Stier, Philip, Susan C van den Heever, Matthew W Christensen, Edward Gryspeerdt, Guy Dagan, Stephen M Saleeby, Massimo Bollasina, Leo J Donner, Kerry A Emanuel, Annica M L Ekman, Graham Feingold, Paul Field, Piers M Forster, Jim M Haywood, Ralph A Kahn, Ilan Koren, Christian Kummerow, Tristan L'Ecuyer, Ülrike Lohmann, Yi Ming, Gunnar Myhre, Johannes Quaas, Daniel Rosenfeld, Bjørn H Samset, Axel Seifert, Graeme L Stephens, and Wei-Kuo Tao, August 2024: Multifaceted aerosol effects on precipitation. Nature Geoscience, 17, DOI:10.1038/s41561-024-01482-6. Abstract
Aerosols have been proposed to influence precipitation rates and spatial patterns from scales of individual clouds to the globe. However, large uncertainty remains regarding the underlying mechanisms and importance of multiple effects across spatial and temporal scales. Here we review the evidence and scientific consensus behind these effects, categorized into radiative effects via modification of radiative fluxes and the energy balance, and microphysical effects via modification of cloud droplets and ice crystals. Broad consensus and strong theoretical evidence exist that aerosol radiative effects (aerosol–radiation interactions and aerosol–cloud interactions) act as drivers of precipitation changes because global mean precipitation is constrained by energetics and surface evaporation. Likewise, aerosol radiative effects cause well-documented shifts of large-scale precipitation patterns, such as the intertropical convergence zone. The extent of aerosol effects on precipitation at smaller scales is less clear. Although there is broad consensus and strong evidence that aerosol perturbations microphysically increase cloud droplet numbers and decrease droplet sizes, thereby slowing precipitation droplet formation, the overall aerosol effect on precipitation across scales remains highly uncertain. Global cloud-resolving models provide opportunities to investigate mechanisms that are currently not well represented in global climate models and to robustly connect local effects with larger scales. This will increase our confidence in predicted impacts of climate change.
Donner, Leo J., December 2023: Deep Convection and Convective Clouds In Fast Processes in Large-Scale Atmospheric Models: Progress, Challenges, and Opportunities (eds Yangang Liu and Pavlos Kollias), American Geophysical Union, DOI:10.1002/9781119529019.ch5121-140.
Global climate models (GCMs) struggle to simulate polar clouds, especially low-level clouds that contain supercooled liquid and closely interact with both the underlying surface and large-scale atmosphere. Here we focus on GFDL's latest coupled GCM–CM4–and find that polar low-level clouds are biased high compared to observations. The CM4 bias is largely due to moisture fluxes that occur within partially ice-covered grid cells, which enhance low cloud formation in non-summer seasons. In simulations where these fluxes are suppressed, it is found that open water with an areal fraction less than 5% dominates the formation of low-level clouds and contributes to more than 50% of the total low-level cloud response to open water within sea ice. These findings emphasize the importance of accurately modeling open water processes (e.g., sea ice lead-atmosphere interactions) in the polar regions in GCMs.
Iipponen, Juho, and Leo J Donner, January 2021: Simple analytic solutions for a convectively driven Walker circulation and their relevance to observations. Journal of the Atmospheric Sciences, 78(1), DOI:10.1175/JAS-D-20-0014.1299-311. Abstract
We present a linear equation for the Walker circulation streamfunction and find its analytic solutions given specified convective heating. In a linear Boussinesq fluid with Rayleigh damping and Newtonian cooling, the streamfunction obeys a Poisson’s equation, forced by gradients in the meridionally averaged diabatic heating and Coriolis force. For an idealized convective heating distribution, analytic solutions for the streamfunction can be found through an analogy with electrostatics. We use these solutions to study the response of the Walker circulation strength (mass transport) to changes in the vertical and zonal scales of convective heating. Robust responses are obtained that depend on how the total convective heating of the atmosphere responds to changing scale. If the total heating remains unchanged, increasing the zonal scale or the vertical scale always leads to a weaker circulation. Conversely, if the total heating grows in proportion to the spatial scale, the circulation becomes stronger with increasing scale. These conclusions are shown to be consistent with a three-dimensional numerical model. Moreover, they are useful in describing the observed seasonal and interannual (ENSO) variability of the Indo-Pacific Walker circulation. On both time scales, the overturning becomes weaker with increasing zonal scale of the convective region, reminiscent of our solutions where we do not vary the total convective heating. Reanalysis data also indicate that the zonal circulation is quite strongly damped, thus yielding a result that the circulation strength is directly proportional to the warm-pool spatial-mean precipitation.
Kuo, Yi-Hung, J David Neelin, C-C Chen, W-T Chen, Leo J Donner, Andrew Gettelman, Xianan Jiang, K-T Kuo, Eric Maloney, C R Mechoso, Yi Ming, K A Schiro, Charles J Seman, Chien-Ming Wu, and Ming Zhao, January 2020: Convective transition statistics over tropical oceans for climate model diagnostics: GCM evaluation. Journal of the Atmospheric Sciences, 77(1), DOI:10.1175/JAS-D-19-0132.1. Abstract
To assess deep-convective parameterizations in a variety of GCMs and examine the fast-timescale convective transition, a set of statistics characterizing the pickup of precipitation as a function of column water vapor (CWV), PDFs and joint-PDFs of CWV and precipitation, and the dependence of the moisture-precipitation relation on tropospheric temperature is evaluated using the hourly output of two versions of GFDL AM4, NCAR CAM5 and superparameterized CAM (SPCAM). The 6-hourly output from the MJOTF/GASS project is also analyzed. Contrasting statistics produced from individual models that primarily differ in representations of moist convection suggest that convective transition statistics can substantially distinguish differences in convective representation and its interaction with the large-scale flow, while models that differ only in spatial-temporal resolution, microphysics, or ocean-atmosphere coupling result in similar statistics. Most of the models simulate some version of the observed sharp increase in precipitation as CWV exceeds a critical value, as well as that convective onset occurs at higher CWV but at lower column RH as temperature increases. While some models quantitatively capture these observed features and associated probability distributions, considerable intermodel spread and departures from observations in various aspects of the precipitation-CWV relationship are noted. For instance, in many of the models, the transition from the low-CWV, non-precipitating regime to the moist regime for CWV around and above critical is less abrupt than in observations. Additionally, some models overproduce drizzle at low CWV, and some require CWV higher than observed for strong precipitation. For many of the models, it is particularly challenging to simulate the probability distributions of CWV at high temperature.
Anber, Usama, S E Giangrande, Leo J Donner, and M P Jensen, August 2019: Updraft Constraints on Entrainment: Insights from Amazonian Deep Convection. Journal of the Atmospheric Sciences, 76(8), DOI:10.1175/JAS-D-18-0234.1. Abstract
Mixing of environmental air into clouds, or entrainment, has been identified as a major contributor to erroneous climate predictions made by modern comprehensive climate and numerical weather prediction models. Despite receiving extensive attention, the ad-hoc treatment of this convective-scale process in global models remains poor. On the other hand, while limited area high-resolution non-hydrostatic models can directly resolve entrainment, their sensitivity to model resolution, especially with the lack of benchmark mass flux observations, limits their applicability. Here, the dataset from the Observations and Modeling of the Green Ocean Amazon (GoAmazon2014/5) campaign focusing on radar retrievals of convective updraft vertical velocities is used with the aid of cloud-resolving model simulations of four deep convective events over the Amazon to provide insights into entrainment.
Entrainment and detrainment are diagnosed from the model simulations by applying the mass continuity equation over cloud volumes, in which grid cells are identified by some thresholds of updraft vertical velocity and cloud condensates, and accounting for the sources and sinks of the air mass. Entrainment is then defined as the environmental air intruding into convective cores causing cloud volume to shrink, while detrainment is defined as cloudy grid-cells departing the convective core and causing cloud volume to expand.
It is found that the diagnosed entrainment from the simulated convective events is strongly correlated to the inverse of the updraft vertical velocities in convective cores, which enables a more robust estimation of the mixing time scale. This highlights the need for improved observational capabilities for sampling updraft velocities across diverse geographic and cloud conditions. Evaluation of a number of assumptions used to represent entrainment in parameterization schemes is also presented, as contrasted against the diagnosed one.
We describe GFDL's CM4.0 physical climate model, with emphasis on those aspects that may be of particular importance to users of this model and its simulations. The model is built with the AM4.0/LM4.0 atmosphere/land model and OM4.0 ocean model. Topics include the rationale for key choices made in the model formulation, the stability as well as drift of the pre‐industrial control simulation, and comparison of key aspects of the historical simulations with observations from recent decades. Notable achievements include the relatively small biases in seasonal spatial patterns of top‐of‐atmosphere fluxes, surface temperature, and precipitation; reduced double Intertropical Convergence Zone bias; dramatically improved representation of ocean boundary currents; a high quality simulation of climatological Arctic sea ice extent and its recent decline; and excellent simulation of the El Niño‐Southern Oscillation spectrum and structure. Areas of concern include inadequate deep convection in the Nordic Seas; an inaccurate Antarctic sea ice simulation; precipitation and wind composites still affected by the equatorial cold tongue bias; muted variability in the Atlantic Meridional Overturning Circulation; strong 100 year quasi‐periodicity in Southern Ocean ventilation; and a lack of historical warming before 1990 and too rapid warming thereafter due to high climate sensitivity and strong aerosol forcing, in contrast to the observational record. Overall, CM4.0 scores very well in its fidelity against observations compared to the Coupled Model Intercomparison Project Phase 5 generation in terms of both mean state and modes of variability and should prove a valuable new addition for analysis across a broad array of applications.
