altitude levels of the datasets vary, but overall they cover the range 100 to 0.4 hPa (approximately 16-55 km). The datasets represent compilations from ground-based instruments (e.g., radiosondes), satellite instruments (brightness temperatures), analyses (which use one or both of the two other data sources), and assimilation analyses (which use a numerical model in the procedure). Most datasets provide temperatures at specific pressure-levels, but some provide data as mean temperatures representative of various pressure-layers. Temperature trends and their estimated errors have been calculated over the basic period 1979 to 1994 from each dataset, and for 1966 to 1994 for those datasets covering the extended period. This paper describes some attributes of the various datasets and plans of comparisons of temperature trend computations from the datasets.

80. Haywood, J M., V Ramaswamy, and Leo J Donner, 1997: A limited-area-model case study of the effects of sub-grid scale variations in relative humidity and cloud upon the direct radiative forcing of sulfate aerosol. Geophysical Research Letters, 24(2), 143-146.
    [ Abstract PDF ]

    limited-area non-hydrostatic model with a horizontal spatial resolution of 2km by 2km is used to assess the importance of sub-grid scale variations in relative humidity and cloud upon the direct radiative forcing (DRF) by tropospheric sulfate aerosols. The DRF from the limited-area model for both clear and cloudy regions is analyzed and the results compared against those obtained using general circulation model (GCM) parameterizations that perform the computations over coarse horizontal grids. In this idealized model study, the GCM calculations underestimate the clear sky DRF by approximately 73% and the cloudy sky DRF by approximately 60%. These results indicate that, for areas where the relative humidity is high and where there is substantial spatial variability in relative humidity and cloud, GCM calculations may considerably underestimate the DRF.

81. Haywood, J M., Ronald J Stouffer, Richard T Wetherald, Syukuro Manabe, and V Ramaswamy, 1997: Transient response of a coupled model to estimated changes in greenhouse gas and sulfate concentrations. Geophysical Research Letters, 24(11), 1335-1338.
    [ Abstract PDF ]

    This study investigates changes in surface air temperature (SAT), hydrology and the thermohaline circulation due to the radiative forcing of anthropogenic greenhouse gases and the direct radiative forcing (DRF) of sulfate aerosols in the GFDL coupled ocean-atmosphere model. Three 300-year model integrations are performed with increasing greenhouse gas concentrations only, increasing sulfate aerosol concentrations only and increasing greenhouse gas and sulfate aerosol concentrations. A control integration is also performed keeping concentrations of sulfate and carbon dioxide fixed. The global annual mean SAT change when both greenhouse gases and sulfate aerosols are included is in better agreement with observations than when greenhouse gases alone are included. When the global annual mean SAT change from a model integration that includes only increases in greenhouse gases is added to that from a model integration that includes only increases in sulfate, the resulting global SAT change is approximately equal to that from a model integration that includes increases in both greenhouse gases and sulfate aerosol throughout the integration period. Similar results are found for global annual mean precipitation changes and for the geographical distribution of both SAT and precipitation changes indicating that the climate response is linearly additive for the two types of forcing considered here. Changes in the mid-continental summer dryness and thermohaline circulation are also briefly discussed.

82. Heintzenberg, J, R J Charlson, A Clarke, C Liousse, V Ramaswamy, K P Shine, M Wendisch, and G Helas, 1997: Measurements and modelling of aerosol single-scattering albedo: Progress, problems and prospects. Contributions to Atmospheric Physics, 70(4), 249-263.
    [ Abstract PDF ]

    The net effect of atmospheric aerosols in the radiation balance is determined by both their scattering and absorption of solar radiation. The combined optical effect is expressed in the single scatter albedo of the particles. Currently available data on the single scatter albedo are insufficient for definitive use in climate models because most of them are not corrected for the method-dependent effect of the scattering portion of the aerosol on the measured absorption, most refer to the dry state of the aerosol, and the coverage of the globe is far from being complete. Standardisation and calibration of the measurements is needed. Modelling exercises using currently available data on the single scatter albedo should clearly state that corrections are required. The purpose of this review is not to suggest a particular range of values for single scatter albedo. Rather, it is to illustrate that the uncertainties are currently imbedded in various data sets because of the lack of calibration, the possibility that may of the extant methods systematically overestimate light absorption coefficients, and the necessity of including the influence of humidity in models.

83. Li, J, Stuart Freidenreich, and V Ramaswamy, 1997: Solar spectral weight at low cloud tops. Journal of Geophysical Research, 102(D10), 11,139-11,143.
    [ Abstract PDF ]

    The spectral distribution of the incoming solar irradiance varies substantially from the top of the atmosphere to the surface. This occurs because of the selective spectral attenuation by the various atmospheric constituents. Using a line-by-line and doubling-adding solar radiative transfer model, we formulate a prescription that accounts for this variation in the spectral solar irradiance and thereby determine the appropriate spectral weights for low clouds. The results are sufficiently general with respect to cloud top heights ranging from 680 to 860 mbar, while the range of applicability in terms of the solar zenith position extends to sun angles less than 75 degrees. On the basis of the results here we suggest a reference cloud top height of 760 mbar and a reference zenith angle of 53 degrees. The error in the radiative quantities relative to the "benchmark" calculations is generally less than 5% in most of the spectral bands. As a simple application, it is found that the enhancement of cloud absorption in a two band cloud optical properties parameterization can be largely avoided by using this simple modification of the solar spectral irradiance incident at the top of low-lying clouds.

84. Ramaswamy, V, and C-T Chen, 1997: Climate forcing-response relationships for greenhouse and shortwave radiative perturbations. Geophysical Research Letters, 24(6), 667-670.
    [ Abstract PDF ]

    The earth's climate system is subject to radiative forcings caused by perturbations in the infrared 'greenhouse' effect and absorbed solar energy. The forcings can be classified as being global in extent (e.g., increase of CO₂) or spatially confined to the northern hemisphere midlatitudes (e.g., anthropogenic sulfate aerosols). Of central importance to climate change assessments are the characteristics of the global and latitudinal changes, and the forcing-response relationships for different kinds of perturbations. Using a general circulation climate model with fixed cloud distributions and microphysical properties, we analyze the equilibrium climate responses to different perturbations representing global and spatially localized radiative forcings. The total climate feedback in the various experiments does not differ significantly, and the global-mean climate sensitivity (ratio of the equilibrium global-mean surface temperature change to the global-mean imposed radiative forcing) behaves in a near-invariant manner for both global and spatially confined forcings. However, relative to the global perturbation cases, forcings confined to the northern hemisphere midlatitudes exhibit a steepening of the meridional gradient of the temperature response in that hemisphere.

