Space Studies General Circulation Model Research Articles

GCAP 2.0: a global 3-D chemical-transport model framework for past, present, and future climate scenarios

Abstract. This paper describes version 2.0 of the Global Change and Air Pollution (GCAP 2.0) model framework, a one-way offline coupling between version E2.1 of the NASA Goddard Institute for Space Studies (GISS) general circulation model (GCM) and the GEOS-Chem global 3-D chemical-transport model (CTM). Meteorology for driving GEOS-Chem has been archived from the E2.1 contributions to phase 6 of the Coupled Model Intercomparison Project (CMIP6) for the pre-industrial era and the recent past. In addition, meteorology is available for the near future and end of the century for seven future scenarios ranging from extreme mitigation to extreme warming. Emissions and boundary conditions have been prepared for input to GEOS-Chem that are consistent with the CMIP6 experimental design. The model meteorology, emissions, transport, and chemistry are evaluated in the recent past and found to be largely consistent with GEOS-Chem driven by the Modern-Era Retrospective analysis for Research and Applications version 2 (MERRA-2) product and with observational constraints.

Simulated Greenland Surface Mass Balance in the GISS ModelE2 GCM: Role of the Ice Sheet Surface

AbstractThe rate of growth or retreat of the Greenland and Antarctic ice sheets remains a highly uncertain component of future sea level change. Here we examine the simulation of Greenland ice sheet surface mass balance (GrIS SMB) in a development branch of the ModelE2 version of the NASA Goddard Institute for Space Studies (GISS) general circulation model (GCM). GCMs are often limited in their ability to represent SMB compared with polar region regional climate models. We compare ModelE2‐simulated GrIS SMB for present‐day (1996–2005) simulations with fixed ocean conditions, at a spatial resolution of 2° latitude by 2.5° longitude (~200 km), with SMB simulated by the Modèle Atmosphérique Régionale (MAR) regional climate model (1996–2005 at a 25‐km resolution). ModelE2 SMB agrees well with MAR SMB on the whole, but there are distinct spatial patterns of differences and large differences in some SMB components. The impacts of changes to the ModelE2 surface are tested, including a subgrid‐scale representation of SMB with surface elevation classes. This has a minimal effect on ice sheet‐wide SMB but corrects local biases. Replacing fixed surface albedo with satellite‐derived values and an age‐dependent scheme has a larger impact, increasing simulated melt by 60%–100%. We also find that lower surface albedo can enhance the effects of elevation classes. Reducing ModelE2 surface roughness length to values closer to MAR reduces sublimation by ~50%. Further work is required to account for meltwater refreezing in ModelE2 and to understand how differences in atmospheric processes and model resolution influence simulated SMB.

Interannual variation, decadal trend, and future change in ozone outflow from East Asia

Abstract. We examine the past and future changes in the O3 outflow from East Asia using a global 3-D chemical transport model, GEOS-Chem. The simulations of Asian O3 outflow for 1986–2006 are driven by the assimilated GEOS-4 meteorological fields, and those for 2000–2050 are driven by the meteorological fields archived by the NASA Goddard Institute for Space Studies (GISS) general circulation model (GCM) 3 under the IPCC SRES A1B scenario. The evaluation of the model results against measurements shows that the GEOS-Chem model captures the seasonal cycles and interannual variations of tropospheric O3 concentrations fairly well with high correlation coefficients of 0.82–0.93 at four ground-based sites and 0.55–0.88 at two ozonesonde sites where observations are available. The increasing trends in surface-layer O3 concentrations in East Asia over the past 2 decades are captured by the model, although the modeled O3 trends have low biases. Sensitivity studies are conducted to examine the respective impacts of meteorological parameters and emissions on the variations in the outflow flux of O3. When both meteorological parameters and anthropogenic emissions varied from 1986–2006, the simulated Asian O3 outflow fluxes exhibited a statistically insignificant decadal trend; however, they showed large interannual variations (IAVs) with seasonal values of 4–9 % for the absolute percent departure from the mean (APDM) and an annual APDM value of 3.3 %. The sensitivity simulations indicated that the large IAVs in O3 outflow fluxes were mainly caused by variations in the meteorological conditions. The variations in meteorological parameters drove the IAVs in O3 outflow fluxes by altering the O3 concentrations over East Asia and by altering the zonal winds; the latter was identified to be the key factor, since the O3 outflow was highly correlated with zonal winds from 1986–2006. The simulations of the 2000–2050 changes show that the annual outflow flux of O3 will increase by 2.0, 7.9, and 12.2 % owing to climate change alone, emissions change alone, and changes in both climate and emissions, respectively. Therefore, climate change will aggravate the effects of the increases in anthropogenic emissions on future changes in the Asian O3 outflow. Future climate change is predicted to greatly increase the Asian O3 outflow in the spring and summer seasons as a result of the projected increases in zonal winds. The findings from the present study help us to understand the variations in tropospheric O3 in the downwind regions of East Asia on different timescales and have important implications for long-term air quality planning in the regions downwind of China, such as Japan and the US.