The clouds in southern hemisphere extratropical cyclones generated by the GFDL climate model are analyzed against MODIS, CloudSat and CALIPSO cloud and precipitation observations. Two model versions are used: one is a developmental version of AM4, a model GFDL will utilize for CMIP6, the other is the same model with a different parameterization of moist convection. Both model versions predict a realistic top-of-atmosphere cloud cover in the southern oceans, within 5% of the observations. However, an examination of cloud cover transects in extratropical cyclones reveals a tendency in the models to overestimate high-level clouds (by differing amounts) and underestimate cloud cover at low-levels (again by differing amounts), especially in the post-cold frontal (PCF) region, when compared to observations. Focusing on only the models, their differences in high and mid-level clouds are consistent with their differences in convective activity and relative humidity (RH), but the same is not true for the PCF region. In this region, RH is higher in the model with less cloud fraction. These seemingly contradictory cloud and RH differences can be explained by differences in the cloud parameterization tuning parameters that ensure radiative balance. In the PCF region, the model cloud differences are smaller than either of the model biases with respect to observations, suggesting other physics changes are needed to address the bias. The process-oriented analysis used to assess these model differences will soon be automated and shared.
In this two-part paper, a description is provided of a version of the AM4.0/LM4.0 atmosphere/land model that will serve as a base for a new set of climate and Earth system models (CM4 and ESM4) under development at NOAA's Geophysical Fluid Dynamics Laboratory (GFDL). This version, with roughly 100km horizontal resolution and 33 levels in the vertical, contains an aerosol model that generates aerosol fields from emissions and a “light” chemistry mechanism designed to support the aerosol model but with prescribed ozone. In Part I, the quality of the simulation in AMIP (Atmospheric Model Intercomparison Project) mode – with prescribed sea surface temperatures (SSTs) and sea ice distribution – is described and compared with previous GFDL models and with the CMIP5 archive of AMIP simulations. The model's Cess sensitivity (response in the top-of-atmosphere radiative flux to uniform warming of SSTs) and effective radiative forcing are also presented. In Part II, the model formulation is described more fully and key sensitivities to aspects of the model formulation are discussed, along with the approach to model tuning.
In Part II of this two-part paper, documentation is provided of key aspects of a version of the AM4.0/LM4.0 atmosphere/land model that will serve as a base for a new set of climate and Earth system models (CM4 and ESM4) under development at NOAA's Geophysical Fluid Dynamics Laboratory (GFDL). The quality of the simulation in AMIP (Atmospheric Model Intercomparison Project) mode has been provided in Part I. Part II provides documentation of key components and some sensitivities to choices of model formulation and values of parameters, highlighting the convection parameterization and orographic gravity wave drag. The approach taken to tune the model's clouds to observations is a particular focal point. Care is taken to describe the extent to which aerosol effective forcing and Cess sensitivity have been tuned through the model development process, both of which are relevant to the ability of the model to simulate the evolution of temperatures over the last century when coupled to an ocean model.
We define a set of 21 atmospheric states, or recurring weather patterns, for a region surrounding the Atmospheric Radiation Measurement Program's Southern Great Plains site using an iterative clustering technique. The states are defined using dynamic and thermodynamic variables from reanalysis, tested for statistical significance with cloud radar data from the Southern Great Plains site, and are determined every 6 h for 14 years, creating a time series of atmospheric state. The states represent the various stages of the progression of synoptic systems through the region (e.g., warm fronts, warm sectors, cold fronts, cold northerly advection, and high-pressure anticyclones) with a subset of states representing summertime conditions with varying degrees of convective activity. We use the states to classify output from the NOAA/Geophysical Fluid Dynamics Laboratory AM3 model to test the model's simulation of the frequency of occurrence of the states and of the cloud occurrence during each state. The model roughly simulates the frequency of occurrence of the states but exhibits systematic cloud occurrence biases. Comparison of observed and model-simulated International Satellite Cloud Climatology Project histograms of cloud top pressure and optical thickness shows that the model lacks high thin cloud under all conditions, but biases in thick cloud are state-dependent. Frontal conditions in the model do not produce enough thick cloud, while fair-weather conditions produce too much. We find that increasing the horizontal resolution of the model improves the representation of thick clouds under all conditions but has little effect on high thin clouds. However, increasing resolution also changes the distribution of states, causing an increase in total cloud occurrence bias.
Fan, Songmiao, D A Knopf, A Heymsfield, and Leo J Donner, November 2017: Modeling of aircraft measurements of ice crystal concentration in the Arctic and a parameterization for mixed-phase cloud. Journal of the Atmospheric Sciences, 74(11), DOI:10.1175/JAS-D-17-0037.1. Abstract
In this study two parameterizations of ice nucleation rate on dust particles are used in a parcel model to simulate aircraft measurements of ice crystal number concentration (Ni) in the Arctic. The parcel model has detailed microphysics for droplet and ice nucleation, growth and evaporation with prescribed vertical air velocities. Three dynamic regimes are considered including large scale ascent, cloud-top generating cells and their combination. With observed meteorological conditions and aerosol concentrations, the parcel model predicts the number concentrations of size-resolved ice crystals, which may be compared to aircraft measurements. Model results show rapid changes with height/time in relative humidity, Ni and thermodynamic phase partitioning, which is not resolved in current climate and weather forecasting models. Parameterizations for ice number and nucleation rate in mixed-phase stratus clouds are thus developed based on the parcel model results to represent the time-integrated effect of some microphysical processes in large-scale models.
Schmidt, Gavin A., David Bader, Leo J Donner, G S Elsaesser, and Jean-Christophe Golaz, et al., September 2017: Practice and philosophy of climate model tuning across six U.S. modeling centers. Geoscientific Model Development, 10(9), DOI:10.5194/gmd-10-3207-2017. Abstract
Model calibration (or "tuning") is a necessary part of developing and testing coupled ocean-atmosphere climate models regardless of their main scientific purpose. There is an increasing recognition that this process needs to become more transparent for both users of climate model output and other developers. Knowing how and why climate models are tuned and which targets are used is essential to avoiding possible misattributions of skillful predictions to data accommodation and vice versa. This paper describes the approach and practice of model tuning for the six major U.S. climate modeling centers. While details differ among groups in terms of scientific missions, tuning targets and tunable parameters, there is a core commonality of approaches. However, practices differ significantly on some key aspects, in particular, in the use of initialized forecast analyses as a tool, the explicit use of the historical transient record, and the use of the present day radiative imbalance vs. the implied balance in the pre-industrial as a target.
Both climate forcing and climate sensitivity persist as stubborn uncertainties limiting the extent to which climate models can provide actionable scientific scenarios for climate change. A key, explicit control on cloud–aerosol interactions, the largest uncertainty in climate forcing, is the vertical velocity of cloud-scale updrafts. Model-based studies of climate sensitivity indicate that convective entrainment, which is closely related to updraft speeds, is an important control on climate sensitivity. Updraft vertical velocities also drive many physical processes essential to numerical weather prediction.
Vertical velocities and their role in atmospheric physical processes have been given very limited attention in models for climate and numerical weather prediction. The relevant physical scales range down to tens of meters and are thus frequently sub-grid and require parameterization. Many state-of-science convection parameterizations provide mass fluxes without specifying vertical velocities, and parameterizations that do provide vertical velocities have been subject to limited evaluation against what have until recently been scant observations. Atmospheric observations imply that the distribution of vertical velocities depends on the areas over which the vertical velocities are averaged. Distributions of vertical velocities in climate models may capture this behavior, but it has not been accounted for when parameterizing cloud and precipitation processes in current models.
New observations of convective vertical velocities offer a potentially promising path toward developing process-level cloud models and parameterizations for climate and numerical weather prediction. Taking account of the scale dependence of resolved vertical velocities offers a path to matching cloud-scale physical processes and their driving dynamics more realistically, with a prospect of reduced uncertainty in both climate forcing and sensitivity.
Li, J, Jingqiu Mao, K-E Min, R A Washenfelder, Steven S Brown, J Kaiser, F N Keutsch, R Volkamer, G M Wolfe, T F Hanisco, I B Pollack, Marta Abalos, M Graus, J B Gilman, B M Lerner, C Warneke, J A de Gouw, Ann M Middlebrook, J Liao, A Welti, B H Henderson, V Faye McNeill, S R Hall, K Ullmann, Leo J Donner, Fabien Paulot, and Larry W Horowitz, August 2016: Observational constraints on glyoxal production from isoprene oxidation and its contribution to organic aerosol over the Southeast United States. Journal of Geophysical Research: Atmospheres, 121(16), DOI:10.1002/2016JD025331. Abstract
We use a 0-D photochemical box model and a 3-D global chemistry-climate model, combined with observations from the NOAA Southeast Nexus (SENEX) aircraft campaign, to understand the sources and sinks of glyoxal over the Southeast United States. Box model simulations suggest a large difference in glyoxal production among three isoprene oxidation mechanisms (AM3ST, AM3B, and MCM v3.3.1). These mechanisms are then implemented into a 3-D global chemistry-climate model. Comparison with field observations shows that the average vertical profile of glyoxal is best reproduced by AM3ST with an effective reactive uptake coefficient γglyx of 2 × 10-3, and AM3B without heterogeneous loss of glyoxal. The two mechanisms lead to 0-0.8 µg m-3 secondary organic aerosol (SOA) from glyoxal in the boundary layer of the Southeast U.S. in summer. We consider this to be the lower limit for the contribution of glyoxal to SOA, as other sources of glyoxal other than isoprene are not included in our model. In addition, we find that AM3B shows better agreement on both formaldehyde and the correlation between glyoxal and formaldehyde (RGF = [GLYX]/[HCHO]), resulting from the suppression of δ-isoprene peroxy radicals (δ-ISOPO2). We also find that MCM v3.3.1 may underestimate glyoxal production from isoprene oxidation, in part due to an underestimated yield from the reaction of IEPOX peroxy radicals (IEPOXOO) with HO2. Our work highlights that the gas-phase production of glyoxal represents a large uncertainty in quantifying its contribution to SOA.