85. Ramaswamy, V, and C-T Chen, 1997: Linear additivity of climate response for combined albedo and greenhouse perturbations. Geophysical Research Letters, 24(5), 567-570.
    [ Abstract PDF ]

    Using an atmospheric general circulation model with fixed cloud amounts and microphysical properties, and coupled to a mixed-layer static ocean, we perform idealized experiments to inquire into the linear characteristics of the modeled climate system's mean response to simultaneous greenhouse and Northern Hemisphere midlatitude albedo perturbations, two forcings deemed to be important during the present times. The two forcings are chosen to be equal and opposite in the global, annual-mean such that a linear behavior would be expected to lead to a complete offset of the global, annual-mean surface temperature change, which is indeed obtained. The monthly and annual zonal-mean surface temperature, and the annual zonal-mean precipitation responses to the combined forcings, also are reasonably similar to the sum of the responses to the individual forcings. The albedo forcing case casts a distinct signature on the circulation and precipitation changes in the northern and southern equatorial regions, which is absent for the greenhouse forcing case. The combined simulation yields a result similar to that for the albedo forcing case, one that is consistent with linear additive expectations.

86. Ramaswamy, V, and Stuart Freidenreich, 1997: Absorption of solar radiation in overcast atmospheres In IRS '96: Current Problems in Atmospheric Radiation, Proceedings of the International Radiation Symposium, Fairbanks, Alaska, 19-24 August 1996. Hampton,, Deepak Publishing, 125-127.
    [ Abstract ]

    investigate the absorption of solar radiation in the near-infrared spectrum in overcast atmospheres containing water vapor and water drops. We compare the results with that for water vapor absorption only, and examine the quantitative dependence of the absorption on the drop optical depth and vertical location of the cloud.

87. Ramaswamy, V, and M Daniel Schwarzkopf, 1997: Stratospheric temperature trends: observations and model simulations In Stratospheric Processes and Their Role in Climate (SPARC), of the First SPARC General Assembly, WMO/TD-No. 814, WCRP-99, Geneva, Switzerland, World Meteorological Organization, 149-152.
88. Schwarzkopf, M D., and V Ramaswamy, 1997: Stratospheric climatic effects due to CH4, N₂O, CFCs and the H₂O infrared continuum: A GCM experiment In IRS '96: Current Problems in Atmospheric Radiation, Proceedings of the International Radiation Symposium, Fairbanks, Alaska, 19-24 August 1996. Hampton,, Deepak Publishing, 336-339.
    [ Abstract ]

    e GFDL longwave radiation parameterization has been modified to employ the CKD 2.1 formulation of the water vapor continuum. A general circulation model (GCM) experiment using the GFDL "SKYHI" model has been performed using the revised algorithm. The calculation also includes the radiative effects of CH₄, N₂O and CFCs. The model-simulated radiative heating rates, fluxes and associated temperture changes are compared to those obtained using the Roberts H₂O continuum formulation.

89. Chen, C-T, and V Ramaswamy, 1996: Sensitivity of simulated global climate to perturbations in low-cloud microphysical properties. Part I: Globally uniform perturbations. Journal of Climate, 9(6), 1385-1402.
    [ Abstract PDF ]

    The sensitivity of the global climate to perturbations in the microphysical properties of low clouds is investigated using a general circulation model coupled to a static mixed layer ocean with fixed cloud distributions and incorporating a new broadband parameterization for cloud radiative properties. A series of GCM experiments involving globally uniform perturbations in cloud liquid water path or effective radius (albedo perturbations), along with one for a doubling of carbon dioxide (greenhouse perturbation), lead to the following results: 1) The model's climate sensitivity (ratio of global-mean surface temperature response to the global-mean radiative forcing) is virtually independent (to ~ 10%) of the sign, magnitude, and the spatial pattern of the forcings considered, thus revealing a linear and invariant nature of the model's global-mean response. 2) Although the total climate feedback is very similar in all the experiments, the strengths of the individual feedback mechanisms (e.g., water vapor, albedo) are different for positive and negative forcings. 3) Changes in moisture, tropospheric static stability, and sea ice extent govern the vertical and zonal patterns of the temperature response, with the spatial distribution of the response being quite different from that of the radiative forcing. 4) The zonal surface temperature response pattern, normalized with respect to the global mean, is different for experiments with positive and negative forcings, particularly in the polar regions of both hemispheres, due to differing changes in sea ice. 5) The change in the surface radiative fluxes is different for the carbon dioxide doubling and cloud liquid water path decrease experiments, even though both cases have the same radiative forcing and a similar global-mean surface temperature response; this leads to differences in the vigor of the hydrologic cycle (evaporation and precipitation rates) in these two experiments.

90. Chen, C-T, and V Ramaswamy, 1996: Sensitivity of simulated global climate to perturbations in low cloud microphysical properties. Part II: Spatially localized perturbations. Journal of Climate, 9(11), 2788-2801.
    [ Abstract PDF ]

    The sensitivity of the global climate to spatially localized (20° - 70°N) perturbations in the microphysical properties of low clouds is investigated using a general circulation model coupled to a mixed layer ocean with fixed cloud distributions. By comparing with earlier experiments involving globally uniform perturbations, insights are obtained into the climate responses to spatially inhomogeneous radiative forcings, such as that due to the contrast in the effective drop radius of land and ocean clouds and the anthropogenic sulfate aerosol-induced alteration of cloud albedo. The main findings of this study are as follows: 1) The model's climate sensitivity (ratio of global-mean surface temperature response to the global-mean radiative forcing) is virtually independent of the distribution and magnitude of forcing. 2) Although the total feedback is very similar in the different experiments, the strengths of the individual feedback mechanisms (water vapor, albedo, lapse rate) are dissimilar. 3) For the localized perturbations, the climate response is essentially confined to the hemisphere in which the forcing occurs, owing to a poor interhemispheric energy exchange. 4) In spite of no forciing in the Southern Hemisphere in the localized experiments, there is a weak "remote" temperature response there. 5) For both global and localized perturbations, the temperature response in the tropical upper troposphere is larger than in the lower troposphere due to moist convective processes; in the localized experiments, while there is a strong vertical gradient in the temperature change at the Northern Hemisphere mid and high latitudes, the temperature change throughout the lower and midtroposphere of the Southern Hemisphere is uniform. 6) The localized experiments induce notable changes in the mean meridional circulation and precipitation near the equator, which are not obtained for the global perturbation cases. 7) The pattern of temperature response of the land and ocean areas in the Northern Hemisphere midlatitudes depends on whether the forcing occurs over both types of surfaces or over land only; the results suggest that the well-known contrast in drop radii between continental and maritime clouds exerts a significant influence on the surface temperature distrigution within the zone and on the manner in which the surface energy balance is maintained.