Configuration and assessment of the GISS ModelE2 contributions to the CMIP5 archive

AbstractWe present a description of the ModelE2 version of the Goddard Institute for Space Studies (GISS) General Circulation Model (GCM) and the configurations used in the simulations performed for the Coupled Model Intercomparison Project Phase 5 (CMIP5). We use six variations related to the treatment of the atmospheric composition, the calculation of aerosol indirect effects, and ocean model component. Specifically, we test the difference between atmospheric models that have noninteractive composition, where radiatively important aerosols and ozone are prescribed from precomputed decadal averages, and interactive versions where atmospheric chemistry and aerosols are calculated given decadally varying emissions. The impact of the first aerosol indirect effect on clouds is either specified using a simple tuning, or parameterized using a cloud microphysics scheme. We also use two dynamic ocean components: the Russell and HYbrid Coordinate Ocean Model (HYCOM) which differ significantly in their basic formulations and grid. Results are presented for the climatological means over the satellite era (1980–2004) taken from transient simulations starting from the preindustrial (1850) driven by estimates of appropriate forcings over the 20th Century. Differences in base climate and variability related to the choice of ocean model are large, indicating an important structural uncertainty. The impact of interactive atmospheric composition on the climatology is relatively small except in regions such as the lower stratosphere, where ozone plays an important role, and the tropics, where aerosol changes affect the hydrological cycle and cloud cover. While key improvements over previous versions of the model are evident, these are not uniform across all metrics.

Model‐based estimation of sampling‐caused uncertainty in aerosol remote sensing for climate research applications

To evaluate the effect of sampling frequency on the global monthly mean aerosol optical thickness (AOT), we use 6 years of geographical coordinates of Moderate Resolution Imaging Spectroradiometer (MODIS) L2 aerosol data, daily global aerosol fields generated by the Goddard Institute for Space Studies General Circulation Model and the chemical transport models Global Ozone Chemistry Aerosol Radiation and Transport, Spectral Radiation‐transport Model for Aerosol Species and Transport Model 5, at a spatial resolution between 1.125°× 1.125° and 2°× 3°: the analysis is restricted to 60°S–60°N geographical latitude. We found that, in general, the MODIS coverage causes an underestimate of the global mean AOT over the ocean. The long‐term mean absolute monthly difference between all and dark target (DT) pixels was 0.01–0.02 over the ocean and 0.03–0.09 over the land, depending on the model dataset. Negative DT biases peak during boreal summers, reaching 0.07–0.12 (30–45% of the global long‐term mean AOT). Addition of the Deep Blue pixels tempers the seasonal dependence of the DT biases and reduces the mean AOT difference over land by 0.01–0.02. These results provide a quantitative measure of the effect the pixel exclusion due to cloud contamination, ocean sun‐glint and land type has on the MODIS estimates of the global monthly mean AOT. We also simulate global monthly mean AOT estimates from measurements provided by pixel‐wide along‐track instruments such as the Aerosol Polarimetry Sensor and the Cloud–Aerosol LiDAR with Orthogonal Polarization. We estimate the probable range of the global AOT standard error for an along‐track sensor to be 0.0005–0.0015 (ocean) and 0.0029–0.01 (land) or 0.5–1.2% and 1.1–4% of the corresponding global means. These estimates represent errors due to sampling only and do not include potential retrieval errors. They are smaller than or comparable to the published estimate of 0.01 as being a climatologically significant change in the global mean AOT, suggesting that sampling density is unlikely to limit the use of such instruments for climate applications at least on a global, monthly scale.