We update and evaluate the treatment of nitrate aerosols in the Geophysical Fluid Dynamics Laboratory (GFDL) atmospheric model (AM3). Accounting for the radiative effects of nitrate aerosols generally improves the simulated aerosol optical depth, although nitrate concentrations at the surface are biased high. This bias can be reduced by increasing the deposition of nitrate to account for the near-surface volatilization of ammonium nitrate or by neglecting the heterogeneous production of nitric acid to account for the inhibition of N2O5 reactive uptake at high nitrate concentrations. Globally, uncertainties in these processes can impact the simulated nitrate optical depth by up to 25 %, much more than the impact of uncertainties in the seasonality of ammonia emissions (6 %) or in the uptake of nitric acid on dust (13 %). Our best estimate for present-day fine nitrate optical depth at 550 nm is 0.006 (0.005–0.008). We only find a modest increase of nitrate optical depth (< 30 %) in response to the projected changes in the emissions of SO2 (−40 %) and ammonia (+38 %) from 2010 to 2050. Nitrate burden is projected to increase in the tropics and in the free troposphere, but to decrease at the surface in the midlatitudes because of lower nitric acid concentrations. Our results suggest that better constraints on the heterogeneous chemistry of nitric acid on dust, on tropical ammonia emissions, and on the transport of ammonia to the free troposphere are needed to improve projections of aerosol optical depth.
Randall, David A., A Del Genio, Leo J Donner, William D Collins, and Stephen A Klein, July 2016: The Impact of ARM on Climate Modeling In The Atmospheric Radiation Measurement (ARM) Program: The First 20 Years, 57, DOI:10.1175/AMSMONOGRAPHS-D-15-0050.1.
Stanfield, R E., J H Jiang, Xiquan Dong, B Xi, H Su, and Leo J Donner, et al., September 2016: A quantitative assessment of precipitation associated with the ITCZ in the CMIP5 GCM simulations. Climate Dynamics, 47(5-6), DOI:10.1007/s00382-015-2937-y. Abstract
According to the Intergovernmental Panel on Climate Change 5th Assessment Report, the broad-scale features of precipitation as simulated by Phase 5 of the Coupled Model Intercomparison Project (CMIP5) are in modest agreement with observations, however, large systematic errors are found in the Tropics. In this study, a new algorithm has been developed to define the North Pacific Intertropical Convergence Zone (ITCZ) through several metrics, including: the centerline position of the ITCZ, the width of the ITCZ, and the magnitude of precipitation along the defined ITCZ. These metrics provide a quantitative analysis of precipitation associated with the ITCZ over the equatorial northern Pacific. Results from 29 CMIP5 Atmospheric Model Intercomparison Project (AMIP) Global Circulation Model (GCM) runs are compared with Global Precipitation Climatology Project (GPCP) and Tropical Rainfall Measuring Mission (TRMM) observations. Similarities and differences between the GCM simulations and observations are analyzed with the intent of quantifying magnitude-, location-, and width-based biases within the GCMs. Comparisons show that most of the GCMs tend to simulate a stronger, wider ITCZ shifted slightly northward compared to the ITCZ in GPCP and TRMM observations. Comparisons of CMIP and AMIP simulated precipitation using like-models were found to be nearly equally distributed, with roughly half of GCMs showing an increase (decrease) in precipitation when coupled (decoupled) from their respective ocean model. Further study is warranted to understand these differences.
Uncertainty in equilibrium climate sensitivity impedes accurate climate projections. While the inter-model spread is known to arise primarily from differences in cloud feedback, the exact processes responsible for the spread remain unclear. To help identify some key sources of uncertainty, we use a developmental version of the next generation Geophysical Fluid Dynamics Laboratory global climate model (GCM) to construct a tightly controlled set of GCMs where only the formulation of convective precipitation is changed. The different models provide simulation of present-day climatology of comparable quality compared to the CMIP5 model ensemble. We demonstrate that model estimates of climate sensitivity can be strongly affected by the manner through which cumulus cloud condensate is converted into precipitation in a model’s convection parameterization, processes that are only crudely accounted for in GCMs. In particular, two commonly used methods for converting cumulus condensate into precipitation can lead to drastically different climate sensitivity, as estimated here with an atmosphere/land model by increasing sea surface temperatures uniformly and examining the response in the top-of-atmosphere energy balance. The effect can be quantified through a bulk convective detrainment efficiency, which measures the ability of cumulus convection to generate condensate per unit precipitation. The model differences, dominated by shortwave feedbacks, come from broad regimes ranging from large-scale ascent to subsidence regions. Given current uncertainties in representing convective precipitation microphysics and our current inability to find a clear observational constraint that favors one version of our model over the others, the implications of this ability to engineer climate sensitivity needs to be considered when estimating the uncertainty in climate projections.
Ao, Chi O., J H Jiang, A J Mannucci, H Su, O Verkhoglyadova, C Zhai, Jason N S Cole, and Leo J Donner, et al., March 2015: Evaluation of CMIP5 upper troposphere and lower stratosphere geopotential height with GPS radio occultation observations. Journal of Geophysical Research: Atmospheres, 120(5), DOI:10.1002/2014JD022239. Abstract
We present a detailed comparison of geopotential height fields between the Coupled Model Inter-Comparison Project phase 5 (CMIP5) models and satellite observations from GPS radio occultation (RO). Our comparison focuses on the annual mean, seasonal cycle, and interannual variability of 200 hPa geopotential height in the years 2002–2008. Using a wide sample of atmosphere-only model runs (AMIP) from the CMIP5 archive, we find that most models agree well with the observations and weather reanalyses in the tropics in both the annual means and interannual variabilities. However, the agreement is poor over the extratropics with the largest model spreads in the high latitudes and the largest bias in the Southern mid to high latitudes that persist all seasons. The models also show excessive seasonal variability over the Northern mid-latitude land areas as well as the Southern Ocean but insufficient variability over the tropics and Antarctica. While the underlying causes for the model discrepancies require further analyses, this study demonstrates that global observations from GPS RO provide accurate benchmark-quality measurements in the upper troposphere and lower stratosphere through which biases in climate models as well as weather reanalyses can be identified.
CLUBB (Cloud Layers Unified by Binormals) is a higher-order closure (HOC) method with an assumed joint probability density function (PDF) for the subgrid variations in vertical velocity, temperature, and moisture. CLUBB has been implemented in the GFDL climate model AM3-CLUBB and successfully unifies the treatment of shallow convection, resolved clouds, and planetary boundary layer (PBL). In this study, we further explore the possibility for CLUBB to unify the deep convection in a new configuration referred as AM3-CLUBB+. AM3-CLUBB+ simulations with prescribed sea surface temperature are discussed. Cloud, radiation, and precipitation fields compare favorably with observations and reanalyses. AM3-CLUBB+ successfully captures the transition from stratocumulus to deep convection and the modulated response of liquid water path to aerosols. Simulations of tropical variability and the Madden-Julian oscillation (MJO) are also improved. Deficiencies include excessive tropical water vapor and insufficient ice clouds in the mid-latitudes.
Jiang, J H., H Su, C Zhai, T Janice Shen, Tongwen Wu, J Zhang, Jason N S Cole, K von Salzen, Leo J Donner, and Charles J Seman, et al., March 2015: Evaluating the diurnal cycle of upper tropospheric ice clouds in climate models using SMILES observations. Journal of the Atmospheric Sciences, 72(3), DOI:10.1175/JAS-D-14-0124.1. Abstract
Upper tropospheric ice cloud measurements from the Superconducting Sub-millimeter Limb Emission Sounder (SMILES) on the International Space Station (ISS) are used to study the diurnal cycle of upper tropospheric ice cloud in the tropics and mid-latitudes (40°S-40°N) and to quantitatively evaluate ice cloud diurnal variability simulated by 10 climate models. Over land, the SMILES-observed diurnal cycle has a maximum at ~18:00 Local Solar Time (LST), while the model-simulated diurnal cycles have phases differing from the observed by −4 to 12 hours. Over ocean, the observations show much smaller diurnal cycle amplitudes than over land with a peak at 12:00 LST, while the modeled diurnal cycle phases are widely distributed throughout the 24-hour period. Most models show smaller diurnal cycle amplitudes over ocean than over land, in agreement with the observations. However, there is a large spread of modeled diurnal cycle amplitudes ranging from 20% to more than 300% of the observed over both land and ocean. Empirical Orthogonal Function (EOF) analysis on the observed and model simulated variations of ice cloud finds that the 1st EOF modes over land from both observation and model simulations explain more than 70% of the ice cloud diurnal variations, and they have similar spatial and temporal patterns. Over ocean, the 1st EOF from observation explains 26.4% of the variance, while the 1st EOF from most models explains more than 70%. The modeled spatial and temporal patterns of the leading EOFs over ocean show large differences from observations, indicating that the physical mechanisms governing the diurnal cycle of oceanic ice clouds are more complicated and not well simulated by the current climate models.
