Bibliography - Leo J Donner

1. Haywood, J M., Leo J Donner, A Jones, and J-C Golaz, March 2009: Global indirect radiative forcing caused by aerosols: IPCC (2007) and beyond In Clouds in the Perturbed Climate System, Jost Heintzenberg and Robert Charlson, eds., MIT Press, 451-467.
   [ Abstract ]

   Anthropogenic aerosols are thought to exert a significant indirect radiative forcing because they act as cloud condensation nuclei in warm cloud processes and ice nuclei in cold cloud processes. While IPCC (2007) discuss many of the processes associated with the perturbation of cloud microphysics by anthropogenic aerosols, they only provide full quantification of the radiative forcing due to the first indirect effect (referred to by IPCC (2007) as the cloud albedo effect). Here we explain that this approach is necessary if one is to compare the radiative forcing from the indirect effect of aerosols with those from other radiative forcing components such as that from changes in well-mixed greenhouse gases. We also highlight the problems in assessing the effect of anthropogenic aerosols upon clouds under the strict definitions of radiative forcing of IPCC (2007). Although results from GCMs at their current state of development suggest analyzing indirect aerosol effects in terms of forcing and feedback is possible, a key rationale for IPCC’s definition of radiative forcing, a straightforward scaling between an agent’s forcing and the temperature change it induces, is significantly compromised. Feedbacks from other radiative forcings are responses to radiative perturbations, while feedbacks from indirect aerosol effects are responses to both radiative and cloud microphysical perturbations. This inherent difference in forcing mechanism breaks down the consistency between forcing and temperature response. It is likely that additional characterization, such as climate efficacy, will be required when comparing indirect aerosol effects with other radiative forcings. We suggest using the radiative flux perturbation associated with a change from pre-industrial to present-day composition, calculated in a GCM with fixed sea-surface temperature and sea ice, as a supplement to IPCC forcing.

2. Lee, S S., Leo J Donner, and V T J Phillips, April 2009: Sensitivity of aerosol and cloud effects on radiation to cloud types: comparison between deep convective clouds and warm stratiform clouds over one-day period. Atmospheric Chemistry and Physics, 9, 2555-2575.
   [ Abstract PDF ]

   Cloud and aerosol effects on radiation in two contrasting cloud types, a deep mesoscale convective system (MCS) and warm stratocumulus clouds, are simulated and compared. At the top of the atmosphere, 45–81% of shortwave cloud forcing (SCF) is offset by longwave cloud forcing (LCF) in the MCS, whereas warm stratiform clouds show the offset of less than ~20%. 28% of increased negative SCF is offset by increased LCF with increasing aerosols in the MCS at the top of the atmosphere. However, the stratiform clouds show the offset of just around 2–5%. Ice clouds as well as liquid clouds play an important role in the larger offset in the MCS. Lower cloud-top height and cloud depth, characterizing cloud types, lead to the smaller offset of SCF by LCF and the offset of increased negative SCF by increased LCF at high aerosol in stratocumulus clouds than in the MCS. Supplementary simulations show that this dependence of modulation of LCF on cloud depth and cloud-top height is also simulated among different types of convective clouds.

3. Donner, Leo J., and W G Large, November 2008: Climate modeling. Annual Review of Environment and Resources, 33, 1-17.
   [ Abstract ]

   Climate models simulate the atmosphere, given atmospheric composition and energy from the sun, and include explicit modeling of, and exchanges with, the underlying oceans, sea ice, and land. The models are based on physical principles governing momentum, thermodynamics, cloud microphysics, radiative transfer, and turbulence. Climate models are evolving into Earth-system models, which also include chemical and biological processes and afford the prospect of links to studies of human dimensions of climate change. Although the fundamental principles on which climate models are based are robust, computational limits preclude their numerical solution on scales that include many processes important in the climate system. Despite this limitation, which is often dealt with by parameterization, many aspects of past and present climate have been successfully simulated using climate models, and climate models are used extensively to predict future climate change resulting from human activity. PDF available from Annual Reviews

4. Lee, S S., Leo J Donner, V T J Phillips, and Yi Ming, 2008: The dependence of aerosol effects on clouds and precipitation on cloud-system organization, shear and stability. Journal of Geophysical Research, 113, D16202, doi:10.1029/2007JD009224.
   [ Abstract ]

   Precipitation suppression due to an increase of aerosol number concentration in stratiform cloud is well-known. It is not certain whether the suppression applies for deep convection. Recent studies have suggested increasing precipitation from deep convection with increasing aerosols under some, but not all, conditions. Increasing precipitation with increasing aerosols can result from strong interactions in deep convection between dynamics and microphysics. High cloud liquid, due to delayed autoconversion, provides more evaporation, leading to more active downdrafts, convergence fields, condensation, collection of cloud liquid by precipitable hydrometeors, and precipitation. Evaporation of cloud liquid is a primary determinant of the intensity of the interactions. It is partly controlled by wind shear modulating the entrainment of dry air into clouds and transport of cloud liquid into unsaturated areas. Downdraft-induced convergence, crucial to the interaction, is weak for shallow clouds, generally associated with low convective available potential energy (CAPE). Aerosol effects on cloud and precipitation can vary with CAPE and wind shear. Pairs of idealized numerical experiments for high and low aerosol cases were run for five different environmental conditions to investigate the dependence of aerosol effect on stability and wind shear. In the environment of high CAPE and strong wind shear, cumulonimbus- and cumulus-type clouds were dominant. Transport of cloud liquid to unsaturated areas was larger at high aerosol, leading to stronger downdrafts. Because of the large vertical extent of those clouds, strong downdrafts and convergence developed for strong interactions between dynamics and microphysics. These led to larger precipitation at high aerosol. Detrainment of cloud liquid and associated evaporation were less with lower CAPE and wind shear, where dynamically weaker clouds dominated. Transport of cloud liquid to unsaturated areas was not as active as in the environment of high CAPE and strong shear. Also, evaporatively driven differences in downdrafts at their level of initial descent were not magnified in clouds with shallow depth as much as in deep convective clouds as they accelerated to the surface over shorter distances. Hence the interaction between dynamics and microphysics was reduced, leading to precipitation suppression at high aerosol. These results demonstrate that increasing aerosol can either decrease or increase precipitation for an imposed large-scale environment supporting cloud development. The implications for larger-scale aspects of the hydrological cycle will require further study with larger-domain models and cumulus parameterizations with advanced microphysics.