Projected effect of 2000–2050 changes in climate and emissions on aerosol levels in China and associated transboundary transport

Abstract. We investigate projected 2000–2050 changes in concentrations of aerosols in China and the associated transboundary aerosol transport by using the chemical transport model GEOS-Chem driven by the Goddard Institute for Space Studies (GISS) general circulation model (GCM) 3 at 4° × 5° resolution. Future changes in climate and emissions projected by the IPCC A1B scenario are imposed separately and together through sensitivity simulations. Accounting for sulfate, nitrate, ammonium, black carbon (BC), and organic carbon (OC) aerosols, concentrations of individual aerosol species change by −1.5 to +0.8 μg m−3, and PM2.5 levels are projected to change by about 10–20% in eastern China as a result of 2000–2050 change in climate alone. With future changes in anthropogenic emissions alone, concentrations of sulfate, BC, and OC are simulated to decrease because of assumed reductions in emissions, and those of nitrate are predicted to increase because of higher NOx emissions combined with decreases in sulfate. The net result is a predicted reduction of seasonal mean PM2.5 concentrations in eastern China by 1–8 μg m−3 (or 10–40%) over 2000–2050. It is noted that current emission inventories for BC and OC over China are judged to be inadequate at present. Transboundary fluxes of different aerosol species show different sensitivities to future changes in climate and emissions. The annual outflow of PM2.5 from eastern China to the western Pacific is estimated to change by −7.0%, −0.7%, and −9.0% over 2000–2050 owing to climate change alone, changes in emissions alone, and changes in both climate and emissions, respectively. The fluxes of nitrate and ammonium aerosols from Europe and Central Asia into western China increase over 2000–2050 in response to projected changes in emissions, leading to a 10.5% increase in annual inflow of PM2.5 to western China with future changes in both emissions and climate. Fluxes of BC and OC from South Asia to China in spring contribute a large fraction of the annual inflow of PM2.5. The annual inflow of PM2.5 from South Asia and Southeast Asia to China is estimated to change by −8%, +281%, and +227% over 2000–2050 owing to climate change alone, changes in emissions alone, and changes in both climate and emissions, respectively. While the 4° × 5° spatial resolution is a limitation of the present study, the direction of predicted changes in aerosol levels and transboundary fluxes still provides valuable insight into future air quality.

Diagnosing Warm Frontal Cloud Formation in a GCM: A Novel Approach Using Conditional Subsetting

Abstract This study analyzes characteristics of clouds and vertical motion across extratropical cyclone warm fronts in the NASA Goddard Institute for Space Studies general circulation model. The validity of the modeled clouds is assessed using a combination of satellite observations from CloudSat, Cloud–Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO), Advanced Microwave Scanning Radiometer for Earth Observing System (AMSR-E), and the NASA Modern-Era Retrospective Analysis for Research and Applications (MERRA) reanalysis. The analysis focuses on developing cyclones, to test the model's ability to generate their initial structure. To begin, the extratropical cyclones and their warm fronts are objectively identified and cyclone-local fields are mapped into a vertical transect centered on the surface warm front. To further isolate specific physics, the cyclones are separated using conditional subsetting based on additional cyclone-local variables, and the differences between the subset means are analyzed. Conditional subsets are created based on 1) the transect clouds and 2) vertical motion; 3) the strength of the temperature gradient along the warm front, as well as the storm-local 4) wind speed and 5) precipitable water (PW). The analysis shows that the model does not generate enough frontal cloud, especially at low altitude. The subsetting results reveal that, compared to the observations, the model exhibits a decoupling between cloud formation at high and low altitudes across warm fronts and a weak sensitivity to moisture. These issues are caused in part by the parameterized convection and assumptions in the stratiform cloud scheme that are valid in the subtropics. On the other hand, the model generates proper covariability of low-altitude vertical motion and cloud at the warm front and a joint dependence of cloudiness on wind and PW.

