Bibliography - M Daniel Schwarzkopf

1. Fiore, Arlene M., J J West, Larry Horowitz, V Naik, and M Daniel Schwarzkopf, April 2008: Characterizing the tropospheric ozone response to methane emission controls and the benefits to climate and air quality. Journal of Geophysical Research, 113, D08307, doi:10.1029/2007JD009162.
   [ Abstract ]

   Reducing methane (CH₄) emissions is an attractive option for jointly addressing climate and ozone (O₃) air quality goals. With multidecadal full-chemistry transient simulations in the MOZART-2 tropospheric chemistry model, we show that tropospheric O₃ responds approximately linearly to changes in CH₄ emissions over a range of anthropogenic emissions from 0–430 Tg CH₄a⁻¹ (0.11–0.16 Tg tropospheric O₃ or ∼11–15 ppt global mean surface O₃ decrease per Tg a⁻¹ CH₄ reduced). We find that neither the air quality nor climate benefits depend strongly on the location of the CH₄ emission reductions, implying that the lowest cost emission controls can be targeted. With a series of future (2005–2030) transient simulations, we demonstrate that cost-effective CH₄ controls would offset the positive climate forcing from CH₄ and O₃ that would otherwise occur (from increases in NO_x and CH₄ emissions in the baseline scenario) and improve O₃ air quality. We estimate that anthropogenic CH₄ contributes 0.7 Wm⁻² to climate forcing and ∼4 ppb to surface O₃ in 2030 under the baseline scenario. Although the response of surface O₃ to CH₄ is relatively uniform spatially compared to that from other O₃ precursors, it is strongest in regions where surface air mixes frequently with the free troposphere and where the local O₃ formation regime is NO_x-saturated. In the model, CH₄ oxidation within the boundary layer (below ∼2.5 km) contributes more to surface O₃ than CH₄ oxidation in the free troposphere. In NO_x-saturated regions, the surface O₃ sensitivity to CH₄ can be twice that of the global mean, with >70% of this sensitivity resulting from boundary layer oxidation of CH₄. Accurately representing the NO_x distribution is thus crucial for quantifying the O₃ sensitivity to CH₄.

2. 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.

3. 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.

4. Shindell, D, Hiram Levy II, M Daniel Schwarzkopf, Larry Horowitz, J F Lamarque, and G Faluvegi, June 2008: Multimodel projections of climate change from short-lived emissions due to human activities. Journal of Geophysical Research, 113, D11109, doi:10.1029/2007JD009152.
   [ Abstract ]

   We use the GISS (Goddard Institute for Space Studies), GFDL (Geophysical Fluid Dynamics Laboratory) and NCAR (National Center for Atmospheric Research) climate models to study the climate impact of the future evolution of short-lived radiatively active species (ozone and aerosols). The models used mid-range A1B emission scenarios, independently calculated the resulting composition change, and then performed transient simulations to 2050 examining the response to projected changes in short-lived species and to changes in both long-lived and short-lived species together. By 2050, two models show that the global mean annual average warming due to long-lived GHGs (greenhouse gases) is enhanced by 20–25% due to the radiatively active short-lived species. One model shows virtually no effect from short-lived species. Intermodel differences are largely related to differences in emissions projections for short-lived species, which are substantial even for a particular storyline. For aerosols, these uncertainties are usually dominant, though for sulfate uncertainties in aerosol physics are also substantial. For tropospheric ozone, uncertainties in physical processes are more important than uncertainties in precursor emissions. Differences in future atmospheric burdens and radiative forcing for aerosols are dominated by divergent assumptions about emissions from South and East Asia. In all three models, the spatial distribution of radiative forcing is less important than that of climate sensitivity in predicting climate impact. Both short-lived and long-lived species appear to cause enhanced climate responses in the same regions of high sensitivity rather than short-lived species having an enhanced effect primarily near polluted areas. Since short-lived species can significantly influence climate, regional air quality emission control strategies for short-lived pollutants may substantially impact climate over large (e.g., hemispheric) scales.

5. 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.

6. West, J J., Arlene M Fiore, V Naik, Larry Horowitz, M Daniel Schwarzkopf, and D L Mauzerall, 2007: Ozone air quality and radiative forcing consequences of changes in ozone precursor emissions. Geophysical Research Letters, 37, L06806, doi:10.1029/2006GL029173.
   [ Abstract ]

   Changes in emissions of ozone (O₃) precursors affect both air quality and climate. We first examine the sensitivity of surface O₃ concentrations (O₃ ^srf) and net radiative forcing of climate (RF_net) to reductions in emissions of four precursors - nitrogen oxides (NO _x ), non-methane volatile organic compounds, carbon monoxide, and methane (CH₄). We show that long-term CH₄-induced changes in O₃, known to be important for climate, are also relevant for air quality; for example, NO _x reductions increase CH₄, causing a long-term O₃ increase that partially counteracts the direct O₃ decrease. Second, we assess the radiative forcing resulting from actions to improve O₃ air quality by calculating the ratio of ΔRF_net to changes in metrics of O₃ ^srf. Decreases in CH₄ emissions cause the greatest RF_net  decrease per unit reduction in O₃ ^srf, while NO _x reductions increase RF_net. Of the available means to improve O₃ air quality, therefore, CH₄ abatement best reduces climate forcing.