5. Lee, S S., Leo J Donner, V T J Phillips, and Yi Ming, 2008: Examination of aerosol effects on precipitation in deep convective clouds during the 1997 ARM summer experiment. Quarterly Journal of the Royal Meteorological Society, 134(634), doi:10.1002/qj.287.
   [ Abstract ]

   It has been generally accepted that increasing aerosols suppress precipitation. The aerosol-induced precipitation suppression was suggested by the study of shallow stratiform clouds. Recent studies of convective clouds showed increasing aerosols could increase precipitation. Those studies showed that intense feedbacks between aerosols and cloud dynamics led to increased precipitation in some cases of convective clouds. This study expanded those studies by analyzing detailed microphysical and dynamical modifications by aerosols leading to increased precipitation. This study focused on three observed cases of mesoscale cloud ensemble (MCE) driven by deep convective clouds, since MCE accounts for a large proportion of the Earth's precipitation and the study of aerosol effects on MCE is at its incipient stage. Those MCEs were observed during the 1997 Atmospheric Radiation Measurement (ARM) summer experiment. Two numerical experiments were performed for each of the MCEs to simulate aerosol effects on deep convection. The first was with high aerosol number concentration, and the second was with low concentration. The results showed an increased precipitation at high aerosol, due to stronger, more numerous updraughts, initiated by stronger convergence lines at the surface in convective regions of the MCE. The stronger convergence lines were triggered by increased evaporation of cloud liquid in the high-aerosol case, made possible by higher values of cloud liquid necessary for autoconversion. The generality of these results requires further investigation. However, they demonstrate that the response of precipitation to increased aerosols in deep convection can be different from that in shallow cloud systems, at least for the cases studied here.

6. Ming, Yi, Paul Ginoux, Leo J Donner, Stuart Freidenreich, Larry Horowitz, Ming Zhao, J-C Golaz, and Shian-Jiann Lin, in press: Transport of European Air Pollution influences Arctic climate. Science. 8/08.
   [ Abstract ]

   Arctic climate is changing at a pace faster than the global average in the recent decades (1, 2). Arctic haze (3) - an accumulation of long-range transported aerosols - enhances longwave emissivity of liquid water clouds both by reducing droplet size (4–6) and by increasing liquid condensate, thus exerting substantial surface warming in winter. The formation of Arctic haze and its influence on local climate are poorly understood, and constitutes an important missing piece of the Arctic climate puzzle. Here we find, with the help of a state-of-the-art global climate model with explicit treatment of pollutant transport and aerosol-cloud interactions, that the poleward transport of European air pollution is controlled strongly by the fluctuation in the second climate mode of the North Atlantic - European region. Though accounting for a smaller fraction of the region’s overall climate variability than the first mode (namely the North Atlantic Oscillation), the second mode has its impacts on Arctic climate amplified through modulating the amount of aerosols reaching the Arctic. This is supported by the fact that the surface aerosol concentrations and longwave downward radiative flux measured at locations lying in the model-projected transport pathway show strong correlation with the second mode. A shift of the mode from negative to positive phases doubles the abundance of Arctic haze, and the resulting increase in cloud liquid condensate alone is estimated to warm the surface by 1.8 K or to reduce the wintertime sea ice by 0.16 m. This finding is essential for understanding Arctic climate variability and change.

7. Salzmann, M, M G Lawrence, V T J Phillips, and Leo J Donner, 2008: Cloud system resolving model study of the roles of deep convection for photo-chemistry in the TOGA COARE/CEPEX region. Atmospheric Chemistry and Physics, 8, 2741-2757.
   [ Abstract PDF ]

   A cloud system resolving model including photo-chemistry (CSRMC) has been developed based on a prototype version of the Weather Research and Forecasting (WRF) model and is used to study influences of deep convection on chemistry in the TOGA COARE/CEPEX region. Lateral boundary conditions for trace gases are prescribed from global chemistry-transport simulations, and the vertical advection of trace gases by large scale dynamics, which is not reproduced in a limited area cloud system resolving model, is taken into account. The influences of deep convective transport and of lightning on NO_x, O₃, and HO_x(=HO₂+OH), in the vicinity of the deep convective systems are investigated in a 7-day 3-D 248×248 km² horizontal domain simulation and several 2-D sensitivity runs with a 500 km horizontal domain. Mid-tropospheric entrainment is more important on average for the upward transport of O₃ in the 3-D run than in the 2-D runs, but at the same time undiluted O₃-poor air from the marine boundary layer reaches the upper troposphere more frequently in the 3-D run than in the 2-D runs, indicating the presence of undiluted convective cores. In all runs, in situ lightning is found to have only minor impacts on the local O₃ budget. Near zero O₃ volume mixing ratios due to the reaction with lightning-produced NO are only simulated in a 2-D sensitivity run with an extremely high number of NO molecules per flash, which is outside the range of current estimates. The fraction of NO_x chemically lost within the domain varies between 20 and 24% in the 2-D runs, but is negligible in the 3-D run, in agreement with a lower average NO_x concentration in the 3-D run despite a greater number of flashes. Stratosphere to troposphere transport of O₃ is simulated to occur episodically in thin filaments in the 2-D runs, but on average net upward transport of O₃ from below ~16 km is simulated in association with mean large scale ascent in the region. Ozone profiles in the TOGA COARE/CEPEX region are suggested to be strongly influenced by the intra-seasonal (Madden-Julian) oscillation.

8. Donner, Leo J., Larry Horowitz, Arlene M Fiore, Charles J Seman, D Blake, and N J Blake, 2007: Transport of radon-222 and methyl iodide by deep convection in the GFDL Global Atmospheric Model AM2. Journal of Geophysical Research, 112, D17303, doi:10.1029/2006JD007548.
   [ Abstract ]

   Transport of radon-222 and methyl iodide by deep convection is analyzed in the Geophysical Fluid Dynamics Laboratory (GFDL) Atmospheric Model 2 (AM2) using two parameterizations for deep convection. One of these parameterizations represents deep convection as an ensemble of entraining plumes; the other represents deep convection as an ensemble of entraining plumes with associated mesoscale updrafts and downdrafts. Although precipitation patterns are generally similar in AM2 with both parameterizations, the deep convective mass fluxes are more than three times larger in the middle- to upper troposphere for the parameterization consisting only of entraining plumes, but do not extend across the tropopause, unlike the parameterization including mesoscale circulations. The differences in mass fluxes result mainly from a different partitioning between convective and stratiform precipitation; the parameterization including mesoscale circulations detrains considerably more water vapor in the middle troposphere and is associated with more stratiform rain. The distributions of both radon-222 and methyl iodide reflect the different mass fluxes. Relative to observations (limited by infrequent spatial and temporal sampling), AM2 tends to simulate lower concentrations of radon-222 and methyl iodide in the planetary boundary layer, producing a negative model bias through much of the troposphere, with both cumulus parameterizations. The shapes of the observed profiles suggest that the larger deep convective mass fluxes and associated transport in the parameterization lacking a mesoscale component are less realistic.

9. 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.