Assessment of the Impacts of Climate Change on European Ozone Levels

The objective of this study is to investigate the potential impact of future climate change on ozone air quality in Europe. To provide a full assessment, simulations with the global chemical transport model GEOS-CHEM driven by the NASA Goddard Institute for Space Studies general circulation model (NASA/GISS GCM) are conducted. To isolate the effects from changes in climate and anthropogenic emissions four types of simulations are performed: (1) present-day climate and emissions (2) future climate following the IPCC Special Report on Emission Scenarios (SRES) A1B scenario and present-day anthropogenic emissions of ozone precursors (3) present-day climate and future emissions and (4) future climate and future emissions. Results indicate that climate change impact on its own leads to an increase of less than 3 ppb in western and central Europe whereas decreases are evident for the rest of the areas with the highest (about 2.5 ppb) in southeastern Europe (Italy, Greece). Increases are attributed to the increases of isoprene biogenic emissions due to increasing temperatures whereas decreases are associated with the increase of water vapor over sea which tends to decrease the lifetime of ozone as well as the increased wind speeds in the 2050 climate. When future emissions are implemented in the future climate simulations, the greatest increases are seen in the southwest and southeast Mediterranean (about 16 ppb) due to the increased isoprene biogenic emissions under higher levels of NO x in the model. Decreases up to 2 ppb of ozone are shown for France, Switzerland, Northern Italy and northern Europe.

Projected Changes in NOxEmissions from Lightning as a Result of 2000–2050 Climate Change

Lightning is one of the most important natural sources of atmospheric NOx.The authors investigate the2000–2050 changes in NOx emissions from lightning using the global three-dimensional Goddard Earth Observing System chemical transport model(GEOS-Chem)driven by meteorological fields from the Goddard Institute for Space Studies(GISS)general circulation model(GCM)3.Projected changes in climate over 2000–2050are based on the Intergovernmental Panel on Climate Change(IPCC)A1B scenario.The global NOx emission from lightning is simulated to be 4.8 Tg N in present day and to increase by about 16.7%over 2000–2050 as a result of the future climate change.The largest present-day emissions and climate-induced changes are found in the upper troposphere in the tropics.Regionally in eastern China(20–55 N,98–125 E),NOx emissions from lighting is simulated to be 0.3 Tg N(6.3%of the global total emission)in present day and to increase by 26.7%over2000–2050.The simulated changes in NOx from lightening correspond well with the projected future changes in convective precipitation.

The Tropical Subseasonal Variability Simulated in the NASA GISS General Circulation Model

Abstract The tropical subseasonal variability simulated by the Goddard Institute for Space Studies general circulation model, Model E2, is examined. Several versions of Model E2 were developed with changes to the convective parameterization in order to improve the simulation of the Madden–Julian oscillation (MJO). When the convective scheme is modified to have a greater fractional entrainment rate, Model E2 is able to simulate MJO-like disturbances with proper spatial and temporal scales. Increasing the rate of rain reevaporation has additional positive impacts on the simulated MJO. The improvement in MJO simulation comes at the cost of increased biases in the mean state, consistent in structure and amplitude with those found in other GCMs when tuned to have a stronger MJO. By reinitializing a relatively poor-MJO version with restart files from a relatively better-MJO version, a series of 30-day integrations is constructed to examine the impacts of the parameterization changes on the organization of tropical convection. The poor-MJO version with smaller entrainment rate has a tendency to allow convection to be activated over a broader area and to reduce the contrast between dry and wet regimes so that tropical convection becomes less organized. Besides the MJO, the number of tropical-cyclone-like vortices simulated by the model is also affected by changes in the convection scheme. The model simulates a smaller number of such storms globally with a larger entrainment rate, while the number increases significantly with a greater rain reevaporation rate.