91. Li, J, and V Ramaswamy, 1996: Four-stream spherical harmonic expansion approximation for solar radiative transfer. Journal of the Atmospheric Sciences, 53(8), 1174-1186.
    [ Abstract PDF ]

    This paper presents a four-stream extensiuon of the sigma-Eddington approximation by considering the higher-order spherical harmonic expansion in radiative intensity. By using the orthogonality relation of the spherical harmonic functions, the derivation of the solution is fairly straightforward. Calculations show that the sigma-four-stream spherical harmonic expansion approximation can reduce the errors in reflection, transmission, and absorption substantially in comparison with the sigma-Eddington approximation. For the conservative scattering case, the error of the new model is generally less than 1% for optical thicknesses greater than unity except for grazing incident solar beam. For nonconservative scattering cases (single scattering albedo omega = 0.9), the error is less than 5% for optical thicknesses greater than unity, in contrast to errors of up to 20% or more under the sigma-Eddington approximation. This model can also predict the azimuthally averaged intensity to a good degree of accuracy. The computational time for this model is not as intensive as for the rigorous numerical methods, owing to the analytical form of the derived solution.

92. Ramaswamy, V, 1996: Longwave radiation In Encyclopedia of Climate and Weather, Vol. 2, New York, Oxford University Press, 478-481.
93. Ramaswamy, V, and J Li, 1996: A line-by-line investigation of solar radiative effects in vertically inhomogeneous low clouds. Quarterly Journal of the Royal Meteorological Society, 122(536), 1873-1890.
    [ Abstract PDF ]

    Using a detailed line-by-line, multiple-scattering solar radiative-transfer model, the influences due to cloud internal inhomogeneity in the vertical upon the solar radiative transfer are investigated. In particular, the consequences due to non-uniform vertical profiles of liquid water and droplet sizes within low clouds are explored in a systematic manner. The fine structure of the spectral overlap between the water droplet and water vapour optical properties, and its effects upon the radiation absorbed within the cloud layer and that reflected at the top of the cloud, are discussed. Without consideration of the in-cloud water vapour, a vertically inhomogeneoous cloud with properties resembling those observed absorbs more solar radiation than an equivalent homogeneous cloud. However, consideration of the effects of the in-cloud vapour, while still leading to a slightly greater absorption for the inhomogeneous case, partly offsets the difference introduced by the vertical distribution of the drop microphysics. The vertical distribution of cloud heating rate is changed substantially because of the inhomogeneity in the microphysics, with the heating rate in the top region of the cloud nearly 50% more than that due to an equivalent vertically homogeneous cloud. Vertical inhomogeneity of cloud microphysics has little influence on the broadband solar albedo, but can cause significant decreases of the cloud reflectance at specific near-infrared wavelengths, i.e., wavelengths greater than 1 µm, (equivalently, wave numbers less than 10,000 cm^-1).

94. Ramaswamy, V, M Daniel Schwarzkopf, and W Randel, 1996: Fingerprint of ozone depletion in the spatial and temporal pattern of recent lower-stratospheric cooling. Nature, 382, 616-618.
    [ Abstract PDF ]

    Observations of air temperatures in the lower stratosphere from 1979 to 1990 reveal a cooling trend that varies both spatially and seasonally. The possible causes of this cooling include changes in concentrations of ozone or of other greenhouse gases, and entirely natural variability, but the relative contributions of such causes are poorly constrained. Here we incorporate the observed decreases in stratospheric ozone concentrations over the same period into a general circulation model of the atmosphere, to investigate the role of the ozone losses in affecting patterns of temperature change. We find that the simulated latitudinal pattern of lower-stratospheric cooling for a given month through the decade corresponds well with the pattern of the observed decadal temperature changes. This result confirms the expectation, from simpler model studies, that the observed ozone depletion exerts a spatially and seasonally varying fingerprint in the decadal cooling of the lower stratosphere, with the influence of increases in concentrations of other greenhouse gases being relatively small. As anthropogenic halocarbon chemicals are important causes of stratospheric ozone depletion, our study suggests a human influence on the patterns of temperature change in the lower stratosphere over this 11-year period.

95. Santer, B D., Abraham H Oort, V Ramaswamy, M Daniel Schwarzkopf, and Ronald J Stouffer, et al., 1996: A search for human influences on the thermal structure of the atmosphere. Nature, 382(6586), 39-46.
    [ Abstract PDF ]

    The observed spatial patterns of temperature change in the free atmosphere from 1963 to 1987 are similar to those predicted by state-of-the-art climate models incorporating various combinations of changes in carbon dioxide, anthropogenic sulphate aerosol and stratospheric ozone concentrations. The degree of pattern similarity between models and observations increases through this period. It is likely that this trend is partially due to human activities, although many uncertainties remain, particularly relating to estimates of natural variability.

96. Schimel, D, and V Ramaswamy, et al., 1996: Radiative forcing of climate change In Climate Change 1995: The Science of Climate Change,, Cambridge, UK, Cambridge University Press, 69-131.
97. Chen, C-T, and V Ramaswamy, 1995: Parameterization of the solar radiative characteristics of low clouds and studies with a general circulation model. Journal of Geophysical Research, 100(D6), 11,611-11,622.
    [ Abstract PDF ]

    A broadband parameterization that improves the quantitative estimates of the solar radiative characteristics of low clouds is developed using reference solutions. The accuracy of the parameterization in determining the shortwave cloud absorption for a wide variety of low-cloud conditions is better than 20%. Other broadband treatments, which do not adequately account for the influences due to above- and in-cloud water vapor and water drop extinction, are also considered to investigate the sensitivity to these factors. The computed northern hemisphere summertime fluxes reveal that (1) the absorbed solar flux in low clouds (F_abs) is overestimated at high latitudes if the effect of attenuation by the above-cloud vapor is ignored in the determination of the water drop absorption, (2) F_abs is underestimated in the tropical regions if in-cloud vapor absorption is not considered, and (3) the conservative scattering assumption for drops yields a substantial underestimate of F_abs at most latitudes. General circulation model simulations with fixed sea surface temperatures and cloud amounts further highlight the significance of the vapor and drop optical properties. Differences in the broadband treatment of the radiative interactions with vapor and drops in low clouds introduce changes in the solar fluxes absorbed by the atmosphere and the surface; for the cases considered here, the solar flux change at the top of the atmosphere differs in sign from that at the surface. The flux differences bring about changes in vertical motion and precipitation; these, in turn, are accompanied by perturbations in the various components of the land surface heat (e.g., latent and sensible heat losses) and moisture (e.g., soil moisture, evaporation) budgets. For approximately similar solar flux differences the changes in the vertical motion, precipitation, and land surface parameters are dissimilar in the tropical and the midlatitude continental regions. Thus because of the adjustments in the atmosphere and the coupling between the atmosphere and the land surface processes, solar flux differences due to biases or deficiencies in the radiative treatment of vapor and drops affect the simulation of the hydrologic fields and the heat balance, including the atmospheric and land surface temperatures.