7. 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.

8. 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

9. 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.

10. 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.

11. 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.

12. 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 ]
13. Stouffer, Ronald J., Thomas L Delworth, Keith W Dixon, Rich Gudgel, Isaac Held, Richard S Hemler, Thomas R Knutson, M Daniel Schwarzkopf, Michael J Spelman, Michael Winton, Anthony J Broccoli, H C Lee, Fanrong Zeng, and Brian J Soden, 2006: GFDL's CM2 Global Coupled Climate Models. Part IV: Idealized Climate Response. Journal of Climate, 19(5), doi:10.1175/JCLI3632.1.
    [ Abstract ]

    The climate response to idealized changes in the atmospheric CO2 concentration by the new GFDL climate model (CM2) is documented. This new model is very different from earlier GFDL models in its parameterizations of subgrid-scale physical processes, numerical algorithms, and resolution. The model was constructed to be useful for both seasonal-to-interannual predictions and climate change research. Unlike previous versions of the global coupled GFDL climate models, CM2 does not use flux adjustments to maintain a stable control climate. Results from two model versions, Climate Model versions 2.0 (CM2.0) and 2.1 (CM2.1), are presented. Two atmosphere–mixed layer ocean or slab models, Slab Model versions 2.0 (SM2.0) and 2.1 (SM2.1), are constructed corresponding to CM2.0 and CM2.1. Using the SM2 models to estimate the climate sensitivity, it is found that the equilibrium globally averaged surface air temperature increases 2.9 (SM2.0) and 3.4 K (SM2.1) for a doubling of the atmospheric CO2 concentration. When forced by a 1% per year CO2 increase, the surface air temperature difference around the time of CO2 doubling [transient climate response (TCR)] is about 1.6 K for both coupled model versions (CM2.0 and CM2.1). The simulated warming is near the median of the responses documented for the climate models used in the 2001 Intergovernmental Panel on Climate Change (IPCC) Working Group I Third Assessment Report (TAR). The thermohaline circulation (THC) weakened in response to increasing atmospheric CO2. By the time of CO2 doubling, the weakening in CM2.1 is larger than that found in CM2.0: 7 and 4 Sv (1 Sv 106 m3 s−1), respectively. However, the THC in the control integration of CM2.1 is stronger than in CM2.0, so that the percentage change in the THC between the two versions is more similar. The average THC change for the models presented in the TAR is about 3 or 4 Sv; however, the range across the model results is very large, varying from a slight increase (+2 Sv) to a large decrease (−10 Sv).

14. 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.

15. 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.

16. 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.

17. 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.

18. 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 ]
19. 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.

20. 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.

21. 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.

22. 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.
23. Soden, Brian J., S Tjernkes, J Schmetz, R Saunders, J Bates, B Ellingson, R Engelen, and M Daniel Schwarzkopf, et al., 2000: An intercomparison of radiation codes for retrieving upper-tropospheric humidity in the 6.3-:m band: A report from the first GVaP Workshop. Bulletin of the American Meteorological Society, 81(4), 797-808.
    [ Abstract PDF ]

    An intercomparison of radiation codes used in retrieving upper-tropospheric humidity (UTH) from observations in the <2 (6.3 :m) water vapor absorption band was performed. This intercomparison is one part of a coordinated effort within the Global Energy and Water Cycle Experiment Water Vapor Project to assess our ability to monitor the distribution and variations of upper-tropospheric moisture from spaceborne sensors. A total of 23 different codes, ranging from detailed line-by-line (LBL) models, to coarser-resolution narrowband (NB) models, to highly parameterized single-band (SB) models participated in the study. Forward calculations were performed using a carefully selected set of temperature and moisture profiles chosen to be representative of a wide range of atmospheric conditions. The LBL model calculations exhibited the greatest consistency with each other, typically agreeing to within 0.5 K in terms of the equivalent blackbody brightness temperature (Tb). The majority of NB and SB models agreed to within ±1 K of the LBL models, although a few older models exhibited systematic Tb biases in excess of 2 K. A discussion of the discrepancies between various models, their association with differences in model physics (e.g., continuum absorption), and their implications for UTH retrieval and radiance assimilation is presented.