A unified turbulence and cloud parameterization based on multi-variate probability density functions (PDFs) has been incorporated into the GFDL atmospheric general circulation model AM3. This PDF-based parameterization not only predicts sub-grid variations in vertical velocity, temperature, and total water, which bridge sub-grid scale processes (such as aerosol activation and cloud microphysics) and grid-scale dynamic and thermodynamic fields, but also unifies the treatment of planetary boundary layer (PBL), shallow convection, and cloud macrophysics. This parameterization is the “Cloud Layers Unified By Binormals” (CLUBB) parameterization. With the incorporation of CLUBB in AM3, coupled with a two-moment cloud microphysical scheme, AM3-CLUBB allows for a more physically-based and self-consistent treatment of aerosol activation, cloud micro- and macro-physics, PBL, and shallow convection.
The configuration and performance of AM3-CLUBB are described. Cloud and radiation fields, as well as most basic climate features, are modeled realistically. Relative to AM3, AM3-CLUBB improves the simulation of coastal stratocumulus, a longstanding deficiency in GFDL models, and their seasonal cycle, especially at higher horizontal resolution, but global skill scores deteriorate slightly. Through sensitivity experiments, we show that (1) the two-moment cloud microphysics helps relieve the deficiency of coastal stratocumulus; (2) using the CLUBB sub-grid cloud water variability in the cloud microphysics has a considerable positive impact on global cloudiness; and (3) the impact of adjusting CLUBB parameters is to improve the overall agreement between model and observations.
Holloway, C E., J C Petch, R J Beare, P Bechtold, G Craig, S H Derbyshire, and Leo J Donner, et al., October 2014: Understanding and representing atmospheric convection across scales: recommendations from the meeting held at Dartington Hall, Devon, UK, 28–30 January 2013. Atmospheric Science Letters, 15(4), DOI:10.1002/asl2.508. Abstract
As weather and climate models move towards higher resolution, there is growing excitement about potential future improvements in the understanding and prediction of atmospheric convection and its interaction with larger-scale phenomena. A meeting in January 2013 in Dartington, Devon was convened to address the best way to maximise these improvements, specifically in a UK context but with international relevance. Specific recommendations included increased convective-scale observations, high-resolution virtual laboratories, and a system of parameterization test beds with a range of complexities. The main recommendation was to facilitate the development of physically based convective parameterizations that are scale-aware, non-local, non-equilibrium, and stochastic.
We use observations and four GFDL AGCMs to analyze the relation between variations in spatial patterns and area-averaged quantities in the top-of-atmosphere radiative fluxes, cloud amount and precipitation related to El Niño over the period 1979-2008. El Niño is associated with an increase in tropical average sea surface temperature of order +0.1K (with a maxima of +0.5K), large local anomalies of +2K (maxima +6K), and tropical tropospheric warming of +0.5K (maxima +1K). We find that model-to-observation biases in the base state translate into corresponding biases in anomalies in response to El Niño. The pattern and amplitude of model biases in reected shortwave (SW) and outgoing longwave radiation (OLR) follows expectations based on their biases in cloud amount: models with a positive cloud amount bias, compared to observations, have too strong local responses to El Niño in cloud amount, SW, OLR and precipitation.
Tropical average OLR increases in response to El Niño in observations and models (correlation coefficients (r) with Niño 3.4 Index in range 0.4 to 0.6). Weaker correlations are found for SW (r: -0.6 to 0), cloud amount (r: -0.2 to +0.1) and precipitation (r: -0.2 to 0). Compositing El Niño events over the period 2001-2007 yields similar results. These results are consistent with El Niño periods being warmer due to a heat pulse from the ocean, and a weak response in clouds and their radiative effect. These weak responses occur despite a large rearrangement in the spatial structure of the tropical circulation, and despite substantial differences in the mean state of observations and models.
Rosenfeld, Daniel, S C Sherwood, R Wood, and Leo J Donner, January 2014: Climate Effects of Aerosol-Cloud Interactions. Science, 343(6169), DOI:10.1126/science.1247490. Abstract
Aerosols counteract part of the warming effects of greenhouse gases, mostly by increasing the amount of sunlight reflected back to space. However, the ways in which aerosols affect climate through their interaction with clouds are complex and incompletely captured by climate models. As a result, the radiative forcing (that is, the perturbation to Earth's energy budget) caused by human activities is highly uncertain, making it difficult to predict the extent of global warming (1, 2). Recent advances have led to a more detailed understanding of aerosol-cloud interactions and their effects on climate, but further progress is hampered by limited observational capabilities and coarse-resolution climate models.
Song, H, W Lin, Yanluan Lin, A Wolf, and Leo J Donner, et al., September 2014: Evaluation of Cloud Fraction Simulated by Seven SCMs against the ARM Observations at the SGP Site. Journal of Climate, 27(17), DOI:10.1175/JCLI-D-13-00555.1. Abstract
This study evaluates the performances of seven single-column models (SCMs) by comparing simulated cloud fraction with observations at the Atmospheric Radiation Measurement Program Southern Great Plains (SGP) site from January 1999 to December 2001. Compared with the three-year mean observational cloud fraction, the ECMWF SCM underestimates cloud fraction at all levels, and the GISS SCM underestimates cloud fraction at levels below 200 hPa. The two GFDL SCMs underestimate lower-to-middle level cloud fraction but overestimate upper-level cloud fraction. The three CAM SCMs overestimate upper-level cloud fraction, and produce lower-level cloud fraction similar to the observations but as a result of compensating overproduction of convective cloud fraction and underproduction of stratiform cloud fraction. Besides, the CAM3 and CAM5 SCMs both overestimate middle-level cloud fraction whereas the CAM4 SCM underestimates. The frequency and partitioning analyses show a large discrepancy among the seven SCMs: contributions of non-stratiform processes to cloud fraction production are mainly in upper-level cloudy events over the cloud cover range 10%-80% in SCMs with prognostic cloud fraction schemes, and in lower-level cloudy events over the cloud cover range 15%-50% in SCMs with diagnostic cloud fraction schemes. Further analysis reveals different relationships between cloud fraction and relative humidity (RH) in the models and observations. The underestimation of lower-level cloud fraction in most SCMs is mainly due to the larger threshold RH used in models. The overestimation of upper-level cloud fraction in the three CAM SCMs and two GFDL SCMs is due to the overestimation of RH, and larger mean cloud fraction of cloudy events plus more occurrences of RH around 40%-80%, respectively.
Benedict, J J., Adam H Sobel, D M W Frierson, and Leo J Donner, January 2013: Tropical Intraseasonal Variability in Version 3 of the GFDL Atmosphere Model. Journal of Climate, 26(2), DOI:10.1175/JCLI-D-12-00103.1. Abstract
Tropical intraseasonal variability is examined in version 3 of the Geophysical Fluid Dynamics Laboratory Atmosphere Model (AM3). Compared to its predecessor AM2, AM3 uses a new treatment of deep and shallow cumulus convection and mesoscale clouds. The AM3 cumulus parameterization is a mass flux-based scheme but also, unlike that in AM2, incorporates subgrid-scale vertical velocities; these play a key role in cumulus microphysical processes. The AM3 convection scheme allows multi-phase water substance produced in deep cumuli to be transported directly into mesoscale clouds, which strongly influence large-scale moisture and radiation fields.
We examine four AM3 simulations, using a control model and three versions with different modifications to the deep convection scheme. In the control AM3, using a convective closure based on CAPE relaxation, both the MJO and Kelvin waves are weak compared to those in observations. By modifying the convective closure and trigger assumptions to inhibit deep cumuli, AM3 produces reasonable intraseasonal variability but a degraded mean state. MJO-like disturbances in the modified AM3 propagate eastward at roughly the observed speed in the Indian Ocean but up to twice the observed speed in the West Pacific. Distinct differences in intraseasonal convective organization and propagation exist among the modified AM3 versions. Differences in vertical diabatic heating profiles associated with the MJO are also found. The two AM3 versions with the strongest intraseasonal signals have a more prominent �bottom-heavy� heating profile leading the disturbance center and �top-heavy� heating profile following the disturbance. The more realistic heating structures are associated with an improved depiction of moisture convergence and intraseasonal convective organization in AM3.
A set of GFDL AM2 sensitivity simulations by varying an entrainment threshold rate to control deep convection occurrence are used to investigate how cumulus parameterization impacts tropical cloud and precipitation characteristics. In the Tropics, model convective precipitation (CP) is frequent and light, while large-scale precipitation (LSP) is intermittent and strong. With deep convection inhibited, CP decreases significantly over land and LSP increases prominently over ocean. This results in an overall redistribution of precipitation from land to ocean. A composite analysis reveals that cloud fraction (low and middle) and cloud condensate associated with LSP is substantially larger than those associated with CP. With about the same total precipitation and precipitation frequency distribution over the Tropics, simulations having greater LSP fraction tend to have larger cloud condensate and low and middle cloud fraction.
Simulations having greater LSP fraction tend to be drier and colder in the upper-troposphere. The induced unstable stratification supports strong transient wind perturbations and LSP. Greater LSP also contributes to greater intraseasonal (20-100 day) precipitation variability. Model LSP has a close connection to the low level convergence via the resolved grid-scale dynamics and thus a close coupling with the surface heat flux. Such wind-evaporation feedback is essential to the development and maintenance of LSP and enhances model precipitation variability. LSP has stronger dependence and sensitivity on column moisture than CP. The moisture-convection feedback, critical to tropical intraseasonal variability, is enhanced in simulations with large LSP. Strong precipitation variability accompanied by the worse mean state implies that an optimal precipitation partitioning is critical to model tropical climate simulation.