10. Phillips, V T., Leo J Donner, and Stephen T Garner, February 2007: Nucleation processes in deep convection simulated by a cloud-system-resolving model with double moment bulk microphysics. Journal of the Atmospheric Sciences, 64(3), doi:10.1175/JAS3869.1.
    [ Abstract ]

    A novel type of limited double-moment scheme for bulk microphysics is presented here for cloud-system-resolving models (CSRMs). It predicts the average size of cloud droplets and crystals, which is important for representing the radiative impact of clouds on the climate system. In this new scheme, there are interactive components for ice nuclei (IN) and cloud condensation nuclei (CCN). For cloud ice, the processes of primary ice nucleation, Hallett–Mossop (HM) multiplication of ice particles (secondary ice production), and homogeneous freezing of aerosols and droplets provide the source of ice number. The preferential evaporation of smaller droplets during homogeneous freezing of cloud liquid is represented for the first time. Primary and secondary (i.e., in cloud) droplet nucleation are also represented, by predicting the supersaturation as a function of the vertical velocity and local properties of cloud liquid. A linearized scheme predicts the supersaturation, explicitly predicting rates of condensation and vapor deposition onto liquid (cloud liquid, rain) and ice (cloud ice, snow, graupel) species. The predicted supersaturation becomes the input for most nucleation processes, including homogeneous aerosol freezing and secondary droplet activation. Comparison of the scheme with available aircraft and satellite data is performed for two cases of deep convection over the tropical western Pacific Ocean. Sensitivity tests are performed with respect to a range of nucleation processes. The HM process of ice particle multiplication has an important impact on the domain-wide ice concentration in the lower half of the mixed-phase region, especially when a lack of upper-level cirrus suppresses homogeneous freezing. Homogeneous freezing of droplets and, especially, aerosols is found to be the key control on number and sizes of cloud particles in the simulated cloud ensemble. Preferential evaporation of smaller droplets during homogeneous freezing produces a major impact on ice concentrations aloft. Aerosols originating from the remote free troposphere become activated in deep convective updrafts and produce most of the supercooled cloud droplets that freeze homogeneously aloft. Homogeneous aerosol freezing is found to occur only in widespread regions of weak ascent while homogeneous droplet freezing is restricted to deep convective updrafts. This means that homogeneous aerosol freezing can produce many more crystals than homogeneous droplet freezing, if conditions in the upper troposphere are favorable. These competing mechanisms of homogeneous freezing determine the overall response of the ice concentration to environmental CCN concentrations in the simulated cloud ensemble. The corresponding sensitivity with respect to environmental IN concentrations is much lower. Nevertheless, when extremely high concentrations of IN are applied, that are typical for plumes of desert dust, the supercooled cloud liquid is completely eliminated in the upper half of the mixed phase region. This shuts down the process of homogeneous droplet freezing.

11. Salzmann, M, M G Lawrence, V T J Phillips, and Leo J Donner, 2007: Model sensitivity studies regarding the role of the retention coefficient for the scavenging and redistribution of highly soluble trace gases by deep convective cloud systems. Atmospheric Chemistry and Physics, 7, 2027-2045.
    [ Abstract PDF ]

    The role of the retention coefficient (i.e. the fraction of a dissolved trace gas which is retained in hydrometeors during freezing) for the scavenging and redistribution of highly soluble trace gases by deep convective cloud systems is investigated using a modified version of the Weather Research and Forecasting (WRF) model. Results from cloud system resolving model runs (in which deep convection is initiated by small random perturbations in association with so-called "large scale forcings (LSF)") for a tropical oceanic (TOGA COARE) and a mid-latitude continental case (ARM) are compared to two runs in which bubbles are used to initiate deep convection (STERAO, ARM). In the LSF runs, scavenging is found to almost entirely prevent a highly soluble tracer initially located in the lowest 1.5 km of the troposphere from reaching the upper troposphere, independent of the retention coefficient. The release of gases from freezing hydrometeors leads to mixing ratio increases in the upper troposphere comparable to those calculated for insoluble trace gases only in the two runs in which bubbles are used to initiate deep convection. A comparison of the two ARM runs indicates that using bubbles to initiate deep convection may result in an overestimate of the influence of the retention coefficient on the vertical transport of highly soluble tracers. It is, however, found that the retention coefficient plays an important role for the scavenging and redistribution of highly soluble trace gases with a (chemical) source in the free troposphere and also for trace gases for which even relatively inefficient transport may be important. The large difference between LSF and bubble runs is attributed to differences in dynamics and microphysics in the inflow regions of the storms. The dependence of the results on the model setup indicates the need for additional model studies with a more realistic initiation of deep convection, e.g., considering effects of orography in a nested model setup.

12. Wilcox, E M., and Leo J Donner, 2007: The frequency of extreme rain events in satellite rain-rate estimates and an atmospheric general circulation model. Journal of Climate, 20(1), doi:10.1175/JCLI3987.1.
    [ Abstract ]

    The frequency distributions of surface rain rate are evaluated in the Tropical Rainfall Measuring Mission (TRMM) and Special Sensor Microwave/Imager (SSM/I) satellite observations and the NOAA/GFDL global atmosphere model version 2 (AM2). Instantaneous satellite rain-rate observations averaged over the 2.5° latitude × 2° longitude model grid are shown to be representative of the half-hour rain rate from single time steps simulated by the model. Rain-rate events exceeding 10 mm h⁻¹ are observed by satellites in most regions, with 1 mm h⁻¹ events occurring more than two orders of magnitude more frequently than 10 mm h⁻¹ events. A model simulation using the relaxed Arakawa–Schubert (RAS) formulation of cumulus convection exhibits a strong bias toward many more light rain events compared to the observations and far too few heavy rain events. A simulation using an alternative convection scheme, which includes an explicit representation of mesoscale circulations and an alternative formulation of the closure, exhibits, among other differences, an order of magnitude more tropical rain events above the 5 mm h⁻¹ rate compared to the RAS simulation. This simulation demonstrates that global atmospheric models can be made to produce heavy rain events, in some cases even exceeding the observed frequency of such events. Additional simulations reveal that the frequency distribution of the surface rain rate in the GCM is shaped by a variety of components within the convection parameterization, including the closure, convective triggers, the spectrum of convective and mesoscale clouds, and other parameters whose physical basis is currently only understood to a limited extent. Furthermore, these components interact nonlinearly such that the sensitivity of the rain-rate distribution to the formulation of one component may depend on the formulation of the others. Two simulations using different convection parameterizations are performed using perturbed sea surface temperatures as a surrogate for greenhouse gas–forced climate warming. Changes in the frequency of rain events greater than 2 mm h⁻¹ associated with changing the convection scheme in the model are greater than the changes in the frequency of heavy rain events associated with a 2-K warming using either model. Thus, uncertainty persists with respect to simulating intensity distributions for precipitation and projecting their future changes. Improving the representation of the frequency distribution of rain rates will rely on refinements in the formulation of cumulus closure and the other components of convection schemes, and greater certainty in predictions of future changes in both total rainfall and in rain-rate distributions will require additional refinements in those parameterizations that determine the cloud and water vapor feedbacks.

13. Folkins, I, P Bernath, C Boone, Leo J Donner, A Eldering, G Lesins, R V Martin, B-M Sinnhuber, and K Walker, 2006: Testing convective parameterizations with tropical measurements of HNO3, CO, H2O, and O3: Implications for the water vapor budget. Journal of Geophysical Research, 111, D23304, doi:10.1029/2006JD007325.
    [ Abstract ]

    The updraft and downdraft mass flux profiles generated by convective parameterizations differ significantly from each other. Most convective parameterizations are tested against temperature and relative humidity profiles from radiosondes. Chemical tracers provide important additional constraints on the vertical redistribution of mass by convective parameterizations. We compile tropical climatologies of water vapor (H2O), ozone (O3), carbon monoxide (CO), and nitric acid (HNO3) from a variety of satellite, aircraft, and balloon-based measurement platforms. These climatologies are compared with the profiles predicted by a variant of the Emanuel convective parameterization, a two-column model of the tropical atmosphere, and by the implementations of the Relaxed Arakawa Schubert (RAS) and Zhang and McFarlane (ZM) parameterizations in a three-dimensional global forecast model. In general, the models with more pronounced convective outflow in the upper troposphere compare more favorably with observations. These models are associated with increased evaporative moistening in the middle and lower troposphere.