98. Freidenreich, Stuart, and V Ramaswamy, 1995: Stratospheric temperature response to improved solar CO₂ and H₂O parameterizations.. Journal of Geophysical Research, 100(D8), 16,721-16,725.
    [ Abstract PDF ]

    A fixed-dynamical heating model is used to investigate the temperature changes in the stratosphere due to improved CO₂ and H₂O shortwave heating parameterizations. Besides being governed by the magnitude of the local heating, the temperature change in any layer due to the improved parameterizations is also dependent on the distribution of the solar heating in other stratospheric layers. This is a consequence of the longwave radiative exchange process, in which the temperature change in other layers, due to the imposed heating perturbations, leads to an exchange of longwave radiative energy with the layer in question, thus affecting its response. Thus the vertical profile of the heating rate becomes a significant factor in determining the stratospheric thermal profile. This investigation also confirms the sensitivity of the temperature response in the lower stratosphere to perturbations in the shortwave CO₂ and H₂O heating.

99. Ramaswamy, V, et al., 1995: Group report: what are the observed and anticipated meteorological and climatic responses to aerosol forcing? In Aerosol Forcing of Climate, Chichester, UK, John Wiley & Sons, 386-399.
100. Santer, B D., Abraham H Oort, V Ramaswamy, M Daniel Schwarzkopf, and Ronald J Stouffer, et al., 1995: A Search for Human Influences on the Thermal Structure of the Atmosphere, Program for Climate Model Diagnosis and Intercomparison, PCMDI Report No. 27, UCRL-ID-121956: Lawrence Livermore, CA, 26 pp.
     [ Abstract ]

     Recent studies have shown that patterns of near-surface temperature change due to combined forcing by CO and anthropogenic sulfate aerosols are easier to identify in the observations than signals due to changes in CO alone (Santer et al., 1995; Mitchell et al., 1995a). Here we extend this work to the vertical structure of atmospheric temperature changes, and additionally consider the possible effects of stratospheric ozone reduction. We compare modelled and observed patterns over the lower troposphere to the lower stratosphere (850 to 50 hPa) and over the low- to mid-troposphere (850 to 500 hPa). In both regions there are strong similarities between observed changes and model-predicted signals. Over 850 to 50 hPa similarities are evident both in CO-only signals and in signals that incorporate the added effects of sulfate aerosols and stratospheric ozone reduction. These similarities are due largely to a common pattern of stratospheric cooling and tropospheric warming in the observations and model experiments. Including the effects of stratospheric ozone reduction results in a more realistic height for the transition between stratospheric cooling and results in a more realistic height for the transition between stratospheric cooling and tropospheric warming. In the low- to mid-troposphere the observations are in better agreement with the temperature-change patterns due to combined forcing than with the CO-only pattern. This is the result of hemispheric-scale temperature-change contrasts that are common to the observations and the combined forcing signal but absent in the CO-only case. The levels of model-versus-observed pattern similarity in both atmospheric regions increase over the period 1963 to 1987. If model estimates of natural internal variability are realistic, it is likely that these trends in pattern similarity are partially due to human activities.

101. Shine, K P., V Ramaswamy, and M Daniel Schwarzkopf, et al., 1995: Radiative forcing due to changes in ozone: A comparison of different codes In Atmospheric Ozone as a Climate Gas, NATO ASI Series I, Vol. 32, Heidelberg, Germany, Springer-Verlag, 373-396.
     [ Abstract ]

     The radiative forcing due to changes in ozone in the troposphere and stratosphere is calculated using a number of different radiative transfer codes and the results are compared. The calculations use a tightly specified set of input parameters. The 14 um band of ozone is shown to make a significant contribution to the forcing for changes in stratospheric ozone, although, because of line overlap, it is of less importance for tropospheric ozone changes. The main cause of the spread in results is differences in the solar forcings; these differences are believed to reflect simplifications used in paramterizations rather than the actual uncertainty in modelling solar irradiances.

102. Shine, K P., and V Ramaswamy, et al., 1995: Radiative Forcing In Climate Change 1994: Radiative Forcing of Climate Change, Cambridge, UK, Cambridge University Press, 163-203.
103. Shine, K P., and V Ramaswamy, et al., 1995: Radiative forcing and temperature trends In Scientific Assessment of Ozone Depletion: 1994, Global Ozone Research and Monitoring Project Report No. 37, Geneva, Switzerland, World Meteorological Organization, 8.1-8.26.
104. Ramaswamy, V, and M M Bowen, 1994: Effect of changes in radiatively active species upon the lower stratospheric temperatures. Journal of Geophysical Research, 99(D9), 18,909-18,921.
     [ Abstract PDF ]

     A one-dimensional radiative-convective model is employed to investigate the thermal effects in the lower stratosphere due to changes in the concentrations of radiatively active species. In particular, we consider the comparative influences due to species that exert surface-troposphere radiative forcings of opposite signs. Two examples of such competing surface-troposphere forcings are (1) increases in the well-mixed greenhouse gases versus increases in tropospheric aerosols and (2) stratospheric ozone loss versus increase in tropospheric ozone. The radiative equilibrium of the lower stratosphere is perturbed both by the local changes in the concentrations of radiatively active species and by the changes in species' concentrations occurring in the troposphere and the middle/upper stratosphere. Perturbations in the concentrations of each of the species, as considered above, leads to a temperature decrease in the lower stratosphere. Relative to the well-mixed greenhouse gases only case, simultaneous increases in these gases and tropospheric aerosols cause a reduction of the net surface-troposphere radiative forcing, thereby diminishing the surface warming. However, since tropospheric aerosols contribute to a cooling of the lower stratosphere, the temperature decrease there is enhanced above that due to trace gases alone, with the aerosol-induced effects scaling approximately linearly with their optical depth. A complete offset of the greenhouse gas surface-troposphere forcing by tropospheric aerosols, while resulting in a null change in the surface temperature, would double the cooling of the lower stratosphere. Increases in tropospheric ozone would enhance the lower stratospheric cooling over and above that caused by the stratospheric ozone depletion. This is in contrast to the cooling and warming effects exerted upon the surface-troposphere system by the stratospheric and the tropospheric ozone changes, respectively. Tropospheric ozone increases of 20% or more can yield a lower stratospheric cooling that is a significant fraction of the effects due to the observed stratospheric ozone loss. Both the surface effects and the enhancement of the lower stratospheric cooling scale approximately linearly with tropospheric ozone increases.

