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David A. Randall
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Harshvardhan, David A. Randall, and Donald A. Dazlich

Abstract

Attempts to map the global longwave surface radiation budget from space have been thwarted by the presence of clouds. Unlike the shortwave, there is no physical relationship between the outgoing longwave and the surface longwave under cloudy skies. Therefore, there is no correlation between spatial and temporal averages of the outgoing longwave radiation and not longwave radiation at the surface. However, in regions where a particular cloud regime exists preferentially, a relationship between the mean longwave cloud radiative forcing (CRF) at the top of the atmosphere and at the surface can he shown to exist. Results from a general circulation model suggest that this relationship for monthly means is coherent over fairly large geographical areas. For example, in tropical convective areas, the longwave CRF at the top is very large, but at the surface it is quite small because of the high opacity of the lowest layers of the atmosphere. On the other hand, in areas of stratus over cool ocean surfaces, the longwave CRF at the top is very small but at the surface, it is quite substantial.

To the extent that the cloudiness simulated in the model mimics the real atmosphere, it may be possible to estimate the monthly mean longwave CRF at the surface from the Earth Radiation Budget Experiment cloud forcing at the top. The net longwave radiation at the surface can then be mapped if monthly means of the clear-sky fluxes are obtained by some independent technique.

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Michael A. Kelly and David A. Randall

Abstract

A simple fixed-SST model of a zonal circulation in the tropical atmosphere has been developed that has separate boxes for the ascending and descending branches of the atmospheric circulation. This circulation resembles the Walker circulation. This is the first box model to determine the fractional widths of the warm and cold pools. The atmospheric model contains an explicit hydrologic cycle, a simplified but physically based radiative transfer parameterization, and interactive clouds.

Results indicate that the intensity of the tropical circulation is crucially dependent on the amount and vertical distribution of water vapor above the cold-pool boundary layer (CPBL). In response to increasing precipitable water over the CPBL, the radiative cooling rate of the free troposphere increases. To a good approximation, subsidence warming balances radiative cooling in the subsiding branches of the circulation. If the fractional width of the cold pool (CP) does not change too much, the circulation must intensify as the subsidence rate increases. To compensate for a stronger circulation and to restore energy balance in the Walker cell, the precipitable water over the warm pool (WP) must decrease. A “moist-outflow” experiment shows that the Walker circulation intensifies if air is advected to the subsiding regions from lower altitudes in the WP. As the advection level decreases, air supplied to the CP becomes warmer and moister, and so the column water vapor in the CP free troposphere increases. The mechanism described above then leads to a strengthening of the circulation. This moist-outflow experiment also shows that when the authors try to moisten the atmosphere by specifying a lower advection level for water vapor, the atmosphere adjusts so as to dry out. This effect is very strong.

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Laura D. Fowler and David A. Randall

Abstract

A prognostic equation for the mass of condensate associated with large-scale cloudiness introduces a direct coupling between the atmospheric moisture budget and the radiation budget through interactive cloud amounts and cloud optical properties. We have compared the cloudiness, the top-of-the-atmosphere and surface radiation budgets, the radiative forcing of clouds, and the atmospheric general circulation simulated with the Colorado State University general circulation model with and without such a prognostic cloud parameterization. In the EAULIQ run, the radiative effects of cloud water, cloud ice, and snow are considered; those of rain are omitted. The cloud optical depth and cloud infrared emissivity depend on the cloud water, cloud ice, and snow paths predicted by a bulk cloud microphysics parameterization. In the CONTROL run, a conventional large-scale condensation scheme is used. Cloud optical properties depend on the mean cloud temperatures. Results are presented in terms of January and July means.