Rosenfeld, Daniel, R Wood, Leo J Donner, and S C Sherwood, August 2013: Aerosol Cloud-Mediated Radiative Forcing: Highly Uncertain and Opposite Effects from Shallow and Deep Clouds In Climate Science for Serving Society: Research, Modeling and Prediction Priorities, DOI:10.1007/978-94-007-6692-1_5. Abstract
Aerosol cloud-mediated radiative forcing, commonly known as the aerosol indirect effect (AIE), dominates the uncertainty in our ability to quantify anthropogenic climate forcing and respectively the climate sensitivity. This uncertainty can be appreciated based on the state of our understanding as presented in this chapter.
Adding aerosols to low clouds generally causes negative radiative forcing by three main mechanisms: redistributing the same cloud water in larger number of smaller drops, adding more cloud water, and increasing the cloud cover. Aerosols affect these components sometimes in harmony but more often in opposite ways. These processes can be highly non-linear, especially in precipitating clouds in which added aerosol can inhibit rain. There is probably little overall sensitivity in most clouds but hyper sensitivity in some, where the processes become highly nonlinear with positive feedbacks, causing changes of cloud regimes in marine stratocumulus under anticyclones. This leads to a complicated and uneven AIE. Process models at high resolution (LES) have reached the stage that they can capture much of this complicated behavior of shallow clouds. The implementation of the processes of cloud aerosol interactions into GCMs is rudimentary due to severe computational limitations and the current state of cloud and aerosol parameterizations, but intense research efforts aimed at improving the realism of cloud-aerosol interaction in GCMs are underway.
Aerosols added to deep clouds generally produce an additional component of positive radiative forcing due to cloud top cooling, expanding, and detraining vapor to the upper troposphere and lower stratosphere. The level of scientific understanding of the AIE on deep clouds is even lower than for the shallow clouds, as mixed phase and ice processes play an important role. Respectively, the parameterization of these processes for GCMs is further away than for the low clouds.
Crucially, the AIE of both shallow and deep clouds must be considered for quantifying anthropogenic climate forcing and inferring climate sensitivity from observations.
While our objective is reducing the uncertainty, it appears that the recently acquired additional knowledge actually increased the uncertainty range of the AIE, as we learn of additional effects that should be quantified.
Song, H, Yanluan Lin, and Leo J Donner, et al., August 2013: Evaluation of Precipitation Simulated by Seven SCMs against the ARM Observation at the SGP Site. Journal of Climate, 26(15), DOI:10.1175/JCLI-D-12-00263.1. Abstract
This study evaluates the performances of seven single column models (SCMs) by comparing simulated surface precipitation with observations at the Atmospheric Radiation Measurement Program Southern Great Plains (SGP) site from January 1999 to December 2001. Results show that although most SCMs can reproduce the observed precipitation reasonably well, there are significant and interesting differences in their details. In cold season, the model-observation differences in the frequency and mean intensity of rain events tend to compensate each other for most SCMs. In warm season, most SCMs produce more rain events in daytime than in nighttime whereas the observation has more rain events in nighttime. The mean intensities of rain events in these SCMs are much stronger in daytime, but weaker in nighttime, than the observation. The higher frequency of rain events during warm season daytime in most SCMs is related to the fact that most SCMs produce a spurious precipitation peak around the regime of weak vertical motions but rich in moisture content. The models also show distinct biases between nighttime and daytime in simulating significant rain events. In nighttime, all the SCMs have lower frequency of moderate-to-strong rain events than the observation for both seasons. In daytime, most SCMs have higher frequency of moderate-to-strong rain events than the observation, especially in warm season. Further analysis reveals distinct meteorological backgrounds for large underestimation and overestimation events. The former occur in the strong ascending regimes with negative low-level horizontal heat and moisture advection whereas the latter occur in the weak or moderate ascending regimes with positive low-level horizontal heat and moisture advection.
Su, H, Leo J Donner, Larry W Horowitz, and Charles J Seman, et al., April 2013: Diagnosis of regime-dependent cloud simulation errors in CMIP5 models using “A-Train” satellite observations and reanalysis data. Journal of Geophysical Research: Atmospheres, 118(7), DOI:10.1029/2012JD018575. Abstract
The vertical distributions of cloud water content (CWC) and cloud fraction (CF) over
the tropical oceans, produced by 13 coupled atmosphere-ocean models submitted to the Phase 5
of Coupled Model Intercomparison Project (CMIP5), are evaluated against CloudSat/CALIPSO
observations as a function of large-scale parameters. Available CALIPSO simulator CF outputs
are also examined. A diagnostic framework is developed to decompose the cloud simulation
errors into large-scale errors, cloud parameterization errors and co-variation errors. We find that
the cloud parameterization errors contribute predominantly to the total errors for all models. The
errors associated with large-scale temperature and moisture structures are relatively greater than
those associated with large-scale mid-tropospheric vertical velocity and lower-level divergence.
All models capture the separation of deep and shallow clouds in distinct large-scale regimes;
however, the vertical structures of high/low clouds and their variations with large-scale
parameters differ significantly from the observations. The CWCs associated with deep
convective clouds simulated in most models do not reach as high in altitude as observed, and
their magnitudes are generally weaker than CloudSat total CWC, which includes the contribution
of precipitating condensates, but are close to CloudSat non-precipitating CWC. All models
reproduce maximum CF associated with convective detrainment, but CALIPSO simulator CFs
generally agree better with CloudSat/CALIPSO combined retrieval than the model CFs,
especially in the mid-troposphere. Model simulated low clouds tend to have little variation with
large-scale parameters except lower-troposphere stability, while the observed low cloud CWC,
CF and cloud top height vary consistently in all large-scale regimes.
Using NASA's A-Train satellite measurements, we evaluate the accuracy of cloud water content (CWC) and water vapor mixing ratio (H2O) outputs from 19 climate models submitted to the Phase 5 of Coupled Model Intercomparison Project (CMIP5), and assess improvements relative to their counterparts for the earlier CMIP3. We find more than half of the models show improvements from CMIP3 to CMIP5 in simulating column-integrated cloud amount, while changes in water vapor simulation are insignificant. For the 19 CMIP5 models, the model spreads and their differences from the observations are larger in the upper troposphere (UT) than in the lower or mid-troposphere (L/MT). The modeled mean CWCs over tropical oceans range from ~3% to ~15× observations in the UT and 40% to 2× observations in the L/MT. For modeled H2Os, the mean values over tropical oceans range from ~1% to 2× of the observations in the UT and within 10% of the observations in the L/MT. The spatial distributions of clouds at 215 hPa are relatively well-correlated with observations, noticeably better than those for the L/MT clouds. Although both water vapor and clouds are better simulated in the L/MT than in the UT, there is no apparent correlation between the model biases in clouds and water vapor. Numerical scores are used to compare different model performances in regards to spatial mean, variance and distribution of CWC and H2O over tropical oceans. Model performances at each pressure level are ranked according to the average of all the relevant scores for that level.
Li, J-L, D E Waliser, W-T Chen, B Guan, T Kubar, Graeme L Stephens, Hsi-Yen Ma, M Deng, Leo J Donner, Charles J Seman, and Larry W Horowitz, August 2012: An observationally based evaluation of cloud ice water in CMIP3 and CMIP5 GCMs and contemporary reanalyses using contemporary satellite data. Journal of Geophysical Research: Atmospheres, 117, D16105, DOI:10.1029/2012JD017640. Abstract
We perform an observationally based evaluation of the cloud ice water content
(CIWC) and path (CIWP) of present-day GCMs, notably 20th century CMIP5 simulations,
and compare these results to CMIP3 and two recent reanalyses. We use three different
CloudSat + CALIPSO ice water products and two methods to remove the contribution
from the convective core ice mass and/or precipitating cloud hydrometeors with variable
sizes and falling speeds so that a robust observational estimate can be obtained for
model evaluations. The results show that for annual mean CIWP, there are factors
of 2–10 in the differences between observations and models for a majority of the GCMs
and for a number of regions. However, there are a number of CMIP5 models, including
CNRM-CM5, MRI, CCSM4 and CanESM2, as well as the UCLA CGCM, that perform
well compared to our past evaluations. Systematic biases in CIWC vertical structure occur
below the mid-troposphere where the models overestimate CIWC, with this bias arising
mostly from the extratropics. The tropics are marked by model differences in the level of
maximum CIWC (250–550 hPa). Based on a number of metrics, the ensemble behavior
of CMIP5 has improved considerably relative to CMIP3, although neither the CMIP5
ensemble mean nor any individual model performs particularly well, and there are still a
number of models that exhibit very large biases despite the availability of relevant
observations. The implications of these results on model representations of the Earth
radiation balance are discussed, along with caveats and uncertainties associated with the
observational estimates, model and observation representations of the precipitating and
cloudy ice components, relevant physical processes and parameterizations.
Lin, Yanluan, Leo J Donner, Stephen A Klein, and Ming Zhao, et al., May 2012: TWP-ICE global atmospheric model intercomparison: convection responsiveness and resolution impact. Journal of Geophysical Research: Atmospheres, 117, D09111, DOI:10.1029/2011JD017018. Abstract
Results are presented from an intercomparison of atmospheric general circulation model
(AGCM) simulations of tropical convection during the Tropical Warm Pool-International Cloud
Experiment (TWP-ICE). The distinct cloud properties, precipitation, radiation, and vertical diabatic
heating profiles associated with three different monsoon regimes (wet, dry, and break) from available
observations are used to evaluate 9 AGCM forecasts initialized daily from realistic global analyses. All
models captured well the evolution of large-scale circulation and thermodynamic fields, but cloud
properties differed substantially among models. Compared with the relatively well simulated top-heavy
heating structures during the wet and break period, most models had difficulty in depicting the bottomheavy
heating profiles associated with cumulus congestus during the dry period. The best performing
models during this period were the ones whose convection scheme was most responsive to the free
tropospheric humidity.