105. Ramaswamy, V, and M M Bowen, 1994: Tropospheric and stratospheric climatic impacts due to increases in tropospheric aerosols and trace gases In Conference on Atmospheric Chemistry, January 23-28, 1994, Nashville, American Meteorological Society, 1-2.
106. Raval, A, Abraham H Oort, and V Ramaswamy, 1994: Observed dependence of outgoing longwave radiation on sea surface temperature and moisture. Journal of Climate, 7(5), 807-821.
     [ Abstract PDF ]

     The authors have empirically examined the dependence of the outgoing longwave radiation (OLR) on sea surface temperature (T_s), precipitable water (W), and height-mean relative humidity (RH). The OLR is obtained from 4 yr of data from the Earth Radiation Budget Experiment (ERBE), while T_s, W, and RH are obtained from objective analyses of rawinsonde and ship data. It is found that in the midlatitudes, the surface temperature explains over 80% of the variability in the clear-sky OLR (F_cs) and almost half of the variability in the total OLR (F_tot). It fails badly in the tropics and subtropics, however, where Ts explains only about 20% of the variability in F_cs and is largely decoupled from Ftot. The two-dimensional contour plot of the OLR binned with respect to T_s and RH is marked by distinct changes in the gradient that are consistent with inferences from earlier investigations. For low values of T_s(<10°C), the OLR depends mainly on T_s. For values of T_s above 10°C, the OLR depends increasingly on RH. Specifically, in the tropics (T_s ~ 25°C), the total and clear-sky OLR depend significantly on both T_s and RH. The well-known drop in OLR in the tropics with increasing T_s correlates directly to an increase in RH, and not to changes in T_s. The authors suggest that the observed dependence of the OLR on T_s and RH be a minimum performance standard for climate models. This approach is illustrated by comparing the observed dependence with the results of a radiative transfer model and an R15 general circulation model, and by discussing the strengths and limitations of using RH to parameterize the OLR.

107. Freidenreich, Stuart, and V Ramaswamy, 1993: Solar radiation absorption by CO₂, overlap with H₂O, and a parameterization for general circulation models. Journal of Geophysical Research, 98(D4), 7255-7264.
     [ Abstract ]

     Line-by-line (LBL) solar radiative solutions are obtained for CO₂-only, H₂O-only, and CO₂ + H₂O atmospheres, and the contributions by the major CO2 and H2O absorption bands to the heating rates in the stratosphere and troposphere are analyzed. The LBL results are also used to investigate the inaccuracies in the absorption by a CO₂ + H₂O atmosphere, arising due to a multiplication of the individual gas transmissions averaged over specific spectral widths (delta v). Errors in absorption generally increase with the value of delta v chosen. However, even when the interval chosen for averaging the individual gas transmissions is the entire solar spectrum, there is no serious degradation in the accuracy of the atmospheric absorbed flux (error < 3%) and the heating rates (errors < 10%). A broadband parameterization for CO₂ absorption, employed in several weather prediction and climate models, is found to substantially underestimate the LBL heating rates throughout the atmosphere, most notably in the stratosphere (errors > 40%). This parameterization is modified such that the resulting errors are less than 20%. When this modified CO₂ parameterization is combined with a recently modified formulation for H₂O vapor absorption, the resulting errors in the heating rates are also less than 20%. The application of the modified solar absorption parameterizations in a general circulation model (GCM) causes an increase in the simulated clear sky diabatic heating rates, ranging from nonnegligible (middle stratosphere and lower troposphere) to significant (lower stratosphere and upper troposphere) additions. The results here should enable a continued use of the older broadband parameterizations in GCMs, albeit in modified forms.

108. Held, Isaac, Richard S Hemler, and V Ramaswamy, 1993: Radiative-convective equilibrium with explicit two-dimensional moist convection. Journal of the Atmospheric Sciences, 50(23), 3909-3927.
     [ Abstract PDF ]

     Radiative-convective statistical equilibria are obtained using a two-dimensional model in which radiative transfer is interactive with the predicted moisture and cloud fields. The domain is periodic in x, with a width of 640 km, and extends from the ground to 26 km. The lower boundary is a fixed-temperature water-saturated surface. The model produces a temperature profile resembling the mean profile observed in the tropics. A number of integrations of several months' duration are described in this preliminary examination of the model's qualitative behavior. The model generates a QBO-like oscillation in the x-averaged winds with an apparent period of ~60 days. This oscillation extends into the troposphere and influences the convective organization. In order to avoid the associated large vertical wind shears, calculations are also performed in which the x-averaged winds are constrained to vanish. The convection then evolves into a pattern in which rain falls only within a small part of the domain. The moisture field appears to provide the memory that localizes the convection. If the vertical shears are fixed in a modest nonzero value, this localization is avoided. Comparing calculations with surface temperatures of 25°C and 30°C, the planetary albedo is found to decrease with increasing temperature, primarily due to a reduction in low-level cloudiness.

109. Ramaswamy, V, and C-T Chen, 1993: An investigation of the global solar radiative forcing due to changes in cloud liquid water path. Journal of Geophysical Research, 98(D9), 16,703-16,712.
     [ Abstract ]

     The instantaneous solar radiative forcing of the surface-atmosphere system associated with a change in the liquid water path (LWP) of low clouds has a significant space-time dependence, owing to the spatial and temporal variations in insolation, solar zenith angle, and surface albedo. This feature is demonstrated by considering globally uniform LWPs and LWP changes. Keeping cloud amounts fixed in space and time, we find that an increase in LWP imparts a distinct meridional gradient to the solar forcing, while the difference between summer and winter forcings introduces a seasonal variation at any given latitude. Relative to the global, annual mean (GAM) value (a negative quantity for an increase in LWP) the forcing is more negative at low latitudes throughout the year and during summer at the high latitudes. In contrast, the forcing is more positive than the GAM value during the winter season at the higher latitudes (poleward of 40°). Thus even the simple assumption of a globally uniform LWP change does not yield a uniform forcing at all latitudes and/or times. However, because of the contrasts in the contributions from the low and high latitudes and over the different seasons the global and annual average of the radiative forcing turns out to be nearly identical to that computed using a global, annual mean atmospheric profile and mean insolation conditions. The annual mean meridional gradient of the forcing is sensitive both to the "control" LWP values and to the changes in those values. A factor that can introduce an additional nonuniformity in the solar forcing is the latitudinal variation in the cloud climatology. We also find that the zonal, annual mean pattern of the forcing due to the cloud LWP change is different from that for carbon dioxide doubling. Thus while a specific globally uniform LWP increase can yield a global, annual mean radiative forcing that is opposite to but has the same magnitude as that for carbon dioxide increases, such a compensation in the forcing cannot be expected to be uniform with latitude or month.