Comparisons with data from the International Satellite Cloud Climatology Project and the Earth Radiation Budget Experiment show that EAULIQ yields improved simulations of the geographical distributions of the simulated cloudiness, the top-of-the-atmosphere radiation budget, and the longwave and shortwave cloud radiative forcings. Differences between EAULIQ and CONTROL are largest in the Tropics and are mostly due to a decrease, in the EAULIQ run, in the amount and optical thickness of upper-tropospheric clouds. In particular, the cold bias in the outgoing longwave radiation and the overestimation of the planetary albedo obtained in the CONTROL run over the tropical convective regions are substantially reduced. Differences in the radiative and latent heating rates between EAULIQ and CONTROL lead to some improvements in the atmospheric general circulation simulated by EAULIQ when compared against statistics on the observed circulation assembled by the European Centre for Medium-Range Weather Forecasts. When compared to CONTROL, EAULIQ yields a warmer troposphere except below 8 km between 3°N and 30°S. The mean meridional circulation is significantly weakened in the EAULIQ run.

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Laura D. Fowler and David A. Randall

Abstract

The inclusion of cloud microphysical processes in general circulation models makes it possible to study the multiple interactions among clouds, the hydrological cycle, and radiation. The gaps between the temporal and spatial scales at which such cloud microphysical processes work and those at which general circulation models presently function force climate modelers to crudely parameterize and simplify the various interactions among the different water species (namely, water vapor, cloud water, cloud ice, rain, and snow) and to use adjustable parameters to which large-scale models can be highly sensitive. Accordingly, the authors have investigated the sensitivity of the climate, simulated with the Colorado State University general circulation model, to various aspects of the parameterization of cloud microphysical processes and its interactions with the cumulus convection and radiative transfer parameterizations.

The results of 120-day sensitivity experiments corresponding to perpetual January conditions have been compared with those of a control simulation in order to 1 ) determine the importance of advecting cloud water, cloud ice, rain, and snow at the temporal and spatial scale resolutions presently used in the model; 2) study the importance of the formation of extended stratiform anvils at the tops of cumulus towers, 3) analyze the role of mixed-phase clouds in determining the partitioning among cloud water, cloud ice, rain, and snow and, hence, their impacts on the simulated cloud optical properties; 4) evaluate the sensitivity of the atmospheric moisture budget and precipitation rates to a change in the fall velocities of rain and snow; 5) determine the model's sensitivity to the prescribed thresholds of autoconversion of cloud water to rain and cloud ice to snow; and 6) study the impact of the collection of supercooled cloud water by snow, as well as accounting for the cloud optical properties of snow.

Results are presented in terms of 30-day mean differences between the sensitivity experiments and control run. The authors find that three-dimensional advection of the water species has little influence on their geographical distributions and globally averaged amounts. The simulated climate remains unchanged when detrained condensed water at the tops of cumulus towers is used as a source of rain and snow rather than as a source of cloud water and cloud ice. In contrast, instantaneously removing cloud water and cloud ice detrained at the tops of cumulus towers in the form of precipitation yields a strong drying of the atmosphere and a significant reduction in the size of the anvils. Altering the partitioning between cloud ice and supercooled cloud water produces significant changes in the vertical distributions of the cloud optical depth and effective cloud fraction, hence producing significant variations in the top-of-the-atmosphere longwave and shortwave cloud radiative forcings. Increasing the fall speeds of rain and snow leads to a decrease in cloudiness and an increase in stratiform rainfall. Increasing the thresholds for autoconversion of cloud water to rain and cloud ice to snow yields a significant increase in middle- and high-level clouds and a reduction of the cumulus precipitation rate. The collection of supercooled cloud water by snow appeared to be an important microphysical process for mixed-phase clouds. Finally, the optical effects of snow have little impact upon the top-of-the-atmosphere radiation budget.

This study illustrates the need for in-depth analysis of the spatial and temporal scale dependence of the different microphysical parameters of the cloud parameterizations used in general circulation models.

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Rachel R. McCrary and David A. Randall

Abstract

Coupled global circulation models (CGCMs) have been widely used to explore potential future climate change. Before these climate projections can be trusted, the ability of the models to simulate present-day climate must be assessed. This study evaluates the ability of three CGCMs that participated in the Fourth Assessment Report of the Intergovernmental Panel on Climate Change to simulate long-term drought over the Great Plains region with the same frequency and intensity as was observed during the twentieth century. The three models evaluated in this study are the Geophysical Fluid Dynamics Laboratory Coupled Model, version 2.0 (CM2.0); the National Centers for Atmospheric Research Community Climate System Model, version 3 (CCSM3); and third climate configuration of the Met Office Unified Model (HadCM3).