24. 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.

25. 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.

26. 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.
27. 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.

28. 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.

29. 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.

30. 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.

31. 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.

32. 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.

33. 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.

34. Feigelson, E M., and M Daniel Schwarzkopf, et al., 1991: Calculation of longwave radiation fluxes in atmospheres. Journal of Geophysical Research, 96(D5), 8985-9001.
    [ Abstract PDF ]

    A technique for the computation of longwave radiative quantities using the line-by-line approach has been developed in the Soviet Union. The method has been applied to obtain fluxes and cooling rates for standard atmospheric profiles used in the Intercomparison of Radiation Codes used in Climate Models (ICRCCM) sponsored by the World Meteorological Organization. The sensistivity of the result to changes in the vertical quadrature scheme, the angular integration, and the spectral line shape is evaluated. Fluxes and cooling rates in the troposphere are in general agreement with those obtained with different line-by-line models. Results from parameterized models, including a wideband statistical model and one employing the integral transmission fuction, have been compared to the line-by-line results. Flux errors in the simplified schemes are of the order of 10 W/m2. The sensitivity of these models to changes in atmospheric profiles, or to an increase in CO₂ amount, is similar to that of the line-by-line calculations.

35. Fels, S, J T Kiehl, A A Lacis, and M Daniel Schwarzkopf, 1991: Infrared cooling rate calculations in operational general circulation models: Comparisons with benchmark computations. Journal of Geophysical Research, 96(D5), 9105-9120.
    [ Abstract PDF ]

    As part of the Intercomparison of Radiation Codes in Climate Models (ICRCCM) project, careful comparisons of the performance of a large number of radiation codes were carried out, and the results compared with those of benchmark calculations. In this paper, we document the performance of a number of parameterized models which have been heavily used in climate and numerical prediction research at three institutions: Geophysical Fluid Dynmaics Laboratory (GFDL), National Center for Atmospheric Research (NCAR), and Goddard Institute for Space Studies (GISS).

36. Ramaswamy, V, M Daniel Schwarzkopf, and S C Liu, 1991: Preface. Journal of Geophysical Research, 96(D5), 8921-8923.
    [ PDF ]
37. Schwarzkopf, M D., and S Fels, 1991: The simplified exchange method revisited: An accurate, rapid method for computation of infrared cooling rates and fluxes. Journal of Geophysical Research, 96(D5), 9075-9096.
    [ Abstract PDF ]

    The performance and construction of a new algorithm for the calculation of infrared cooling rates and fluxes in terrestrial general circulation models are described in detail. The computational method, which is suitable for use in models of both the troposphere and the middle atmosphere, incorporates effects now known to be important, such as an extended water vapor epsilon-type continuum, careful treatment of water vapor lines, of water-carbon dioxide overlap, and of Voigt line shape. The competing requirements of accuracy and speed are both satisfied by extensive use of a generalization of the simplified exchange approximation of Fels and Schwarzkopf (1975). Cooling rates and fluxes are validated by comparison with benchmark line-by-line calculations on standard atmospheric profiles obtained for the Intercomparison of Radiation Codes Used in Climate Models (ICRCCM). Results indicate that the new algorithm is substantially more accurate than any previously used at the Geophysical Fluid Dynamics Laboratory.

38. Schwarzkopf, M D., and S Fels, 1989: GFDL radiation codes: The next generation In IRS '88: Current Problems in Atmospheric Radiation, A. Deepak Publishing, 433-435.
39. Crisp, D, S Fels, and M Daniel Schwarzkopf, 1986: Approximate methods for finding CO₂ 15-µm band transmission in planetary atmospheres. Journal of Geophysical Research, 91(D11), 11,851-11,866.
    [ Abstract PDF ]

    The CO₂ 15-µm band provides an important source of thermal opacity in the atmospheres of Venus, Earth, and Mars. Efficient and accurate methods for finding the transmission in this band are therefore needed before complete, self-consistent physical models of these atmospheres can be developed. In this paper we describe a hierarchy of such methods. The most versatile and accurate of these is an "exact" line-by-line model (Fels and Schwarzkopf, 1981). Other methods described here employ simplifying assumptions about the structure of the 15-µm band which significantly improve their efficiency. Because such approximations can reduce the accuracy of a model, as well as its computational expense, we established the range of validity of these simpler models by comparing their results to those generated by the line-by-line model. Pressures and absorber amounts like those encountered in the atmospheres of Venus, Earth, and Mars were used in these tests. Physical band models based on the Goody (1952) random model compose the first class of approximate methods. These narrow-band models include a general random model and other more efficient techniques that employ the Malkmus (1967) line-strength distribution. Two simple strategies for including Voigt and Doppler line-shape effects are tested. We show that the accuracy of these models at low pressures is very sensitive to the line-strength distribution as well as the line shape. The second class of approximate methods is represented by an exponential wideband model. This physical band model is much more efficient than those described above, since it can be used to find transmission functions for broad sections of the CO₂ 15-µm band in a single step. When combined with a simple Voigt parameterization, this method produces results almost as accurate as those obtained from the more expensive narrow-band random models. The final class of approximate methods tested here includes the empirical logarithmic wideband models that have been used extensively in climate-modeling studies (Kiehl and Ramanathan, 1983; Pollack, et al., 1981). These methods are very efficient, but their range of validity is more limited than that of the other methods tested here. These methods should therefore be used with caution.