110. Schwarzkopf, M D., and V Ramaswamy, 1993: Radiative forcing due to ozone in the 1980s: Dependence on altitude of ozone change. Geophysical Research Letters, 20(2), 205-208.
     [ Abstract PDF ]

     The radiative forcing of the surface-troposphere system caused by the changes in ozone in the 1980s is sensitive to the altitude profile of these changes. In the tropics, inclusion of lower stratospheric ozone depletions observed by SAGE results in a substantial negative radiative ozone forcing. In mid-latitudes, the magnitude of the negative stratospheric ozone forcing diminishes as the altitude of ozone depletion is raised above the tropopause. By contrast, the radiative forcing corresponding to the decadal tropospheric ozone increases observed at certain Northern Hemisphere mid-latitude locations is strongly positive. The magnitude and sign of the total (tropospheric + stratospheric) ozone forcing in Northern Hemisphere mid-latitudes is criticaly dependent on the vertical profile of the tropospheric ozone increases and the lower stratospheric losses near the tropopause.

111. Isaksen, I, V Ramaswamy, H Rodhe, and T M L Wigley, 1992: Radiative forcing of climate In Climate Change 1992: The Supplementary Report to the IPCC Scientific Assessment, Cambridge, UK, Cambridge University Press, 47-67.
112. Ramaswamy, V, and Stuart Freidenreich, 1992: A study of broadband parameterizations of the solar radiative interactions with water vapor and water drops. Journal of Geophysical Research, 97(D11), 11,487-11,512.
     [ Abstract PDF ]

     Reference radiative transfer solutions in the near-infrared spectrum, which account for the spectral absorption characteristics of the water vapor molecule and the absorbing-scattering features of water drops, are employed to investigate and develop broadband treatments of solar water vapor absorption and cloud radiative effects. The conceptually simple and widely used Lacis-Hansen parameterization for solar water vapor absorption is modified so as to yield excellent agreement in the clear sky heating rates. The problem of single cloud decks over a nonreflecting surface is used to highlight the factors involved in the development of broadband overcast sky parameterizations. Three factors warrant considerable attention: (1) the manner in which the spectrally dependent drop single-scattering values are used to obtain the broadband cloud radiative properties, (2) the effect of the spectral attenuation by the vapor above the cloud on the determination of the broadband drop reflection and transmission, and (3) the broadband treatment of the spectrally dependent absorption due to drops and vapor inside the cloud. The solar flux convergence in clouds is very sensitive to all these considerations. Ignoring effect 2 tends to overestimate the cloud heating, particularly for low clouds, while a poor treatment of effect 3 tends to an underestimate. A new parameterization that accounts for the aforementioned considerations is accurate to within ~ 30% over a wide range of overcast sky conditions, including solar zenith angles and cloud characteristics (altitudes, drop models, optical depths, and geometrical thicknesses), with the largest inaccuracies occurring for geometrically thick, extended cloud systems containing large amounts of vapor. Broadband methods that treat improperly one or more of the above considerations can yield substantially higher errors (>35%) for some overcast sky conditions while having better agreements over limited portions of the parameter range. For example, a technique that considers effect 3 but ignores effect 2 yields a partial compensation of errors of opposite sign, such that the resulting inaccuracy for geometrically thick clouds can be less than 20%. In contrast to the marked sensitivity of the cloud heating rates, the maximum relative errors in the reflected flux at the top of the overcast atmosphere and the transmitted flux at the surface do not vary appreciably under the various broadband treatments; with the new parameterization, the relative errors are less than 15%. In applying the broadband concept to overcast atmospheres and multiple cloud decks, there are cases when the errors can be larger than stated above. Hence a general use of broadband methods in weather prediction and climate models (e.g., general circulation models) should be accompanied by a realization of the potential inaccuracies that can occur for specific overcast sky cases.

113. Ramaswamy, V, M Daniel Schwarzkopf, and K P Shine, 1992: Radiative forcing of climate from halocarbon-induced global stratospheric ozone loss. Nature, 355, 810-812.
     [ Abstract PDF ]

     Observations from satellite and ground-based instruments indicate that between 1979 and 1990 there have been statistically significant losses of ozone in the lower stratosphere of the middle to high latitudes in both hemispheres. Here we determine the radiative forcing of the surface-troposphere system due to the observed decadal ozone losses, and compare it with that due to the increased concentrations of the other main radiatively active gases (CO₂, CH₄, N₂O and chlorofluorocarbons) over the same time period. Our results indicate that a significant negative radiative forcing results from ozone losses in middle to high latitudes caused by the CFCs and other gases. As the anthropogenic emissions of CFCs and other halocarbons are thought to be largely responsible for the observed ozone depletions, our results suggest that the net decadal contribution of CFCs to the greenhouse climate forcing is substantially less than previously estimated.

114. Fouquart, Y, B Bonnel, and V Ramaswamy, 1991: Intercomparing shortwave radiation codes for climate studies. Journal of Geophysical Research, 96(D5), 8955-8968.
     [ Abstract PDF ]

     As a second step of the international program of Intercomparison of Radiation Codes Used in Climate Models (ICRCCM), an intercomparison of shortwave radiation models was initiated. Among the 26 codes that participated in the comparison were very detailed (line-by-line), narrow-band (high-spectral resolution), as well as highly parameterized (low-spectral resolution) models. A considerable spread was detected in the response of these models to a set of well-defined atmospheric profiles. Substantial discrepancies exist among models even for the simplest case of pure water vapor absorption with standard deviation ranging from 1% to 3% for the downward fluxes at the surface and from 6% to 11% for the total atmospheric absorption. The divergences in downward surface flux increase to nearly 4% when all absorbers and the molecular scattering are considered. In cloudy conditions the divergences range from 4% to 10%, depending on the cloud optical thickness. Another major uncertainty that has been identified is the spectral averaging of the scattering properties which can result in very significant errors for low spectral resolution codes. Since these errors appear to be systematic, they may induce unrealistic feedback mechanisms in numerical climate models. The amplitude of the differences between models is in many cases larger than the accuracy required for the achievements of several objectives of the World Climate Research Program. While reference solutions for the absorption and scattering in atmospheres can be obtained based on the state-of-the-art spectroscopic knowledge and rigorous computational techniques, the absolute tests of the validity of the radiation algorithms would be comprehensive field experiments in which the radiative and all relevant atmospheric parameters are measured to a high degree of accuracy.