Characteristics of Mesoscale Organization in WRF Simulations of Convection during TWP-ICE

Abstract Compared to satellite-derived heating profiles, the Goddard Institute for Space Studies general circulation model (GCM) convective heating is too deep and its stratiform upper-level heating is too weak. This deficiency highlights the need for GCMs to parameterize the mesoscale organization of convection. Cloud-resolving model simulations of convection near Darwin, Australia, in weak wind shear environments of different humidities are used to characterize mesoscale organization processes and to provide parameterization guidance. Downdraft cold pools appear to stimulate further deep convection both through their effect on eddy size and vertical velocity. Anomalously humid air surrounds updrafts, reducing the efficacy of entrainment. Recovery of cold pool properties to ambient conditions over 5–6 h proceeds differently over land and ocean. Over ocean increased surface fluxes restore the cold pool to prestorm conditions. Over land surface fluxes are suppressed in the cold pool region; temperature decreases and humidity increases, and both then remain nearly constant, while the undisturbed environment cools diurnally. The upper-troposphere stratiform rain region area lags convection by 5–6 h under humid active monsoon conditions but by only 1–2 h during drier break periods, suggesting that mesoscale organization is more readily sustained in a humid environment. Stratiform region hydrometeor mixing ratio lags convection by 0–2 h, suggesting that it is strongly influenced by detrainment from convective updrafts. Small stratiform region temperature anomalies suggest that a mesoscale updraft parameterization initialized with properties of buoyant detrained air and evolving to a balance between diabatic heating and adiabatic cooling might be a plausible approach for GCMs.

Modeling global secondary organic aerosol formation and processing with the volatility basis set: Implications for anthropogenic secondary organic aerosol

The volatility basis set, a computationally efficient framework for the description of organic aerosol partitioning and chemical aging, is implemented in the Goddard Institute for Space Studies General Circulation Model II′ for a coupled global circulation and chemical transport model to simulate secondary organic aerosol (SOA) formation. The latest smog chamber information about the yields of anthropogenic and biogenic precursors is incorporated in the model. SOA formation from monoterpenes, sesquiterpenes, isoprene, and anthropogenic precursors is estimated as 17.2, 3.9, 6.5, and 1.6 Tg yr−1, respectively. Reducing water solubility of secondary organic gas from 105 to 103 mol L−1 atm−1 (1 atm = 1.01325 × 105 N m−2) leads to a 90% increase in SOA production and an increase of over 340% in total atmospheric burden, from 0.54 to 2.4 Tg. Increasing the temperature sensitivity of SOA leads to a 30% increase in production, to 38.2 Tg yr−1. Since the additional SOA is formed at high altitude, where deposition time scales are longer, the average lifetime is doubled from 6.8 to 14.3 days, resulting in a near tripling of atmospheric burden to 1.5 Tg. Chemical aging of anthropogenic SOA by gas‐phase reaction of the SOA components with the hydroxyl radical adds an additional 2.7–9.3 Tg yr−1 of anthropogenic SOA to the above production rates and nearly doubles annual average total SOA burdens. The possibility of such high anthropogenic SOA production rates challenges the assumption that anthropogenic volatile organic compounds are not important SOA precursors on a global scale. Model predictions with and without SOA aging are compared with data from two surface observation networks: the Interagency Monitoring of Protected Visual Environments for the United States and the European Monitoring and Evaluation Programme.

Effect of chemistry‐aerosol‐climate coupling on predictions of future climate and future levels of tropospheric ozone and aerosols

We explore the extent to which chemistry‐aerosol‐climate coupling influences predictions of future ozone and aerosols as well as future climate using the Goddard Institute for Space Studies (GISS) general circulation model II' with on‐line simulation of tropospheric ozone‐NOx‐hydrocarbon chemistry and sulfate, nitrate, ammonium, black carbon, primary organic carbon, and secondary organic carbon aerosols. Based on IPCC scenario A2, year 2100 ozone, aerosols, and climate simulated with full chemistry‐aerosol‐climate coupling are compared with those simulated from a stepwise approach. In the stepwise method year 2100 ozone and aerosols are first simulated using present‐day climate and year 2100 emissions (denoted as simulation CHEM2100sw) and year 2100 climate is then predicted using offline monthly fields of O3 and aerosols from CHEM2100sw (denoted as simulation CLIM2100sw). The fully coupled chemistry‐aerosol‐climate simulation predicts a 15% lower global burden of O3 for year 2100 than the simulation CHEM2100sw which does not account for future changes in climate. Relative to CHEM2100sw, year 2100 column burdens of all aerosols in the fully coupled simulation exhibit reductions of 10–20 mg m−2 in DJF and up to 10 mg m−2 in JJA in mid to high latitudes in the Northern Hemisphere, reductions of up to 20 mg m−2 over the eastern United States, northeastern China, and Europe in DJF, and increases of 30–50 mg m−2 over populated and biomass burning areas in JJA. As a result, relative to year 2100 climate simulated from CLIM2100sw, full chemistry‐aerosol‐climate coupling leads to a stronger net global warming by greenhouse gases, tropospheric ozone and aerosols in year 2100, with a global and annual mean surface air temperature higher by 0.42 K. For simulation of year 2100 aerosols, we conclude that it is important to consider the positive feedback between future aerosol direct radiative forcing and future aerosol concentrations; increased aerosol concentrations lead to reductions in convection and precipitation (or wet deposition of aerosols), further increasing lower tropospheric aerosol concentrations.