Compared with the large impact of cloud and convective parameterizations on model cloud and
precipitation characteristics, resolution has relatively minor impact on simulated cloud properties.
However, one feature that was influenced by resolution in several models was the diurnal cycle of
precipitation. Peaking at a different time from convective precipitation, large-scale precipitation
generally increases in high resolution forecasts and modulates the total precipitation diurnal cycle.
Overall, the study emphasizes the need for convection parameterizations that are more responsive to
environmental conditions as well as the substantial diversity among large-scale cloud and precipitation
schemes in current AGCMs. This experiment has demonstrated itself to be a very useful testbed for
those developing cloud and convection schemes for AGCMs.
The Geophysical Fluid Dynamics Laboratory (GFDL) has developed a coupled general circulation model (CM3) for atmosphere, oceans, land, and sea ice. The goal of CM3 is to address emerging issues in climate change, including aerosol-cloud interactions, chemistry-climate interactions, and coupling between the troposphere and stratosphere. The model is also designed to serve as the physical-system component of earth-system models and models for decadal prediction in the near-term future, for example, through improved simulations in tropical land precipitation relative to earlier-generation GFDL models. This paper describes the dynamical core, physical parameterizations, and basic simulation characteristics of the atmospheric component (AM3) of this model.
Relative to GFDL AM2, AM3 includes new treatments of deep and shallow cumulus convection, cloud-droplet activation by aerosols, sub-grid variability of stratiform vertical velocities for droplet activation, and atmospheric chemistry driven by emissions with advective, convective, and turbulent transport. AM3 employs a cubed-sphere implementation of a finite-volume dynamical core and is coupled to LM3, a new land model with eco-system dynamics and hydrology.
Most basic circulation features in AM3 are simulated as realistically, or more so, than in AM2. In particular, dry biases have been reduced over South America. In coupled mode, the simulation of Arctic sea ice concentration has improved. AM3 aerosol optical depths, scattering properties, and surface clear-sky downward shortwave radiation are more realistic than in AM2. The simulation of marine stratocumulus decks and the intensity distributions of precipitation remain problematic, as in AM2.
The last two decades of the 20th century warm in CM3 by .32°C relative to 1881-1920. The Climate Research Unit (CRU) and Goddard Institute for Space Studies analyses of observations show warming of .56°C and .52°C, respectively, over this period. CM3 includes anthropogenic cooling by aerosol cloud interactions, and its warming by late 20th century is somewhat less realistic than in CM2.1, which warmed .66°C but did not include aerosol cloud interactions. The improved simulation of the direct aerosol effect (apparent in surface clear-sky downward radiation) in CM3 evidently acts in concert with its simulation of cloud-aerosol interactions to limit greenhouse gas warming in a way that is consistent with observed global temperature changes.
Donner, Leo J., W Schubert, and R C J Somerville, February 2011: The Development of Atmospheric General Circulation Models: Complexity, Synthesis and Computation, New York, NY: Cambridge University Press, 255pp.
The fourth version of the Community Climate System Model (CCSM4) was recently completed and released to the climate community. This paper describes developments to all CCSM components, and documents fully coupled preindustrial control runs compared to the previous version, CCSM3. Using the standard atmosphere and land resolution of 1° results in the sea surface temperature biases in the major upwelling regions being comparable to the 1.4°-resolution CCSM3. Two changes to the deep convection scheme in the atmosphere component result in CCSM4 producing El Niño–Southern Oscillation variability with a much more realistic frequency distribution than in CCSM3, although the amplitude is too large compared to observations. These changes also improve the Madden–Julian oscillation and the frequency distribution of tropical precipitation. A new overflow parameterization in the ocean component leads to an improved simulation of the Gulf Stream path and the North Atlantic Ocean meridional overturning circulation. Changes to the CCSM4 land component lead to a much improved annual cycle of water storage, especially in the tropics. The CCSM4 sea ice component uses much more realistic albedos than CCSM3, and for several reasons the Arctic sea ice concentration is improved in CCSM4. An ensemble of twentieth-century simulations produces a good match to the observed September Arctic sea ice extent from 1979 to 2005. The CCSM4 ensemble mean increase in globally averaged surface temperature between 1850 and 2005 is larger than the observed increase by about 0.4°C. This is consistent with the fact that CCSM4 does not include a representation of the indirect effects of aerosols, although other factors may come into play. The CCSM4 still has significant biases, such as the mean precipitation distribution in the tropical Pacific Ocean, too much low cloud in the Arctic, and the latitudinal distributions of shortwave and longwave cloud forcings.
The recently developed GFDL Atmospheric Model version 3 (AM3), an atmospheric general circulation model (GCM), incorporates a prognostic treatment of cloud drop number to simulate the aerosol indirect effect. Since cloud drop activation depends on cloud-scale vertical velocities, which are not reproduced in present-day GCMs, additional assumptions on the subgrid variability are required to implement a local activation parameterization into a GCM.
This paper describes the subgrid activation assumptions in AM3 and explores sensitivities by constructing alternate configurations. These alternate model configurations exhibit only small differences in their present-day climatology. However, the total anthropogenic radiative flux perturbation (RFP) between present-day and preindustrial conditions varies by ±50% from the reference, because of a large difference in the magnitude of the aerosol indirect effect. The spread in RFP does not originate directly from the subgrid assumptions but indirectly through the cloud retuning necessary to maintain a realistic radiation balance. In particular, the paper shows a linear correlation between the choice of autoconversion threshold radius and the RFP.
Climate sensitivity changes only minimally between the reference and alternate configurations. If implemented in a fully coupled model, these alternate configurations would therefore likely produce substantially different warming from preindustrial to present day.
This paper documents time mean simulation characteristics from the ocean and sea ice components in a new coupled climate model developed at NOAA's Geophysical Fluid Dynamics Laboratory (GFDL). The climate model, known as CM3, is formulated with effectively the same ocean and sea ice components as the earlier GFDL climate model, CM2.1, yet with extensive developments made to the atmosphere and land model components. Both CM2.1 and CM3 show stable mean climate indices, such as large scale circulation and sea surface temperatures (SSTs). There are notable improvements in the CM3 climate simulation relative to CM2.1, including a modified SST bias pattern and reduced biases in the Arctic sea ice cover. We anticipate SST differences between CM2.1 and CM3 in lower latitudes through analysis of the atmospheric fluxes at the ocean surface in corresponding Atmospheric Model Intercomparison Project (AMIP) simulations. In contrast, SST changes in the high latitudes are dominated by ocean and sea ice effects absent in AMIP simulations. The ocean interior simulation in CM3 is generally warmer than CM2.1, which adversely impacts the interior biases.
Successful simulation of aerosol indirect effects in climate models requires parameterizations that capture the full range of cloud-aerosol interactions, including positive and negative liquid water path (LWP) responses to increasing aerosol concentrations, as suggested by large eddy simulations (LESs). A parameterization based on multi-variate probability density functions with dynamics (MVD PDFs) has been incorporated into the single-column version of GFDL AM3, extended to treat aerosol activation, and coupled with a two-moment microphysics scheme. We use it to explore cloud-aerosol interactions. In agreement with LESs, our single-column simulations produce both positive and negative LWP responses to increasing aerosol concentrations, depending on precipitation and free atmosphere relative humidity. We have conducted sensitivity tests to vertical resolution and droplet sedimentation parameterization. The dependence of sedimentation on cloud droplet size is essential to capture the full LWP responses to aerosols. Further analyses reveal that the MVD PDFs are able to represent changes in buoyancy profiles induced by sedimentation as well as enhanced entrainment efficiency with aerosols comparable to LESs.
Lee, S S., and Leo J Donner, August 2011: Effects of Cloud Parameterization on Radiation and Precipitation: A comparison between single-moment microphysics and double-moment microphysics. Terrestrial Atmospheric and Oceanic Sciences, 22(4), DOI:10.3319/TAO.2011.03.03.01(A). Abstract
This study compares a single-moment microphysics scheme to a double-moment microphysics scheme using four observed cases of a mesoscale cloud system. Previous studies comparing a single-moment microphysics scheme to a double-moment microphysics scheme have focused on microphysical processes or overall dynamics, precipitation and morphology of cloud systems. However, they have not focused on how the different representation of microphysical processes between a single- and double-moment microphysics scheme affects precipitation. This study shifts its focus from that of previous studies to the effect of the different representation of microphysics on precipitation. In addition, this study examines the effect of the different representation of microphysical processes on different radiation budgets between single- and double-moment microphysics schemes.
The temporal evolution of precipitation simulated by a single-moment microphysics scheme is significantly different from that by a double-moment microphysics scheme in this study. This is mostly due to different physical representations of key processes (i.e., autoconversion, saturation, and nucleation). Also, a simulation by a single-moment microphysics scheme results in different radiation budgets compared to a double-moment microphysics scheme. More reflection of incident solar radiation in a simulation with a double-moment microphysics scheme than that with a single-moment microphysics scheme is simulated.