115. Ramaswamy, V, and Stuart Freidenreich, 1991: Solar radiative line-by-line determination of water vapor absorption and water cloud extinction in inhomogeneous atmospheres. Journal of Geophysical Research, 96(D5), 9133-9157.
     [ Abstract PDF ]

     The complete available spectral features (line-by-line, or LBL) of the water vapor molecule in the solar spectrum and a precise treatment of particulate scattering are employed to obtain and analyze the solar radiative fluxes and heating rates in plane-parallel, vertically inhomogeneous model atmospheres containing vapor only, water cloud only, and vapor-plus-cloud present simultaneously. These studies are part of the Intercomparison of Radiation Codes in Climate Models (ICRCCM) project and constitute useful benchmark computations against which results from simpler radiation algorithms can be compared. The "exact" solution of the radiative transfer equation for cloudy atmospheres with the cloud in a single model layer consumes an exorbitant amount of computational resources (~ 100 hours on a Cyber 205). Two other techniques that are considerably more economical are also investigated. These techniques, too, are based on the LBL spectral features of the H₂O molecule but consist of an approximation in either the vapor optical depth or in the multiple-scattering process. The technique involving the "binning" of the vapor optical depths yields extremely accurate fluxes and heating rates for both the vapor and vapor-plus-cloud cases; in particular, it is a practical alternative for obtaining benchmark solutions to the solar radiative transfer in overcast atmospheres (3.8 hours). In contrast, the multiple-scattering approximation technique does not yield precise results; however, considering its computational efficiency (0.5 hours), it offers a rapid means to obtain a first-order approximation of the spectrally integrated quantities. The analyses of the alternate techniques suggest their potential use for high spectral resolution sensitivity studies of the radiative effects due to various types of clouds.

116. Ramaswamy, V, M Daniel Schwarzkopf, and S C Liu, 1991: Preface. Journal of Geophysical Research, 96(D5), 8921-8923.
     [ PDF ]
117. Wetherald, Richard T., V Ramaswamy, and Syukuro Manabe, 1991: A comparative study of the observations of high clouds and simulations by an atmospheric general circulation model. Climate Dynamics, 5, 135-143.
     [ Abstract PDF ]

     The importance of clouds in the upper troposphere (cirrus) for the sensitivity of the Earth's climate e.g., requires that these clouds be modeled accurately in general circulation model (GCM) studies of the atmosphere. Bearing in mind the lack of unambiguous quantitative information on the geographical distribution and properties of high clouds, the simulated distribution of upper tropospheric clouds in a spectral GCM is compared with several satellite-derived datasets that pertain to high clouds only, for both winter and summer seasons. In the model, clouds are assumed to occupy an entire grid box whenever the relative humidity exceeds 99%: otherwise the grid box is assumed to be free of cloud. Despite the simplicity of the cloud prediction scheme, the geographical distribution of the maxima in the model's upper tropospheric cloud cover coincides approximately with the regions of the observed maxima in the high cloud amount and their frequency of occurrence (e.g., intertropical convergence zone and the monsoon areas). These areas exhibit a minimum in the outgoing longwave radiation (OLR; Nimbus-7) and are also coincident with regions of heavy precipitation. The model, with its relatively simple cloud formation scheme, appears to capture the principal large-scale features of the tropical convective processes that are evident in the satellite and precipitation datasets, wherein the intense, upward motion is accompanied by condensation and the spreading of thick upper tropospheric layers of high relative humidity and.cloudiness in the vicinity of the tropical rainbelt regions.

118. Ramaswamy, V, and V Ramanathan, 1989: Solar absorption by cirrus clouds and the maintenance of the tropical upper troposphere thermal structure. Journal of the Atmospheric Sciences, 46(14), 2293-2310.
     [ Abstract PDF ]

     Radiative transfer calculations employing observed values of the ice crystal size distribution demonstrate that the absorption of solar radiation by cirrus clouds can make a significant contribution to the diabatic heating of the upper troposphere. The effects due to this absorption on the upper tropospheric (100-300 mb) thermal profile are investigated in a general circulation model (GCM) with interactive clouds; guided by observations, two experiments are performed assuming vastly different vertical profiles of the ice water density. Solar heating rates within the extensive cirrus decks associated with monsoon and other convective clouds reach values of 1.5 K day-1. Thus, cirrus solar heating can be an important source for east-west asymmetries in the tropical diabatic heating. Furthermore, because of the latitudinal gradients in the solar insolation, cirrus solar absorption can also influence the meridional heating gradients within the upper troposphere. In spite of the significant east-west asymmetries in the imposed cirrus solar heating, the change in the GCM tropical temperatures is nearly zonally uniform. The magnitude of the zonal mean tropical temperature changes in the GCM (up to 5 degrees K at P ~ 165 mb) indicate that lack of cirrus solar heating may be one reason for the cold bias of the GCMs. Furthermore, the shortwave heating can also account for the observed lapse rate stabilization in the upper tropophere. In addition to the solar effect, the longwave radiative effects of cirrus can also be important but their sign and magnitude are very sensitive to the vertical distribution of clouds. Cirrus longwave heating rates can range from large negative values (cooling) when overlying optically thick clouds (for example, in "deep" extended systems with base below the upper troposphere) to large positive values (heating) for "anvil" type cirrus located in the upper troposphere and with no other clouds below. For the overcast portions of the tropics, if "anvil" type cirri are the only clouds of significance in the upper troposphere, the longwave heating would be the dominant radiative component and this effect becomes more pronounced with increasing altitude of cloud location. Hence, for the tropical zone as a whole, the sign and magnitude of the longwave effect depends on the relative composition of the "deep" and "anvil" clouds. Radiation model calculations that employ climatological values of the vertical distribution of clouds yield a longwave heating effect for the cirrus with the magnitude being comparable to the solar effect. Thus, our results suggest a significant role for the cirrus radiative effects in maintaining the zonal mean thermal structure of the upper troposphere. This influence should be contrasted with the notion that the steep positive gradient in the tropical upper-troposphere potential temperatures is maintained by the latent heat released in penetrating cumulus towers.