Distinguishing Aerosol Impacts on Climate over the Past Century

Abstract Aerosol direct (DE), indirect (IE), and black carbon–snow albedo (BAE) effects on climate between 1890 and 1995 are compared using equilibrium aerosol–climate simulations in the Goddard Institute for Space Studies General Circulation Model coupled to a mixed layer ocean. Pairs of control (1890)–perturbation (1995) with successive aerosol effects allow isolation of each effect. The experiments are conducted both with and without concurrent changes in greenhouse gases (GHG). A new scheme allowing dependence of snow albedo on black carbon snow concentration is introduced. The fixed GHG experiments global surface air temperature (SAT) changed by −0.2°, −1.0°, and +0.2°C from the DE, IE, and BAE. Ice and snow cover increased 1% from the IE and decreased 0.3% from the BAE. These changes were a factor of 4 larger in the Arctic. Global cloud cover increased by 0.5% from the IE. Net aerosol cooling effects are about half as large as the GHG warming, and their combined climate effects are smaller than the sum of their individual effects. Increasing GHG did not affect the IE impact on cloud cover, however they decreased aerosol effects on SAT by 20%, and on snow/ice cover by 50%; they also obscure the BAE on snow/ice cover. Arctic snow, ice, cloud, and shortwave forcing changes occur mostly during summer–fall, but SAT, sea level pressure, and longwave forcing changes occur during winter. An explanation is that aerosols impact the cryosphere during the warm season but the associated SAT effect is delayed until winter.

Effects of 2000–2050 global change on ozone air quality in the United States

We investigate the effects on U.S. ozone air quality from 2000–2050 global changes in climate and anthropogenic emissions of ozone precursors by using a global chemical transport model (GEOS‐Chem) driven by meteorological fields from the NASA Goddard Institute for Space Studies general circulation model (NASA/GISS GCM). We follow the Intergovernmental Panel on Climate Change A1B scenario and separate the effects from changes in climate and anthropogenic emissions through sensitivity simulations. The 2000–2050 changes in anthropogenic emissions reduce the U.S. summer daily maximum 8‐hour ozone by 2–15 ppb, but climate change causes a 2–5 ppb positive offset over the Midwest and northeastern United States, partly driven by decreased ventilation from convection and frontal passages. Ozone pollution episodes are far more affected by climate change than mean values, with effects exceeding 10 ppb in the Midwest and northeast. We find that ozone air quality in the southeast is insensitive to climate change, reflecting compensating effects from changes in isoprene emission and air pollution meteorology. We define a “climate change penalty” as the additional emission controls necessary to meet a given ozone air quality target. We find that a 50% reduction in U.S. NOx emissions is needed in the 2050 climate to reach the same target in the Midwest as a 40% reduction in the 2000 climate. Emission controls reduce the magnitude of this climate change penalty and can even turn it into a climate benefit in some regions.