Lin, Yanluan, Leo J Donner, and B A Colle, March 2011: Parameterization of riming intensity and its impact on ice fall speed using ARM data. Monthly Weather Review, 139(3), DOI:10.1175/2010MWR3299.1. Abstract
Riming within mixed-phase clouds can have a large impact on the prediction of clouds and precipitation within weather and climate models. The increase of ice particle fall speed due to riming has not been considered in most General Circulation Models (GCMs), and many weather models only consider ice particles that are either unrimed or heavily rimed (not a continuum of riming amount). Using the Atmospheric Radiation Measurement (ARM) program dataset at Southern Great Plains (SGP) of the United States, a new parameterization for riming is derived, which includes a diagnosed rimed mass fraction and its impact on the ice particle fall speed. When evaluated against a vertical-pointing Doppler radar for stratiform mixed-phase clouds, the new parameterization produces better ice fall speeds than a conventional parameterization.
The new parameterization is tested in the recently developed Geophysical Fluid Dynamics Laboratory (GFDL) atmospheric model (AM3) using prescribed sea surface temperature (SST) simulations. Compared with the standard (CTL) simulation, the new parameterization increases ice amount aloft by ~20–30% globally, which reduces the global mean outgoing longwave radiation (OLR) by ~2.8 Wm−2 and the top of atmosphere (TOA) short-wave absorption by ~1.5 Wm−2. Global mean precipitation is also slightly reduced, especially over the Tropics. Overall, the new parameterization produces a comparable climatology with the CTL simulation and it improves the physical basis for using a fall velocity larger than a conventional parameterization in the current AM3.
Guo, Huan, Jean-Christophe Golaz, Leo J Donner, D P Schanen, and B M Griffin, October 2010: Multi-variate probability density functions with dynamics for cloud droplet activation in large-scale models: Single column tests. Geoscientific Model Development, 3(2), DOI:10.5194/gmd-3-475-2010. Abstract
Successful simulation of cloud-aerosol interactions (indirect aerosol effects) in climate models requires relating grid-scale aerosol, dynamic, and thermodynamic fields to small-scale processes like aerosol activation. A turbulence and cloud parameterization, based on multi-variate probability density functions of sub-grid vertical velocity, temperature, and moisture, has been extended to treat aerosol activation. Multi-variate probability density functions with dynamics (MVD PDFs) offer a solution to the problem of the gap between the resolution of climate models and the scales relevant for aerosol activation and a means to overcome the limitations of diagnostic estimates of cloud droplet number concentration based only on aerosol concentration.
Incorporated into the single-column version of GFDL AM3, the MVD PDFs successfully simulate cloud properties including precipitation for cumulus, stratocumulus, and cumulus-under-stratocumulus. The extension to treat aerosol activation predicts droplet number concentrations in good agreement with large eddy simulations (LES). The droplet number concentrations from the MVD PDFs match LES results more closely than diagnostic relationships between aerosol concentration and droplet concentration.
In the single-column model simulations, as aerosol concentration increases, droplet concentration increases, precipitation decreases, but liquid water path can increase or decrease.
Lee, S S., Leo J Donner, and Joyce Penner, July 2010: Thunderstorm and stratocumulus: how does their contrasting morphology affect their interactions with aerosols?Atmospheric Chemistry and Physics, 10(14), DOI:10.5194/acp-10-6819-2010. Abstract
It is well-known that aerosols affect clouds and that the effect of aerosols on clouds is critical for understanding human-induced climate change. Most climate model studies have focused on the effect of aerosols on warm stratiform clouds (e.g., stratocumulus clouds) for the prediction of climate change. However, systems like the Asian and Indian Monsoon, storm tracks, and the intertropical convergence zone, play important roles in the global hydrological cycle and in the circulation of energy and are driven by thunderstorm-type convective clouds. Here, we show that the different morphologies of these two cloud types lead to different aerosol-cloud interactions. Increasing aerosols are known to suppress the conversion of droplets to rain (i.e., so-called autoconversion). This increases droplets as a source of evaporative cooling, leading to an increased intensity of downdrafts. The acceleration of the intensity of downdrafts is larger in convective clouds due to their larger cloud depths (providing longer paths for downdrafts to follow to the surface) than in stratiform clouds. More accelerated downdrafts intensify the gust front, leading to significantly increased updrafts, condensation and thus the collection of cloud liquid by precipitation, which offsets the suppressed autoconversion. This leads to an enhancement of precipitation with increased aerosols in convective clouds. However, the downdrafts are less accelerated in stratiform clouds due to their smaller cloud depths, and they are not able to induce changes in updrafts as large as those in convective clouds. Thus, the offset is not as effective, and this allows the suppression of precipitation with increased aerosols. Thus aerosols affect these cloud systems differently. The dependence of the effect of aerosols on clouds on the morphology of clouds should be taken into account for a more complete assessment of climate change.
A new stratiform cloud scheme including a two-moment bulk microphysics module, a cloud cover parameterization allowing ice supersaturation, and an ice nucleation parameterization has been implemented into the recently developed GFDL AM3 general circulation model (GCM) as part of an effort to treat aerosol-cloud-radiation interactions more realistically. Unlike the original scheme, the new scheme facilitates the study of cloud-ice-aerosol interactions via influences of dust and sulfate on ice nucleation. While liquid and cloud ice water path associated with stratiform clouds are similar for the new and the original scheme, column integrated droplet numbers and global frequency distributions (PDFs) of droplet effective radii differ significantly. This difference is in part due to a difference in the implementation of the Wegener-Bergeron-Findeisen (WBF) mechanism, which leads to a larger contribution from super-cooled droplets in the original scheme. Clouds are more likely to be either completely glaciated or liquid due to the WBF mechanism in the new scheme. Super-saturations over ice simulated with the new scheme are in qualitative agreement with observations, and PDFs of ice numbers and effective radii appear reasonable in the light of observations. Especially, the temperature dependence of ice numbers qualitatively agrees with in-situ observations. The global average long-wave cloud forcing decreases in comparison to the original scheme as expected when super-saturation over ice is allowed. Anthropogenic aerosols lead to a larger decrease in short-wave absorption (SWABS) in the new model setup, but outgoing long-wave radiation (OLR) decreases as well, so that the net effect of including anthropogenic aerosols on the net radiation at the top of the atmosphere (netradTOA = SWABS-OLR) is of similar magnitude for the new and the original scheme.
Haywood, Jim M., Leo J Donner, A Jones, and Jean-Christophe 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.
Lee, S S., Leo J Donner, and Vaughan 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(7), DOI:10.5194/acp-9-2555-2009. Abstract
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.
Lee, S S., Leo J Donner, and Vaughan T J Phillips, October 2009: Impacts of aerosol chemical composition on microphysics and precipitation in deep convection. Atmospheric Research, 94(2), DOI:10.1016/j.atmosres.2009.05.015. Abstract
Aerosols affect precipitation by modifying cloud properties such as cloud droplet number concentration (CDNC). Aerosol effects on CDNC depend on aerosol properties such as number concentration, size spectrum, and chemical composition. This study focuses on the effects of aerosol chemical composition on CDNC and, thereby, precipitation in a mesoscale cloud ensemble (MCE) driven by deep convective clouds. The MCE was observed during the 1997 department of energy's Atmospheric Radiation Measurement (ARM) summer experiment. Double-moment microphysics with explicit nucleation parameterization, able to take into account those three properties of aerosols, is used to investigate the effects of aerosol chemical composition on CDNC and precipitation. The effects of aerosol chemical compositions are investigated for both soluble and insoluble substances in aerosol particles. The effects of soluble substances are examined by varying mass fractions of two representative soluble components of aerosols in the continental air mass: sulfate and organics. The increase in organics with decreasing sulfate lowers critical supersaturation (Sc) and leads to higher CDNC. Higher CDNC results in smaller autoconversion of cloud liquid to rain. This provides more abundant cloud liquid as a source of evaporative cooling, leading to more intense downdrafts, low-level convergence, and updrafts. The resultant stronger updrafts produce more condensation and thus precipitation, as compared to the case of 100% sulfate aerosols. The conventional assumption of sulfate aerosol as a surrogate for the whole aerosol mass can be inapplicable for the case with the strong sources of organics. The less precipitation is simulated when an insoluble substance replaces organics as compared to when it replaces sulfate. When the effects of organics on the surface tension of droplet and solution term in the Köhler curve are deactivated by the insoluble substance, Sc is raised more than when the effects of sulfate on the solution term are deactivated by the insoluble substance. This leads to lower CDNC and, thus, larger autoconversion of cloud liquid to rain, providing less abundant cloud liquid as a source of evaporative cooling. The resultant less evaporative cooling produces less intense downdrafts, weaker low-level convergence, updrafts, condensation and, thereby, less precipitation in the case where organics is replaced by the insoluble substance than in the case where sulfate is replaced by the insoluble substance. The variation of precipitation caused by the change in the mass fraction between the soluble and insoluble substances is larger than that caused by the change in the mass fraction between the soluble substances.
Ntelekos, A A., and Leo J Donner, et al., August 2009: The effects of aerosols on intense convective precipitation in the northeastern United States. Quarterly Journal of the Royal Meteorological Society, 135(643), DOI:10.1002/qj.476. Abstract
A fully coupled meteorology-chemistry-aerosol mesoscale model (WRF-Chem) is used to assess the effects of aerosols on intense convective precipitation over the northeastern United States. Numerical experiments are performed for three intense convective storm days and for two scenarios representing typical and low aerosol conditions. The results of the simulations suggest that increasing concentrations of aerosols can lead to either enhancement or suppression of precipitation. Quantification of the aerosol effect is sensitive to the metric used due to a shift of rainfall accumulation distribution when realistic aerosol concentrations are included in the simulations. Maximum rainfall accumulation amounts and areas with rainfall accumulations exceeding specified thresholds provide robust metrics of the aerosol effect on convective precipitation. Storms developing over areas with medium to low aerosol concentrations showed a suppression effect on rainfall independent of the meteorological environment. Storms developing in areas of relatively high particulate concentrations showed enhancement of rainfall when there were simultaneous high values of convective available potential energy, relative humidity and wind shear. In these cases, elevated aerosol concentrations resulted in stronger updraughts and downdraughts and more coherent organization of convection. For the extreme case, maximum rainfall accumulation differences exceeded 40 mm. The modelling results suggest that areas of the northeastern US urban corridor that are close to or downwind of intense sources of aerosols, could be more favourable for rainfall enhancement due to aerosols for the aerosol concentrations typical of this area.