The models are shown to capture the broad features of the climatology of the Great Plains, with maximum precipitation occurring in early summer, as observed. However, all of the models overestimate annual precipitation rates. Also, in CCSM3, precipitation and evapotranspiration experience unrealistic decreases between the months of June and August.

Long-term droughts are found in each simulation of the twentieth century that are comparable in duration, severity, and spatial extent as has been observed. However, the processes found to be associated with simulated long-term droughts vary among the models. In both CM2.0 and HadCM3, low-frequency variations in Great Plains precipitation are found to correspond with low-frequency variations in tropical Pacific SSTs. In CCSM3, on the other hand, there appears to be no significant correlation between tropical Pacific SST variability and Great Plains precipitation. Strong land–atmosphere coupling in CCSM3 may explain the persistence of long-term droughts in this model.

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Charlotte A. DeMott, David A. Randall, and Marat Khairoutdinov

Abstract

Implied ocean heat transport (To) based on net surface energy budgets is computed for two versions of the Community Atmospheric Model (CAM, version 3.0) general circulation model (GCM). The first version is the standard CAM with parameterized convection. The second is the multiscale modeling framework (MMF), in which parameterized convection is replaced with a two-dimensional cloud-resolving model in each GCM grid column. Although global-mean net surface energy totals are similar for both models, differences in the geographic distributions of the component errors lead to distinctly different To for each model, with CAM’s To generally agreeing with observationally based To estimates, and the MMF’s To producing northward transport at all latitudes north of ∼50°S.

Analysis of component error sources in the To calculation identifies needed improvements in the MMF. Net surface shortwave radiation and latent heat fluxes over the oceans are the primary causes of To errors in the MMF. Surface shortwave radiation biases in the MMF are associated with liquid and/or ice water content biases in tropical and extratropical convection and a deficit of marine stratocumulus clouds. It is expected that tropical ice water contents in the MMF can be made more realistic via improvements to the cloud microphysics parameterization. MMF marine stratocumulus clouds are overly sensitive to low-level relative humidity and form only with nearly saturated conditions and a shallow boundary layer. Latent heat flux errors in the MMF are amplifications of those found in the CAM and are concentrated in the trade wind regime and the Asian monsoon region and the adjacent western Pacific Ocean.

Potential improvements to To are estimated by replacing either simulated net surface shortwave or latent heat fluxes with those from observations and recomputing To. When observed shortwave fluxes are used, both CAM and MMF produce greatly improved To curves for both hemispheres. When To is computed using observed latent heat fluxes, CAM To degrades slightly and MMF To improves, especially in the sign of Southern Hemisphere transport.

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Laura D. Fowler, David A. Randall, and Steven A. Rutledge

Abstract

Microphysical processes responsible for the formation and dissipation of water and ice clouds have been incorporated into the Colorado State University General Circulation Model in order to 1) yield a more physically based representation of the components of the atmospheric moisture budget, 2) link the distribution and optical properties of the model-generated clouds to the predicted cloud water and ice amounts, and 3) produce more realistic simulations of cloudiness and the earth's radiation budget.

The bulk cloud microphysics scheme encompasses five prognostic variables for the mass of water vapor, cloud water, cloud ice, rain, and snow. Graupel and hail are neglected. Cloud water and cloud ice are predicted to form through large-scale condensation and deposition processes and also through detrainment at the tops of cumulus towers. The production of rain and snow occur through autoconversion of cloud water and cloud ice. Rain drops falling through clouds can grow by collecting cloud water, and falling snow can collect both cloud water and cloud ice. These collection processes are formulated using the continuous collection equation. Evaporation of cloud water, cloud ice, rain, and snow are allowed in subsaturated layers. Melting and freezing are included. We also provide a coupling between convective clouds and stratiform anvils through the detrainment of cloud water and cloud ice at the tops of cumulus towers. Interactive cloud optical properties provide the link between the cloud n-microphysics and radiation parameterizations; the optical depths and infrared emissivities of large-scale stratiform clouds are parameterized in terms of the cloud water and cloud ice paths.