Evaluation of tropical cloud regimes in observations and a general circulation model

Tropical cloud regimes defined by cluster analysis of International Satellite Cloud Climatology Project (ISCCP) cloud top pressure (CTP)–optical thickness distributions and ISCCP-like Goddard Institute for Space Studies (GISS) general circulation model (GCM) output are analyzed in this study. The observations are evaluated against radar–lidar cloud-top profiles from the atmospheric radiation measurement (ARM) Program active remote sensing of cloud layers (ARSCL) product at two tropical locations and by placing them in the dynamical context of the Madden–Julian oscillation (MJO). ARSCL highest cloud-top profiles indicate that differences among some of the six ISCCP regimes may not be as prominent as suggested by ISCCP at the ARM tropical sites. An experimental adjustment of the ISCCP CTPs to produce cloud-top height profiles consistent with ARSCL eliminates the independence between those regimes. Despite these ambiguities, the ISCCP regime evolution over different phases of the MJO is consistent with existing MJO mechanisms, but with a greater mix of cloud types in each phase than is usually envisioned. The GISS Model E GCM produces two disturbed and two suppressed regimes when vertical convective condensate transport is included in the model’s cumulus parameterization. The primary model deficiencies are the absence of an isolated cirrus regime, a lack of mid-level cloud relative to ARSCL, and a tendency for occurrences of specific parameterized processes such as deep and shallow convection and stratiform low cloud formation to not be associated preferentially with any single cloud regime.

Simulated effects of climate change on summertime nitrogen deposition in the eastern US

It is anticipated that climate change may impact regional-scale air quality and atmospheric deposition in the coming decades. To simulate the effects of climate change on nitrogen (N) deposition across numerous watersheds in the eastern US, we applied the NASA Goddard Institute for Space Studies General Circulation Model (GISS-GCM), Fifth Generation Pennsylvania State University/National Center for Atmospheric Research Mesoscale Model (MM5), Sparse Matrix Operator Kernel Emissions (SMOKE) modeling system, and the US Environmental Protection Agency Community Multiscale Air Quality (CMAQ) Model. Keeping chemical initial and boundary conditions, land use, and anthropogenic area and point source emissions fixed, this modeling system was applied over five summers (June–August) from 1993 to 1997 and five summers from 2053 to 2057. Over these eastern US watersheds, the modeling system estimated 3–14% increases in summertime N deposition as a result of climate change. This increase is primarily due to the direct effects of climate change on atmospheric conditions and chemistry. Wet N deposition is predicted to increase as a result of increased precipitation, while dry N deposition is predicted to increase as higher surface temperatures favor gas-phase nitric acid to particulate nitrate. The simulated increase suggests that additional reductions in N oxides and/or ammonia may be needed to fully realize the anticipated benefits of planned reduction strategies, including the Clean Air Interstate Rule (CAIR).

Future climate impacts of direct radiative forcing of anthropogenic aerosols, tropospheric ozone, and long‐lived greenhouse gases

Long‐lived greenhouse gases (GHGs) are the most important driver of climate change over the next century. Aerosols and tropospheric ozone (O3) are expected to induce significant perturbations to the GHG‐forced climate. To distinguish the equilibrium climate responses to changes in direct radiative forcing of anthropogenic aerosols, tropospheric ozone, and GHG between present day and year 2100, four 80‐year equilibrium climates are simulated using a unified tropospheric chemistry‐aerosol model within the Goddard Institute for Space Studies (GISS) general circulation model (GCM) II′. Concentrations of sulfate, nitrate, primary organic (POA) carbon, secondary organic (SOA) carbon, black carbon (BC) aerosols, and tropospheric ozone for present day and year 2100 are obtained a priori by coupled chemistry‐aerosol GCM simulations, with emissions of aerosols, ozone, and precursors based on the Intergovernmental Panel on Climate Change (IPCC) Special Report on Emissions Scenario (SRES) A2. Changing anthropogenic aerosols, tropospheric ozone, and GHG from present day to year 2100 is predicted to perturb the global annual mean radiative forcing by +0.18 (considering aerosol direct effects only), +0.65, and +6.54 W m−2 at the tropopause, and to induce an equilibrium global annual mean surface temperature change of +0.14, +0.32, and +5.31 K, respectively, with the largest temperature response occurring at northern high latitudes. Anthropogenic aerosols, through their direct effect, are predicted to alter the Hadley circulation owing to an increasing interhemispheric temperature gradient, leading to changes in tropical precipitation. When changes in both aerosols and tropospheric ozone are considered, the predicted patterns of change in global circulation and the hydrological cycle are similar to those induced by aerosols alone. GHG‐induced climate changes, such as amplified warming over high latitudes, weakened Hadley circulation, and increasing precipitation over the Tropics and high latitudes, are consistent with predictions of a number of previous GCM studies. Finally, direct radiative forcing of anthropogenic aerosols is predicted to induce strong regional cooling over East and South Asia. Wintertime rainfall over southeastern China and the Indian subcontinent is predicted to decrease because of the increased atmospheric stability and decreased surface evaporation, while the geographic distribution of precipitation is also predicted to be altered as a result of aerosol‐induced changes in wind flow.