Quaas, Johannes, Yi Ming, Leo J Donner, and Paul Ginoux, et al., November 2009: Aerosol indirect effects – general circulation model intercomparison and evaluation with satellite data. Atmospheric Chemistry and Physics, 9(22), DOI:10.5194/acp-9-8697-2009. Abstract
Aerosol indirect effects continue to constitute one of the most important uncertainties for anthropogenic climate perturbations. Within the international AEROCOM initiative, the representation of aerosol-cloud-radiation interactions in ten different general circulation models (GCMs) is evaluated using three satellite datasets. The focus is on stratiform liquid water clouds since most GCMs do not include ice nucleation effects, and none of the model explicitly parameterises aerosol effects on convective clouds. We compute statistical relationships between aerosol optical depth (τa) and various cloud and radiation quantities in a manner that is consistent between the models and the satellite data. It is found that the model-simulated influence of aerosols on cloud droplet number concentration (Nd) compares relatively well to the satellite data at least over the ocean. The relationship between τa and liquid water path is simulated much too strongly by the models. This suggests that the implementation of the second aerosol indirect effect mainly in terms of an autoconversion parameterisation has to be revisited in the GCMs. A positive relationship between total cloud fraction (fcld) and τa as found in the satellite data is simulated by the majority of the models, albeit less strongly than that in the satellite data in most of them. In a discussion of the hypotheses proposed in the literature to explain the satellite-derived strong fcld–τa relationship, our results indicate that none can be identified as a unique explanation. Relationships similar to the ones found in satellite data between τa and cloud top temperature or outgoing long-wave radiation (OLR) are simulated by only a few GCMs. The GCMs that simulate a negative OLR–τa relationship show a strong positive correlation between τa and fcld. The short-wave total aerosol radiative forcing as simulated by the GCMs is strongly influenced by the simulated anthropogenic fraction of τa, and parameterisation assumptions such as a lower bound on Nd. Nevertheless, the strengths of the statistical relationships are good predictors for the aerosol forcings in the models. An estimate of the total short-wave aerosol forcing inferred from the combination of these predictors for the modelled forcings with the satellite-derived statistical relationships yields a global annual mean value of −1.5±0.5 Wm−2. In an alternative approach, the radiative flux perturbation due to anthropogenic aerosols can be broken down into a component over the cloud-free portion of the globe (approximately the aerosol direct effect) and a component over the cloudy portion of the globe (approximately the aerosol indirect effect). An estimate obtained by scaling these simulated clear- and cloudy-sky forcings with estimates of anthropogenic τa and satellite-retrieved Nd–τa regression slopes, respectively, yields a global, annual-mean aerosol direct effect estimate of −0.4±0.2 Wm−2 and a cloudy-sky (aerosol indirect effect) estimate of −0.7±0.5 Wm−2, with a total estimate of −1.2±0.4 Wm−2.
The Madden–Julian oscillation (MJO) interacts with and influences a wide range of weather and climate
phenomena (e.g., monsoons, ENSO, tropical storms, midlatitude weather), and represents an important, and
as yet unexploited, source of predictability at the subseasonal time scale. Despite the important role of the
MJO in our climate and weather systems, current global circulation models (GCMs) exhibit considerable
shortcomings in representing this phenomenon. These shortcomings have been documented in a number of
multimodel comparison studies over the last decade. However, diagnosis of model performance has been
challenging, and model progress has been difficult to track, because of the lack of a coherent and standardized
set of MJO diagnostics. One of the chief objectives of the U.S. Climate Variability and Predictability
(CLIVAR) MJO Working Group is the development of observation-based diagnostics for objectively
evaluating global model simulations of the MJO in a consistent framework. Motivation for this activity is
reviewed, and the intent and justification for a set of diagnostics is provided, along with specification for their
calculation, and illustrations of their application. The diagnostics range from relatively simple analyses of
variance and correlation, to more sophisticated space–time spectral and empirical orthogonal function
analyses. These diagnostic techniques are used to detect MJO signals, to construct composite life cycles, to
identify associations of MJO activity with the mean state, and to describe interannual variability of the MJO.
Donner, Leo J., and William 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
Lee, S S., Leo J Donner, Vaughan 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.
Lee, S S., Leo J Donner, Vaughan 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.
Salzmann, M, M G Lawrence, Vaughan 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(10), DOI:10.5194/acp-8-2741-2008. Abstract
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 NOx, O3, and
HOx(=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 NOx 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 NOx
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.
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.
Ming, Yi, V Ramaswamy, Leo J Donner, Vaughan T J Phillips, Stephen A Klein, Paul Ginoux, and Larry W 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 (Nd) 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 Nd 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 Nd with a characteristic maximum in the lower troposphere; this feature differs from the profile that would be obtained if Ndis diagnosed from the sulfate mass concentration based on an often-used empirical relationship. Prognosticated Nd 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.
Phillips, Vaughan 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.
Salzmann, M, M G Lawrence, Vaughan 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.
Wilcox, E M., and Leo J Donner, January 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.
Folkins, I, P Bernath, C Boone, Leo J Donner, A Eldering, G Lesins, Randall 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.
Lin, J-L, G N Kiladis, B E Mapes, K M Weickmann, Kenneth R Sperber, W Lin, Matthew 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.
Ming, Yi, V Ramaswamy, Leo J Donner, and Vaughan 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/JAS3686.1. 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).
Phillips, Vaughan T., and Leo J Donner, October 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.05.171. 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
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. PDF
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, Wei-Kuo Tao, R Atlas, Y Gu, K N Liou, A Del Genio, Marat Khairoutdinov, and Andrew 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.
Phillips, Vaughan 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. PDF
Phillips, Vaughan 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. PDF
Salzmann, M, M G Lawrence, Vaughan T J Phillips, and Leo J Donner, 2005: Mass flux diagnostics in CRM studies. Geophysical Research Abstracts, 7, 06799. PDF
Salzmann, M, M G Lawrence, Vaughan 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. PDF
for climate research developed at the Geophysical Fluid Dynamics Laboratory (GFDL) are presented. The atmosphere model, known as AM2, includes a new gridpoint dynamical core, a prognostic cloud scheme, and a multispecies aerosol climatology, as well as components from previous models used at GFDL. The land model, known as LM2, includes soil sensible and latent heat storage, groundwater storage, and stomatal resistance. The performance of the coupled model AM2–LM2 is evaluated with a series of prescribed sea surface temperature (SST) simulations. Particular focus is given to the model's climatology and the characteristics of interannual variability related to E1 Niño– Southern Oscillation (ENSO).
One AM2–LM2 integration was performed according to the prescriptions of the second Atmospheric Model Intercomparison Project (AMIP II) and data were submitted to the Program for Climate Model Diagnosis and Intercomparison (PCMDI). Particular strengths of AM2–LM2, as judged by comparison to other models participating in AMIP II, include its circulation and distributions of precipitation. Prominent problems of AM2– LM2 include a cold bias to surface and tropospheric temperatures, weak tropical cyclone activity, and weak tropical intraseasonal activity associated with the Madden–Julian oscillation.
An ensemble of 10 AM2–LM2 integrations with observed SSTs for the second half of the twentieth century permits a statistically reliable assessment of the model's response to ENSO. In general, AM2–LM2 produces a realistic simulation of the anomalies in tropical precipitation and extratropical circulation that are associated with ENSO.
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. PDF
Salzmann, M, M G Lawrence, Vaughan T J Phillips, and Leo J Donner, September 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.
Donner, Leo J., 2003: Multiple scales in cumulus convection and their implications for cumulus parameterization in large-scale models. Geophysical Research Abstracts, 5, 02917. PDF
Donner, Leo J., 2003: Tracer transport by parameterized convection. Geophysical Research Abstracts, 5, 02733. PDF
Donner, Leo J., and Vaughan T J Phillips, November 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
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.
Phillips, Vaughan T., and Leo J Donner, 2003: The validity of a new closure assumption for the parameterisation of deep convection. Geophysical Research Abstracts, 5, 07384. 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.
Xu, K-M, R T Cederwall, Leo J Donner, W W Grabowski, F Guichard, D E Johnson, Marat Khairoutdinov, S K Krueger, J C Petch, David A Randall, Charles J Seman, Wei-Kuo Tao, D-P Wang, Shang-Ping 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.
Donner, Leo J., Charles J Seman, Richard S Hemler, and Songmiao 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.
Redelsperger, J L., P R A Brown, F Guichard, C Hoff, M Kawasima, S Lang, T Montmerle, K Nakamura, K Saito, Charles J Seman, Wei-Kuo 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.
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.
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.
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.
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.
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.
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.
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.
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. PDF
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.
Haywood, Jim 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.
Donner, Leo J., 1996: Conditional and convective instability In Encyclopedia of Climate and Weather, Vol. 1, New York, Oxford University Press, 186-191.
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.
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.
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.
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
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.
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.
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.
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.