14. Lin, J-L, G N Kiladis, B E Mapes, K M Weickmann, K R Sperber, W Lin, M C Wheeler, and Leo J Donner, et al., 2006: Tropical Intraseasonal Variability in 14 IPCC AR4 Climate Models. Part I: Convective Signals. Journal of Climate, 19(12), doi:10.1175/JCLI3735.1.
    [ Abstract ]

    This study evaluates the tropical intraseasonal variability, especially the fidelity of Madden–Julian oscillation (MJO) simulations, in 14 coupled general circulation models (GCMs) participating in the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4). Eight years of daily precipitation from each model’s twentieth-century climate simulation are analyzed and compared with daily satellite-retrieved precipitation. Space–time spectral analysis is used to obtain the variance and phase speed of dominant convectively coupled equatorial waves, including the MJO, Kelvin, equatorial Rossby (ER), mixed Rossby–gravity (MRG), and eastward inertio–gravity (EIG) and westward inertio–gravity (WIG) waves. The variance and propagation of the MJO, defined as the eastward wavenumbers 1–6, 30–70-day mode, are examined in detail. The results show that current state-of-the-art GCMs still have significant problems and display a wide range of skill in simulating the tropical intraseasonal variability. The total intraseasonal (2–128 day) variance of precipitation is too weak in most of the models. About half of the models have signals of convectively coupled equatorial waves, with Kelvin and MRG–EIG waves especially prominent. However, the variances are generally too weak for all wave modes except the EIG wave, and the phase speeds are generally too fast, being scaled to excessively deep equivalent depths. An interesting result is that this scaling is consistent within a given model across modes, in that both the symmetric and antisymmetric modes scale similarly to a certain equivalent depth. Excessively deep equivalent depths suggest that these models may not have a large enough reduction in their “effective static stability” by diabatic heating. The MJO variance approaches the observed value in only 2 of the 14 models, but is less than half of the observed value in the other 12 models. The ratio between the eastward MJO variance and the variance of its westward counterpart is too small in most of the models, which is consistent with the lack of highly coherent eastward propagation of the MJO in many models. Moreover, the MJO variance in 13 of the 14 models does not come from a pronounced spectral peak, but usually comes from part of an overreddened spectrum, which in turn is associated with too strong persistence of equatorial precipitation. The two models that arguably do best at simulating the MJO are the only ones having convective closures/triggers linked in some way to moisture convergence.

15. 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).

16. Phillips, V T., and Leo J Donner, 2006: Cloud microphysics, radiation and vertical velocities in two- and three-dimensional simulations of deep convection. Quarterly Journal of the Royal Meteorological Society, 132(621C), doi:10.1256/qj.05.1713011-3033.
    [ Abstract ]

    This study investigates the importance of dimensionality for the characteristics of simulations performed with cloud-system resolving models (CSRMs). In addition to intrinsic questions related to dimensionality in CSRMs, the issue has gained added interest since CSRMs can be utilized instead of conventional cloud parametrizations to represent deep convection within global climate models. Such CSRMs may be either two- or three-dimensional. CSRM simulations of five observed cases of deep convection are performed in both two and three dimensions (2D and 3D) with the aim of elucidating the impact of dimensionality on overall cloud statistics. Observed profiles of the large-scale average of advection of temperature and humidity are applied to initiate and maintain the convection. Two of the cases are from tropical oceanic regions. The other three cases are continental. The average ascent rate in deep convective, cloudy updraughts is about 20-50% higher at mid-levels of the troposphere in 3D than in 2D, for all cases. This corresponds to an increase by a similar percentage in the vertical mass flux of deep updraughts in the oceanic cases. Furthermore, the weak ascent (0.1<1 m s-1) outside the deep convective updraughts is much less prevalent in 3D than in 2D, with vertical velocities being about 20% lower for a given cumulative frequency and a lower vertical mass flux. Downdraughts are weaker in 3D, for most cases. There is a substantial sensitivity of the vertical profiles of cloud liquid and cloud ice, and of other microphysical species, to dimensionality. This is consistent with the sensitivity of the dynamics of convection. Corresponding changes in radiative transfer, especially in the short-wave band, result from the cloud-radiative interactions. In particular, the peak in domain-averaged cloud liquid content in the melting layer is about 50% higher in most of the 2D simulations. The land cases display more sensitivity of the short-wave radiative flux to the choice of orientation of the vertical plane of 2D simulations.

17. Donner, Leo J., 2005: Cloud-system resolving models (CSRMs) and their roles in understanding interactions between convection and large-scale flows. Geophysical Research Abstracts, 7, 02887.
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18. Li, J-L, D E Waliser, J H Jiang, D L Wu, W Read, J W Waters, A M Tompkins, Leo J Donner, J-D Chern, W K Tao, R Atlas, Y Gu, K N Liou, A Del Genio, M Khairoutdinov, and A Gettelman, 2005: Comparisons of EOS MLS cloud ice measurements with ECMWF analyses and GCM simulations: Initial results. Geophysical Research Letters, 32, L18710, doi:10.1029/2005GL023788.
    [ Abstract ]

    To assess the status of global climate models (GCMs) in simulating upper-tropospheric ice water content (IWC), a new set of IWC measurements from the Earth Observing System's Microwave Limb Sounder (MLS) are used. Comparisons are made with ECMWF analyses and simulations from several GCMs, including two with multi-scale-modeling framework. For January 2005 monthly and daily mean values, the spatial agreement between MLS and ECMWF is quite good, although MLS estimates are higher by a factor of 2–3 over the Western Pacific, tropical Africa and South America. For the GCMs, the model-data agreement is within a factor of 2–4 with larger values of disagreement occurring over Eastern Pacific and Atlantic ITCZs, tropical Africa and South America. The implications arising from sampling and uncertainties in the observations, the modeled values and their comparison are discussed. These initial results demonstrate the potential usefulness of this data set for evaluating GCM performance and guiding development efforts.