Two annual-cycle numerical simulations are performed to assess the impact of cloud microphysics on the hydrological cycle. In the “EAULIQ” run, large-scale moist processes and cloud optical properties are driven by the bulk cloud microphysics parameterization. In the “CONTROL” run, condensed water is immediately removed from the atmosphere in the form of rain, which may evaporate as it falls through subsaturated layers. Stratiform ice clouds are not considered in CONTROL. When clouds are present, cloud optical depths and cloud infrared emissivities are dependent on the mean cloud temperatures.

Results are presented in terms of January and July monthly averages. Emphasis is placed on the spatial distributions of cloud water, cloud ice, rain, and snow produced by the cloud microphysics scheme. In EAULIQ, cloud water and cloud ice are more abundant in the middle latitudes than in the Tropics, suggesting that large-scale condensation contributes a major part to the production of condensed water. Comparisons between the simulated vertically integrated cloud water and die columnar cloud water retrievals from satellite microwave measurements over the global oceans indicate a reasonable agreement. Interactions between the cloud micro- physics and cumulus convection parameterizations lead to smaller, more realistic precipitation rates. In particular, the cumulus precipitation rate is strongly reduced when compared to CONTROL.

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Melissa A. Burt, David A. Randall, and Mark D. Branson

Abstract

As the Arctic sea ice thins and ultimately disappears in a warming climate, its insulating power decreases. This causes the surface air temperature to approach the temperature of the relatively warm ocean water below the ice. The resulting increases in air temperature, water vapor, and cloudiness lead to an increase in the surface downwelling longwave radiation (DLR), which enables a further thinning of the ice. This positive ice–insulation feedback operates mainly in the autumn and winter. A climate change simulation with the Community Earth System Model shows that, averaged over the year, the increase in Arctic DLR is 3 times stronger than the increase in Arctic absorbed solar radiation at the surface. The warming of the surface air over the Arctic Ocean during fall and winter creates a strong thermal contrast with the colder surrounding continents. Sea level pressure falls over the Arctic Ocean, and the high-latitude circulation reorganizes into a shallow “winter monsoon.” The resulting increase in surface wind speed promotes stronger surface evaporation and higher humidity over portions of the Arctic Ocean, thus reinforcing the ice–insulation feedback.

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Charlotte A. DeMott, Cristiana Stan, and David A. Randall

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Mechanisms for the northward propagation (NP) of the boreal summer intraseasonal oscillation (BSISO) and associated Asian summer monsoon (ASM) are investigated using data from the interim ECMWF Re-Analysis (ERA-Interim, herein called ERAI) and the superparameterized Community Climate System Model (SP-CCSM). Analyzed mechanisms are 1) destabilization of the lower troposphere by sea surface temperature anomalies, 2) boundary layer moisture advection, and boundary layer convergence associated with 3) SST gradients and 4) barotropic vorticity anomalies. Mechanism indices are regressed onto filtered OLR anomaly time series to study their relationships to the intraseasonal oscillation (ISO) and to equatorial Rossby (ER) waves.

Northward propagation in ERAI and SP-CCSM is promoted by several mechanisms, but is dominated by boundary layer moisture advection and the barotropic vorticity effect. SST-linked mechanisms are of secondary importance but are nonnegligible. The magnitudes of NP mechanisms vary from the Indian Ocean to the west Pacific Ocean, implying that NP is accomplished by different mechanisms across the study area.

SP-CCSM correctly simulates observed NP mechanisms over most of the ASM domain except in the Arabian Sea during the early stages of the monsoon life cycle. Reduced NP in the Arabian Sea arises from weaker-than-observed easterly shear, reducing the effectiveness of the barotropic vorticity mechanism. The ability of SP-CCSM to correctly simulate NP mechanisms in other regions results from the model’s ability to simulate reasonable mean wind and moisture fields, a realistic spectrum of variability, and the capability of convection to respond to boundary layer changes induced by large-scale NP mechanisms.

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