40. Schwarzkopf, M D., and S Fels, 1985: Improvements to the algorithm for computing CO2 transmissivities and cooling rates. Journal of Geophysical Research, 90(C10), 10,541-10,550.
    [ Abstract PDF ]

    A new interpolation algorithm is derived for obtaining CO₂ 15-μm transmissivities at any pressure from tables of transmission functions at standard pressures. The new method is a revision of the Fels-Schwarzkopf (1981) technique. Improvements to the standard transmissivity tables are also discussed. An extension of these methods to calculate transmissivities at CO₂ concentrations other than those used for the tables is described.

41. Fels, S, and M Daniel Schwarzkopf, 1981: An efficient, accurate algorithm for calculating an efficient, accurate algorithm for calculating CO₂ 15 um band cooling rates. Journal of Geophysical Research, 86(C2), 1205-1232.
    [ Abstract PDF ]

    A fast, accurate method for obtaining atmospheric carbon dioxide transmission functions for the 15-Mum band is presented. Tables of transmissivities for m band is presented. Tables of transmissivities for CO₂ mixing ratios of 330 and 660 ppmv on standard pressure grids comprising geopotential heights ranging from 0 to 80 km and using standard temperatures are included. An algorithm for interpolating from these values to any desired temperature profile and to any other pressure grid is detailed.

42. Fels, S, Jerry D Mahlman, M Daniel Schwarzkopf, and R W Sinclair, 1980: Stratospheric sensitivity to perturbations in ozone and carbon dioxide: Radiative and dynamical response. Journal of the Atmospheric Sciences, 37(10), 2265-2297.
    [ Abstract PDF ]

    We have attempted to assess the stratospheric effects of two different perturbations: 1) a uniform 50% reduction in ozone; and 2) a uniform doubling of carbon dioxide. The primary studies employ an annual mean insolation version of the recently developed GFDL 40-level general circulation model (GCM). Supporting the auxilliary calculations using purely radiative models are also presented. One of these, in which the thermal sensitivity is computed using the assumption that heating by dynamical processes is unaffected by changed composition, gives results which generally are in excellent agreement with those from the GCM. Exceptions to this occur in the ozone reduction experiment at the tropical tropopause and the tropical mesosphere. The predicted response to the ozone reduction is largest at 50 km in the tropics, where the temperature decreases by 25 K; at the tropical tropopause, the decrease is 5 K. The carbon dioxide increase results in a 10 K decrease at 50 km, decreasing to zero at the tropopause. The temperature change in the CO₂ experiment is remarkably uniform in latitude.

43. Fels, S, and M Daniel Schwarzkopf, 1975: The simplified exchange approximation: a new method for radiative transfer calculations. Journal of the Atmospheric Sciences, 32(7), 1475-1488.
    [ Abstract ]

    A new scheme for the efficient calculation of longwave radiative heating rates is proposed. Its speed and accuracy make it attractive for use in large atmospheric circulation models. The approximation suggested isA new scheme for the efficient calculation of longwave radiative heating rates is proposed. Its speed and accuracy make it attractive for use in large atmospheric circulation models. The approximation suggested is q ~ q^e - q^e CTS + qCTS,CTS, where q is the heating rate, q^e an "emissivity" heating rate calculated using the strong-line approximation and neglecting variation of line intensity with temperature, q^e CTS the heating rate calculated using the cool-to-space approximation and the emissivity assumption, and qCTS the heating rate calculated by the cool-to-space approximation. Tests using a variety of soundings indicate that for both clear sky and cloudy cases the new approximation is substantially more accurate than either the emissivity or the cool-to-space approximations alone. Deviations from exact calculations are generally under 0.05 K day^-1. Errors in the calculated flux at the surface are also shown to be small especially with the inclusion of a "heat from ground" term in the approximation. Some alternate schemes using similar approximations are presented and their utility discussed.

Direct link to page: http://www.gfdl.noaa.gov/bibliography/resultstest.php?author=1090