19. Phillips, V T., and Leo J Donner, 2005: Cloud microphysics, radiation and dynamics in two-and three-dimensional simulations of deep convection. Geophysical Research Abstracts, 7, 10376.
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20. Phillips, V T., and Leo J Donner, 2005: Effects of aerosol concentration on a cloud field simulated by a cloud-resolving model with a double-moment bulk microphysics scheme and fully interactive radiation. Geophysical Research Abstracts, 7, 10336.
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21. Salzmann, M, M G Lawrence, V T J Phillips, and Leo J Donner, 2005: Mass flux diagnostics in CRM studies. Geophysical Research Abstracts, 7, 06799.
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22. Salzmann, M, M G Lawrence, V T J Phillips, and Leo J Donner, 2005: Modelling tracer transport by a cumulus ensemble: Mean ascent and lateral boundary conditions. Geophysical Research Abstracts, 7, 00048.
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23. Donner, Leo J., 2004: Global and regional distributions of tracers: impact of deep convective towers and associated upper-tropospheric stratiform clouds. Geophysical Research Abstracts, 6, 01266, .
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24. Salzmann, M, M G Lawrence, V T J Phillips, and Leo J Donner, 2004: Modelling tracer transport by a cumulus ensemble: lateral boundary conditions and large-scale ascent. Atmospheric Chemistry and Physics, 4, 1797-1811.
    [ Abstract PDF ]

    The vertical transport of tracers by a cumulus ensemble at the TOGA-COARE site is modelled during a 7 day episode using 2-D and 3-D cloud-resolving setups of the Weather Research and Forecast (WRF) model. Lateral boundary conditons (LBC) for tracers, water vapour, and wind are specified and the horizontal advection of trace gases across the lateral domain boundaries is considered. Furthermore, the vertical advection of trace gases by the large-scale motion (short: vertical large-scale advection of tracers, VLSAT) is considered. It is shown, that including VLSAT partially compensates the calculated net downward transport from the middle and upper troposphere (UT) due to the mass balancing mesoscale subsidence induced by deep convection. Depending on whether the VLSAT term is added or not, modelled domain averaged vertical tracer profiles can differ significantly. Differences between a 2-D and a 3D model run were mainly attributed to an increase in horizontal advection across the lateral domain boundaries due to the meridional wind component not considered in the 2-D setup.

25. Andronache, C, Leo J Donner, Charles J Seman, and Richard S Hemler, 2003: Interactions between the ITCZ and aerosols during INDOEX. Geophysical Research Abstracts, 5, 02719.
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26. Donner, Leo J., 2003: Multiple scales in cumulus convection and their implications for cumulus parameterization in large-scale models. Geophysical Research Abstracts, 5, 02917.
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27. Donner, Leo J., 2003: Tracer transport by parameterized convection. Geophysical Research Abstracts, 5, 02733.
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28. Donner, Leo J., and V T J Phillips, 2003: Boundary layer control on convective available potential energy: Implications for cumulus parameterization. Journal of Geophysical Research, 108(D22), 4701, doi:10.1029/2003JD003773.
    [ Abstract PDF ]

    Convective available potential energy (CAPE), frequently regarded as an indicator of the potential intensity of deep convection, is strongly controlled by the properties of the planetary boundary layer (BL). Variations in CAPE observed during field experiments in midcontinent North America, the tropical east Atlantic, and the tropical west Pacific, can be accounted for mostly by changes in the temperature and humidity in the BL. The coupling between CAPE and the BL holds for both convective and nonconvective conditions. The coupling under conditions of deep convection implies a constraint on the intensity of deep convection which can be used as a closure for cumulus parameterization. This constraint requires equilibrium in the environment of the parcel used as a basis for calculating CAPE. Over many cases, parcel-environment equilibrium is observed to hold more robustly than equilibrium of CAPE itself. When observational uncertainties are considered, it is uncertain whether quasi-equilibrium, in which the rate of change of CAPE is substantially less than the rate at which mean advection and BL fluxes change CAPE, holds at subdiurnal timescales in the eastern Atlantic and the western Pacific. Quasi-equilibrium is a poor approximation at subdiurnal timescales in midcontinent North America. At timescales approaching diurnal, quasi-equilibrium holds in all cases. Cumulus parameterizations based on quasi-equilibrium may be limited in their ability to model diurnal cycles as a result. CAPE fluctuations related to large, subdiurnal variations in surface fluxes are much sharper than CAPE fluctuations related to changes in mean advection above the BL, especially over land. The strong BL control on CAPE indicates that deep convection does not equilibrate rapid, high-amplitude variations in CAPE originating there.

29. Phillips, V T., and Leo J Donner, 2003: The validity of a new closure assumption for the parameterisation of deep convection. Geophysical Research Abstracts, 5, 07384.
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30. Andronache, C, Leo J Donner, Charles J Seman, and Richard S Hemler, 2002: A study of the impact of the Intertropical Convergence Zone on aerosols during INDOEX. Journal of Geophysical Research, 107(D19), doi:10.1029/2001JD900248.
    [ Abstract PDF ]

    We report model simulations of the effect of deep convection on aerosol under typical Intertropical Convergence Zone (ITCZ) conditions in the tropical Indian Ocean as encountered during the Indian Ocean Experiment (INDOEX). Measurements taken during various phases of INDOEX showed significant aerosol mass concentrations of nss-sulfate, carbonaceous, and mineral dust over the northern Indian Ocean. During the winter dry season aerosol species accumulate and are transported long distances to the tropical regions. In contrast, aerosol measurements south of the ITCZ exhibit significantly lower aerosol concentrations, and the convective activity, mixing, and wet removal in the ITCZ are responsible for their depletion. Our results, based on a cloud-resolving model, driven by National Centers for Environmental Prediction analysis, show that convection and precipitation can remove significant amounts of aerosol, as observed in the Indian Ocean ITCZ. The aerosol lifetime in the boundary layer (BL) is of the order of hours in intense convection with precipitation, but on average is in the range of 1-3 days for the case studied here. Since the convective events occur in a small fraction of the ITCZ area, the aerosol lifetime can vary significantly due to variability of precipitation. Our results show that the decay in concentration of various species of aerosols is comparable with in situ measurements and that the ITCZ can act to reduce the transport of polluted air masses into the Southern Hemisphere especially in cases with significant precipitation. Another finding is that aerosol loading typical to north of ITCZ tends to induce changes in cloud microphysical properties. We found that a difference between clean air masses as those encountered south of the ITCZ to aerosol polluted air masses as encountered north of the ITCZ is associated with a slight decrease of the cloud droplet effective radius (average changes of about 2 :m) and an increase in cloud droplet number concentration (average changes by about 40 to 100 cm-3 ) consistent with several in situ measurements. Thus polluted air masses from the northern Indian Ocean are associated with altered microphysics, and the extent of these effects is dependent on the efficiency of aerosol removal by ITCZ precipitation and dilution by mixing with pristine air masses from the Southern Hemisphere.