The Tropical Atmospheric El Niño Signal in Satellite Precipitation Data and a Global Climate Model

AbstractAspects of the tropical atmospheric response to El Niño related to the global energy and water cycle are examined using satellite retrievals from the Tropical Rainfall Measuring Mission and the Advanced Microwave Scanning Radiometer-E and simulations from the Goddard Institute for Space Studies (GISS) general circulation model (GCM). The El Niño signal is extracted from climate fields using a linear cross-correlation technique that captures local and remote in-phase and lagged responses. Passive microwave and radar precipitation anomalies for the 1997/98 and 2002/03 El Niños and the intervening La Niña are highly correlated, but anomalies in stratiform–convective rainfall partitioning in the two datasets are not. The GISS GCM produces too much rainfall in general over ocean and too little over land. Its atmospheric response to El Niño is weaker and decays a season too early. Underestimated stratiform rainfall fraction (SRF) and convective downdraft mass flux in the GISS GCM and excessive shallow convective and low stratiform cloud result in latent heating that peaks at lower altitudes than inferred from the data. The GISS GCM also underestimates the column water vapor content throughout the Tropics, which causes it to overestimate outgoing longwave radiation. The response of both quantities to interannual Hadley circulation anomalies is too weak. The GISS GCM’s Walker circulation also exhibits a weak remote response to El Niño, especially over the Maritime Continent and western Indian Ocean. This appears to be a consequence of weak static stability due to the model’s lack of upper-level stratiform anvil heating, excessive low-level heating, and excessive dissipation due to cumulus momentum mixing. Our results suggest that parameterizations of mesoscale updrafts, convective downdrafts, and cumulus-scale pressure gradient effects on momentum transport are keys to a reasonable GISS GCM simulation of tropical interannual variability.

Biogenic secondary organic aerosol over the United States: Comparison of climatological simulations with observations

Understanding the effects of global climate change on regional air quality is central in future air quality planning. We report here on the use of the Goddard Institute for Space Studies (GISS) general circulation model (GCM) III to drive the GEOS‐CHEM global atmospheric chemical transport model to simulate climatological present‐day aerosol levels over the United States. Evaluation of model predictions using surface measurements from the Interagency Monitoring of Protected Visual Environments (IMPROVE) network indicates that the GISS GCM III/GEOS‐CHEM model is a suitable tool for simulating aerosols over the United States in the present climate. The model reproduces fairly well the concentrations of sulfate (mean bias of −0.36 μg m−3, normalized mean bias (NMB) of −25.9%), black carbon (−0.004 μg m−3, −1.9%), organic carbon that comprises primary and secondary components (−0.56 μg m−3, −34.2%), and PM2.5 (−0.87 μg m−3, −20.4%). Nitrate concentrations are overpredicted in the western United States (west of 95°W) with a NMB of +75.6% and underestimated in the eastern United States with a NMB of −54.4%. Special attention is paid to biogenic secondary organic aerosol (SOA). The highest predicted seasonal mean SOA concentrations of 1–2 μg m−3 and 0.5–1.5 μg m−3 are predicted over the northwestern and southeastern United States, respectively, in the months of June–July–August. Isoprene is predicted to contribute 49.5% of the biogenic SOA burden over the United States, with the rest explained by the oxidation of terpenes. Predicted biogenic SOA concentrations are in reasonable agreement with inferred SOA levels from IMPROVE measurements. On an annual basis, SOA is predicted to contribute 10–20% of PM2.5 mass in the southeastern United States, as high as 38% in the northwest and about 5–15% in other regions, indicating the important role of SOA in understanding air quality and visibility over the United States.