119. Ramaswamy, V, 1988: Aerosol radiative forcing and model responses In Aerosols and Climate, A. Deepak Publishing, 349-372.
     [ Abstract PDF ]

     The effects on the surface-atmosphere system due to aerosols are surveyed, with an emphasis on the radiative aspects of aerosols generated by noncatastrophic, nonepisodic events. Solar absorption and scattering by one type of "nominal" tropospheric aerosols is chosen as an example to illustrate the nature of the perturbations in the radiative fluxes at the surface, at the top of atmosphere, and in the atmospheric layers. The range in the magnitude of the perturbations can be quite large when catastrophic or episodic events are also considered. The effects of the aerosol perturbations in the solar spectrum are analyzed by considering the compensation effects due to radiative and radiative-convective mechanisms. These constitute idealistic atmospheric responses, wherein all the mechanisms are confined in a local vertical column and changes in the hydrologic cycle are ignored. Estimates of these responses are compared with those reported from global three-dimensional general circulation model simulations. The results suggest that an aerosol-induced radiative forcing can cause changes in the local energy balance and in the circulation, besides causing changes in the atmospheric thermal profile. The coupling of the aerosol microphysics and radiative and dynamical mechanisms has been achieved thus far only in the study of catastrophic and episodic events. Future investigations would need to focus more on these aspects for all types of aerosols. Robust estimates of the sources and optical properties as well as a better understanding of the microphysical and transport processes are needed to assess the aerosol-induced radiative-dynamical-microphysical interactions unambiguously.The coupling of the aerosol microphysics and radiative and dynamical mechanisms has been achieved thus far only in the study of catastrophic and episodic events. Future investigations would need to focus more on these aspects for all types of aerosols. Robust estimates of the sources and optical properties as well as a better understanding of the microphysical and transport processes are needed to assess the aerosol-induced radiative-dynamical-microphysical interactions unambiguously.

120. Ramaswamy, V, 1988: Dehydration mechanism in the antarctic stratosphere during winter. Geophysical Research Letters, 15(8), 863-866.
     [ Abstract PDF ]

     The growth of ice nuclei through deposition of water vapor at temperatures below frost point is investigated in the context of the Antarctic winter stratosphere. The altitude and the ambient water vapor mixing ratio, as well as the size of the nuclei determine the ice particle growth rate, with higher altitudes requiring colder temperatures for ice deposition. The magnitude of the temperature decrease below the frost point and its evolution over the winter determine the residence time of the growing ice particles and the loss of water vapor at any altitude. A winter-long simulation, using the observed South Pole daily temperatures, suggests that, in the limit of weak latitudinal mixing over the sustral winter, considerable dehydration can occur within the polar vortex, with the higher altitudes (above 22 km.) experiencing the least losses.

121. Ramaswamy, V, 1988: Evolution of polar stratospheric clouds during the Antarctic winter In Polar Ozone Workshop, Proceedings of the Polar Ozone Workshop held in Snowmass, CO, May 9-13, 1988, NASA Conference Public, NASA, 83-84.
122. Thompson, S L., V Ramaswamy, and C Covey, 1987: Atmospheric effects of nuclear war aerosols in general circulation model simulations: influence of smoke optical properties. Journal of Geophysical Research, 92(D9), 10,942-10,960.
     [ Abstract PDF ]

     A global atmospheric general circulation model (GCM) is modified to include radiative transfer parameterizations for the absorption and scattering of solar radiation and the absorption of thermal infrared (IR) radiation by smoke aerosols. The solar scattering modifications include a parameterization for diagnosing smoke optical properties as a function of the time-and space-dependent smoke particle radii. The aerosol IR modifications allow for both the "grey" absorber approximation and a broadband approximation that resolves the aerosol absorption in four spectral intervals. We examine the sensitivity of some GCM-simulated atmospheric and climatic effects to the optical properties and radiative transfer parameterizations used in studies of massive injections of smoke. Specifically, we test the model response to solar scattering versus nonscattering smoke, variations in prescribed smoke single scattering albedo and IR specific absorption, and interactive versus fixed smoke optical properties. Hypothetical nuclear war created smoke scenarios assume the July injection of 60 or 180 Tg of smoke over portions of the mid-latitude land areas of the northern hemisphere. Atmospheric transport and scavenging of the smoke are included. Nonscattering smoke cases produce roughly 40 Wm-2 more Earth-atmosphere solar irradiance absorption over the northern hemisphere, when compared to scattering smoke cases having equivalent specific absorption efficiencies. Varying the elemental carbon content of smoke over a plausible range produces a 4° - 6° C change in average mid-latitude land surface temperature, and a variation of about 0.1 in zonally averaged planetary albedo in the northern hemisphere. The inclusion of IR absorption by smoke (IR specific absorption to visible specific extinction ratio of 0.1) produces mid-latitude July temperature decreases that are 4° - 6°C smaller in magnitude than produced by IR-transparent cases. Thus the smoke IR opacity effect can make a substantial relative change in land surface temperature estimates when compared to July mid-latitude land temperature decreases of 15° - 20°C found in IR-transparent cases.

123. Ramaswamy, V, and A Detwiler, 1986: Interdependence of radiation and microphysics in cirrus clouds. Journal of the Atmospheric Sciences, 43(21), 2289-2301.
     [ Abstract PDF ]

     The important microphysical relationships determining the radiative properties and growth of ice crystals in stratiform cirrus clouds are investigated. A horizontally infinite cloud layer is modeled in the midlatitude upper troposphere. Optical properties of spheres of equal surface area are assumed to represent the scattering characteristics of nonspherical crystals, while the delta-Eddington approximation is used to solve the radiative transfer equations. Classical expressions for ice particle growth and sublimation are coupled to those for radiative energy exchange in order to follow ice particle evolution within the cloud. The radiative properties of the clouds influence the balance among the cloud physical processes within the cloud. In the top 5 percent of optically thin clouds, the ice particle energy balance is essentially between latent and heat diffusion. In the case of clouds with large optical depths, the energy balance is between latent heat and radiation, i.e., radiative cooling enhances particle growth by vapor deposition. In the lower 5 percent of optically thin or thick clouds, latent heat and radiation are balanced by the diffusion of heat from the particle to the environment. Here, upwelling radiation enhances particle sublimation at cloud base. Environmental ice saturation ratio is the primary factor determining the energy balance during growth of ice crystals. When the ice saturation ratio is ~ 1, crystal growth rates are small, and radiative heating/cooling exercises a strong influence. However, for ice saturation ratios more than a percentage above or below unity, radiative influences on growth rates of crystals with lengths less than 200 um are negligible. We have followed the one-dimensional temporal evolution of 1-km thick cirrus cloud layers subsiding in still air. Crystals at cloud top grow larger with time while those at cloud base sublimate as the cloud settles into dry air, with the vertical fall distance greater for larger initial crystal lengths. The temporal evolution of the cloud microphysical characteristics results in modification of the radiation fields, both within the cloud and at the cloud boundaries.

Direct link to page: http://www.gfdl.noaa.gov/bibliography/resultstest.php?author=1114