31. Xu, K-M, R T Cederwall, Leo J Donner, W W Grabowski, F Guichard, D E Johnson, M Khairoutdinov, S K Krueger, J C Petch, D A Randall, Charles J Seman, W K Tao, D-P Wang, S Xie, J J Yio, and M Zhang, 2002: An intercomparison of cloud-resolving models with the Atmospheric Radiation Measurement summer 1997 Intensive Observation Period data. Quarterly Journal of the Royal Meteorological Society, 128(580), 593-624.
    [ Abstract PDF ]

    This paper reports an intercomparison study of midlatitude continental cumulus convection simulated by eight two-dimensional and two three-dimensional cloud-resolving models (CRMs), driven by observed large-scale advective temperature and moisture tendencies, surface turbulent fluxes, and radiative-heating profiles during three sub-periods of the summer 1997 Intensive Observation Period of the U.S. Department of Energy's Atmospheric Radiation Measurement (ARM) program. Each sub-period includes two or three precipitation events of various intensities over a span of 4 or 5 days. The results can be summarized as follows: #CRMs can reasonably simulate midlatitude continental summer convection observed in the ARM Cloud and Radiation Testbed site in terms of the intensity of convective activity, and the temperature and specific-humidity evolution. Delayed occurrences of the initial precipitation events are a common feature for all three sub-cases among the models. Cloud mass fluxes, condensate mixing ratios and hydrometeor fractions produced by all CRMs are similar. Some of the simulated cloud properties such as cloud liquid-water path and hydrometeor fraction are rather similar to available observations. All CRMs produce large downdraught mass fluxes with magnitudes similar to those of updraughts, in contrast to CRM results for tropical convection. Some inter-model differences in cloud properties are likely to be related to those in the parameterization of microphysical processes. #There is generally a good agreement between the CRMs and observations with CRMs being significantly better than single-column models (SCMs), suggesting that current results are suitable for use in improving parameterizations in SCMs. However, improvements can still be made in the CRM simulations; these include the proper initialization of the CRMs and a more proper method of diagnosing cloud boundaries in model outputs for comparison with satellite and radar cloud observations.

32. Donner, Leo J., Charles J Seman, Richard S Hemler, and Song-Miao Fan, 2001: A cumulus parameterization including mass fluxes, convective vertical velocities, and mesoscale effects: thermodynamic and hydrological aspects in a general circulation model. Journal of Climate, 14(16), 3444-3463.
    [ Abstract PDF ]

    A cumulus parameterization based on mass fluxes, convective-scale vertical velocities, and mesoscale effects has been incorporated in an atmospheric general circulation model (GCM). Most contemporary cumulus parameterizations are based on convective mass fluxes. This parameterization augments mass fluxes with convective-scale vertical velocities as a means of providing a method for incorporating cumulus microphysics using vertical velocities at physically appropriate (subgrid) scales. Convective-scale microphysics provides a key source of material for mesoscale circulations associated with deep convection, along with mesoscale in situ microphysical processes. The latter depend on simple, parameterized mesoscale dynamics. Consistent treatment of convection, microphysics, and radiation is crucial for modeling global-scale interactions involving clouds and radiation. Thermodynamic and hydrological aspects of this parameterization in integrations of the Geophysical Fluid Dynamics Laboratory SKYHI GCM are analyzed. Mass fluxes, phase changes, and heat and moisture transport by the mesoscale components of convective systems are found to be large relative to those of convective (deep tower) components, in agreement with field studies. Partitioning between the convective and mesoscale components varies regionally with large-scale flow characteristics and agrees well with observations from the Tropical Rainfall Measuring Mission (TRMM) satellite. The effects of the mesoscale components of convective systems include stronger Hadley and Walker circulations, warmer upper-tropospheric Tropics, and moister Tropics. The mass fluxes for convective systems including mesoscale components differ appreciably in both magnitude and structure from those for convective systems consisting of cells only. When mesoscale components exist, detrainment is concentrated in the midtroposphere instead of the upper troposphere, and the magnitudes of mass fluxes are smaller. The parameterization including mesoscale components is consistent with satellite observations of the size distribution of convective systems, while the parameterization with convective cells only is not. The parameterization of convective vertical velocities is an important control on the intensity of the mesoscale stratiform circulations associated with deep convection. The mesoscale components are less intense than in TRMM observations if spatially and temporally invariant convective vertical velocities are used instead of parameterized, variable velocities.

33. Redelsperger, J L., P R A Brown, F Guichard, C Hoff, M Kawasima, S Lang, T Montmerle, K Nakamura, K Saito, Charles J Seman, W K Tao, and Leo J Donner, 2000: A GCSS model intercomparison for a tropical squall line observed during TOGA-COARE. 1. Cloud-resolving models. Quarterly Journal of the Royal Meteorological Society, 126(564), 823-863.
    [ Abstract PDF ]

    Results from eight cloud-resolving models are compared for the first time for the case of an oceanic tropical squall line observed during theTropical Ocean/Global Atmosphere Coupled Ocean-Atmosphere Response Experiment. There is broad agreement between all the models in describing the overall structure and propagation of the squall line and some quantitative agreement in the evolution of rainfall. There is also a more qualitative agreement between the models in describing the vertical structure of the apparent heat and moisture sources. The three-dimensional (3D) experiments with an active ice-phase and open lateral boundary conditions along the direction of the system propagation show good agreement for all parameters. The comparison of 3D simulated fields with those obtained from two different analyses of airborne Doppler radar data indicates that the 3D models are able to simulate the dynamical structure of the squall line, including the observed double-peaked updraughts. However, the second updraught peak at around 10 km in height is obtained only when the ice phase is represented. The 2D simulations with an ice-phase parameterization also exhibit this structure, although with a larger temporal variability. In the 3D simulations, the evolution of the mean wind profile is in the sense of decreasing the shear, but the 2D simulations are unable to reproduce this behavior.

34. 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.

35. 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.
36. Donner, Leo J., Charles J Seman, and Richard S Hemler, 1999: Ice microphysics and radiative transfer in deep convective systems In 10th Conference on Atmospheric Radiation, 28 June-2 July 1999, Madison, WI, American Meteorological Society, 611-614.
37. Donner, Leo J., Charles J Seman, and Richard S Hemler, 1999: Three-dimensional cloud-system modeling of GATE convection. Journal of the Atmospheric Sciences, 56(12), 1885-1912.
    [ Abstract PDF ]

    Deep convection and its associated mesoscale circulations are modeled using a three-dimensional elastic model with bulk microphysics and interactive radiation for a composite easterly wave from the Global Atmospheric Research Program Atlantic Tropical Experiment. The energy and moisture budgets, large-scale heat sources and moisture sinks, microphysics, and radiation are examined. The modeled cloud system undergoes a life cycle dominated by deep convection in its early stages, followed by an upper-tropospheric mesoscale circulation. The large-scale heat sources and moisture sinks associated with the convective system agree broadly with diagnoses from field observations. The modeled upper-tropospheric moisture exceeds observed values. Strong radiative cooling at the top of the mesoscale circulation can produce overturning there. Qualitative features of observed changes in large-scale convective available potential energy and convective inhibition are found in the model integrations, although quantitative magnitudes can differ, especially for convection inhibition. Radiation exerts a strong influence on the microphysical properties of the cloud system. The three-dimensional integrations exhibit considerably less sporadic temporal behavior than corresponding two-dimensional integrations. While the third dimension is less important over timescales longer than the duration of a phase of an easterly wave in the lower and middle troposphere, it enables stronger interactions between radiation and dynamics in the upper-tropospheric mesoscale circulation over a substantial fraction of the life cycle of the convective system.

38. Donner, Leo J., and Charles J Seman, 1999: The role of ice sedimentation in the microphysical and radiative budgets of COARE convective systems In Proceedings of a Conference on the TOGA Coupled Ocean-Atmosphere Response Experiment (COARE),, Boulder, CO, USA, 7-14 July 1998, COARE-98, WCRP-107, WMO/TD-No. 940, World Meteorological Organization, 227-232.
39. 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.

40. Haywood, J M., V Ramaswamy, and Leo J Donner, 1998: Reply. Geophysical Research Letters, 25(7), 1041.
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41. Donner, Leo J., Charles J Seman, Richard S Hemler, and John P Sheldon, 1997: Radiative transfer in a three-dimensional cloud-system-resolving model In IRS '96: Current Problems in Atmospheric Radiation, Proceedings of the International Radiation Symposium, Fairbanks, Alaska, 19-24 August 1996. Hampton,, Deepak Publishing, 109-112.
    [ Abstract ]

    A three-dimensional, non-hydrostatic cloud-system-resolving model is used to study radiative transfer in convective systems. The model domain covers approximately 50,000 km2. Prognostic equations determine the evolution of liquid and ice mixing ratios. The three-dimensional distribution of liquid and ice is used in shortwave and long-wave radiative-transfer calculations. A tropical convective system with a mesoscale anvil circulation is analyzed. The distribution of radiative forcing is examined, and its role in the evolution of the convective system is considered.

42. Donner, Leo J., Charles J Seman, and John P Sheldon, 1997: Cloud-radiative interactions in high-resolution cloud-resolving models In 9th Conference on Atmospheric Radiation, Boston, MA, American Meteorological Society, 47-48.
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43. Donner, Leo J., Charles J Seman, Brian J Soden, Richard S Hemler, J C Warren, J Ström, and K N Liou, 1997: Large-scale ice clouds in the GFDL SKYHI general circulation model. Journal of Geophysical Research, 102(D18), 21,745-21,768.
    [ Abstract ]

    Ice clouds associated with large-scale atmospheric processes are studied using the SKYHI general circulation model (GCM) and parameterizations for their microphysical and radiative properties. The ice source is deposition from vapor, and the ice sinks are gravitational settling and sublimation. Effective particle sizes for ice distributions are related empirically to temperature. Radiative properties are evaluated as functions of ice path and effective size using approximations to detailed radiative-transfer solutions (Mie theory and geometric ray tracing). The distributions of atmospheric ice and their impact on climate and climate sensitivity are evaluated by integrating the SKYHI GCM (developed at the Geophysical Fluid Dynamics Laboratory) for six model months. Most of the major climatological cirrus regions revealed by satellite observations appear in the GCM. The radiative forcing associated with ice clouds acts to warm the Earth-atmosphere system. Relative to a SKYHI integration without these clouds, zonally averaged temperatures are warmer in the upper tropical troposphere with ice clouds. The presence of ice produced small net changes in the sensitivity of SKYHI climate to radiative perturbations, but this represents an intricate balance among changes in clear-, cloud-, solar-, and longwave-sensitivity components. Deficiencies in the representation of ice clouds are identified as results of biases in the large-scale GCM fields which drive the parameterization and neglect of subgrid variations in these fields, as well as parameterization simplifications of complex microphysical and radiative processes.

44. 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.

45. Donner, Leo J., 1996: Conditional and convective instability In Encyclopedia of Climate and Weather, Vol. 1, New York, Oxford University Press, 186-191.
46. Donner, Leo J., Brian J Soden, and Charles J Seman, 1996: Use of ISCCP data to evaluate a GCM parameterization for ice clouds In International Workshop on Research Uses of ISCCP Datasets, World Climate Research Programme, WCRP-97, WMO/TD No. 790, World Meteorological Organization, 11.39.
47. Donner, Leo J., 1995: Validating cumulus parameterizations using cloud (system)-resolving models In 21st Conference on Hurricanes and Tropical Meteorology, Boston, MA, American Meteorological Society, 564-566.
48. Donner, Leo J., J C Warren, and J Ström, 1995: Implementing microphysics at physically appropriate scales in GCMs In Workshop on Cloud Microphysics Parameterizations in Global Atmospheric Circulation Models, WCRP-90, WMO/TD-No. 713, Geneva, Switzerland, World Meteorological Organization, 133-139.
49. Donner, Leo J., 1994: Radiative forcing by parameterized ice clouds in a general circulation model In The Eighth Conference on Atmospheric Radiation, Boston, MA, American Meteorological Society, 110-112.
    [ PDF ]
50. Kasahara, A, A P Mizzi, and Leo J Donner, 1994: Diabatic initialization for improvement in the tropical analysis of divergence and moisture using satellite radiometric imagery data. Tellus A, 46A(3), 242-264.
    [ Abstract PDF ]

    To improve the quality of horizontal divergence and moisture analyses in the tropics, a diabatic initialization scheme is developed to incorporate information on convective activity and the proxy data of precipitation obtained from satellite radiometric imagery data. The tropical precipitation rates are estimated by developing a relationship between the pentad precipitation data of the Global Precipitation Climatology Project with daily outgoing longwave radiation data. The tropical belt from 35°S to 25°N (for January 1988) is divided in to 3 parts: convective, convective fringe, and downward-motion (clear-air) areas. In the convective region, the algorithm adjusts the horizontal divergence and humidity fields such that a version of the Kuo cumulus parameterization will yield the precipitation rates closest to the proxy data. The temperature in the planetary boundary layer is also adjusted, if necessary, to ensure the initiation of cumulus convection. In the downward-motion region, the divergence field is adjusted to yield descending motion expected from the thermodynamic balance between radiative cooling and adiabatic warming. In the convective fringe region, where convective criteria are not met, the divergence field is adjusted only to satisfy the global conservation of divergence. The humidity field is left intact in both the downward-motion and convective fringe regions. This adjustment scheme will ameliorate problems associated with spinup of precipitation in a numerical prediction model with the same cumulus parameterization as used in the initialization. This initialization scheme may be used as a method of quality control for first-guess fields in four-dimensional data assimilation by means of satellite radiometric imagery data.

51. Soden, Brian J., and Leo J Donner, 1994: Evaluation of a GCM cirrus parameterization using satellite observations. Journal of Geophysical Research, 99(D7), 14,401-14,413.
    [ Abstract PDF ]

    This study applies a simple yet effective methodology to validate a general circulation model parameterization of cirrus ice water path. The methodology combines large-scale dynamic and thermodynamic fields from operational analyses with prescribed occurrence of cirrus clouds from satellite observations to simulate a global distribution of ice water path. The predicted cloud properties are then compared with the corresponding satellite measurements of visible optical depth and infrared cloud emissivity to evaluate the reliability of the parameterization. This methodology enables the validation to focus strictly on the water loading side of the parameterization by eliminating uncertainties involved in predicting the occurrence of cirrus internally within the parameterization. Overall, the parameterization performs remarkably well in capturing the observed spatial patterns of cirrus optical properties. Spatial correlations between the observed and the predicted optical depths are typically greater than 0.7 for the tropics and northern hemisphere midlatitudes. The good spatial agreement largely stems from the strong dependence of the ice water path upon the temperature of the environment in which the clouds form. Poorer correlation (r ~ 0.3) are noted over the southern hemisphere midlatitudes, suggesting that additional processes not accounted for by the parameterization may be important there. Quantitative evaluation of the parameterization is hindered by the present uncertainty in the size distribution of cirrus ice particles. Consequently, it is difficult to determine if discrepancies between the observed and the predicted optical properties are attributable to errors in the parameterized ice water path or to geographic variations in effective radii.

52. Donner, Leo J., 1993: A cumulus parameterization including mass fluxes, vertical momentum dynamics, and mesoscale effects. Journal of the Atmospheric Sciences, 50(6), 889-906.
    [ Abstract PDF ]

    A formulation for parameterizing cumulus convection, which treats cumulus vertical momentum dynamics and mass fluxes consistently, is presented. This approach predicts the penetrative extent of cumulus updrafts on the basis of their vertical momentum and provides a basis for treating cumulus microphysics using formulations that depend on vertical velocity. Treatments for cumulus microphysics are essential if the water budgets of convective systems are to be evaluated for treating mesoscale stratiform processes associated with convection, which are important for radiative interactions influencing climate. The water budget (both condensed and vapor) of the cumulus updrafts is used to drive a semi-empirical parameterization for the large-scale effects of the mesoscale circulations associated with deep convection The parameterization for mesoscale effects invokes mesoscale ascent to redistribute vertically water detrained at the tops of the cumulus updrafts. The local cooling associated with this mesoscale ascent is probably larger than radiative heating of the mesoscale anvil clouds, and the mesoscale ascent may be in part a response to such radiative heating. The parameterization was applied to two tropical thermodynamic profiles whose diagnosed forcing by convective systems differed significantly. A spectrum of cumulus updrafts was allowed. The deepest of the updrafts penetrated the upper troposphere, while the shallower updrafts penetrated into the region of the mesoscale anvil. The relative numbers of cumulus updrafts of characteristic vertical velocities comprising the parameterized ensemble corresponded well with available observations. However, the large-scale heating produced by the ensemble without mesoscale circulations was concentrated at lower heights than observed or was characterized by excessive peak magnitudes. Also, an unobserved large-scale source of water vapor was produced in the middle troposphere. When the parameterization for mesoscale effects was added, the large-scale thermal and moisture forcing predicted by the parameterization agreed well with observations for both cases. The significance of mesoscale processes, some of which may depend in part on radiative forcing, suggests that future cumulus parameterization development will need to treat some radiative processes. Further, the long time scale of the mesoscale processes relative to that of the cumulus cells indicates a possible requirement for carrying some characteristics of the convective system in time as cumulus parameterizations are incorporated in large-scale models whose resolutions remain too large to capture explicitly the mesoscale processes. A formulation for parameterizing cumulus convection, which treats cumulus vertical momentum dynamics and mass fluxes consistently, is presented. This approach predicts the penetrative extent of cumulus updrafts on the basis of their vertical momentum and provides a basis for treating cumulus microphysics using formulations that depend on vertical velocity. Treatments for cumulus microphysics are essential if the water budgets of convective systems are to be evaluated for treating mesoscale stratiform processes associated with convection, which are important for radiative interactions influencing climate. The water budget (both condensed and vapor) of the cumulus updrafts is used to drive a semi-empirical parameterization for the large-scale effects of the mesoscale circulations associated with deep convection. The parameterization for mesoscale effects invokes mesoscale ascent to redistribute vertically water detrained at the tops of the cumulus updrafts. The local cooling associated with this mesoscale ascent is probably larger than radiative heating of the mesoscale anvil clouds, and the mesoscale ascent may be in part a response to such radiative heating. The parameterization was applied to two tropical thermodynamic proiles whose diagnosed forcing by convective systems differed significantly. A spectrum of cumulus updrafts was allowed. The deepest of the updrafts penetrated the upper troposphere, while the shallower updrafts penetrated into the region of the mesoscale anvil. The relative numbers of cumulus updrafts of characteristic vertical velocities comprising the parameterized ensemble corresponded well with available observations. However, the large-scale heating produced by the ensemble without mesoscale circulations was concentrated at lower heights than observed or was characterized by excessive peak magnitudes. Also, an unobserved large-scale source of water vapor was produced in the middle troposphere. When the parameterization for mesoscale effects was added, the large-scale thermal and moisture forcing predicted by the parameterization agreed well with observations for both cases. The significance of mesoscale processes, some of which may depend in part on radiative forcing, suggests that future cumulus parameterization development will need to treat some radiative processes. Further, the long time scale of the mesoscale processes relative to that of the cumulus cells indicates a possible requirement for carrying some characteristics of the convective system in time as cumulus parameterizations are incorporated in large-scale models whose resolutions remain too large to capture explicitly the mesoscale processes.

53. Donner, Leo J., 1993: Radiative interactions with convective systems: implications for cumulus parameterization In IRS '92: Current Problems in Atmospheric Radiation, Hampton, VA, Deepak Publishing, 27-31.
    [ Abstract ]

    The requirements which must be satisfied by cumulus parameterizations, if they are to be useful for treating cloud-radiative interactions involving convective systems, are considered. The primary requirement for a cumulus parameterization to calculate the large-scale tendencies of temperature and humidity is the distribution of mass fluxes associated with the parameterized ensemble. To understand radiative transfer in a convective system, the areas and microphysical properties of the cumulus ensemble must be known. A basis for their evaluation is provided by simultaneously determining the distribution of both mass fluxes and vertical velocities in a cumulus ensemble. Treating radiative-convective interactions also requires that the mesoscale stratiform circulations associated with deep convection be represented. A parameterization which satisfies these criteria is discussed.

54. Donner, Leo J., and W G Large, 0000: Submitted 2008. Climate Modeling. Annual Review of Earth and Planetary Sciences, .
    [ Abstract PDF ]

    Climate models simulate the atmosphere, given atmospheric composition and energy from the sun, and include explicit modeling of, and exchanges with, the underlying oceans, sea ice, and land. The models are based on phys- ical principles governing momentum, thermodynamics, cloud microphysics, radiative transfer, and turbulence. Climate models are evolving into earth- system models which will also include chemical and biological processes and aord the prospect of links to studies of human dimensions of climate and climate change. Although the fundamental principles on which climate models are based are quite robust, computational limits preclude their numerical solution on scales which include many processes important for the determination of cli- mate. Despite this limitation, many aspects of past and present climate and recent climate change have been successfully simulated using climate models, and climate models are used extensively to predict future climate change due to human activity.

55. Donner, Leo J., and W G Large, 0000: Submitted 2008 - Climate Modeling. Annual Review of Environment and Resources, .
    [ Abstract PDF ]

    Climate models simulate the atmosphere, given atmospheric composition and energy from the sun, and include explicit modeling of, and exchanges with, the underlying oceans, sea ice, and land. The models are based on physical principles governing momentum, thermodynamics, cloud microphysics, radiative transfer, and turbulence. Climate models are evolving into earth-system models which will also include chemical and biological processes and afford the prospect of links to studies of human dimensions of climate and climate change. Although the fundamental principles on which climate models are based are quite robust, computational limits preclude their numerical solution on scales which include many processes important for the determination of cli- mate. Despite this limitation, many aspects of past and present climate and recent climate change have been successfully simulated using climate models, and climate models are used extensively to predict future climate change due to human activity.

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