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Kuan-Man Xu and David A. Randall

Abstract

Existing cloudiness parameterizations based on specified probability distribution functions (PDFs) and large-scale relative humidity (RH) in climate-models are evaluated with data produced from explicit simulations of observed tropical cloud systems and subtropical stratocumuli. PDF-based parameterizations were originally intended for use in cloud-resolving models, where fractional cloudiness is only associated with turbulence-scale motion. It is demonstrated with simulated data that most PDF-based parameterizations are not adequate for predicting fractional cloudiness in climate models because their performance is dependent upon the cloud regimes. Modifications to some PDF-based formulations are suggested, especially with regard to the inclusion of skewness of conservative variables. The skewness factors are found to be highly dependent upon which scales of motion coexist within a grid cell. RH-based parameterizations are not readily supported due to a wide range of variations of clear-region averaged RHs with height and the grid size of climate models, as well as their wide range of variations at a given height.

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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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David A. Randall, James A. Abeles, and Thomas G. Corsetti

Abstract

The UCLA general circulation model (GCM) has been used to simulate the seasonally varying planetary boundary layer (PBL), as well as boundary-layer stratus and stratocumulus clouds. The PBL depth is a prognostic variable of the GCM, incorporated through the use of a vertical coordinate system in which the PBL is identified with the lowest model layer.

Stratocumulus clouds are assumed to occur whenever the upper portion of the PBL becomes saturated, provided that the cloud-top entrainment instability does not occur. As indicated by Arakawa and Schubert, cumulus clouds are assumed to originate at the PBL top, and tend to make the PBL shallow by drawing on its mass.

Results are presented from a three-year simulation, starting from a 31 December initial condition obtained from an earlier run with a different version of the model. The simulated seasonally varying climates of the boundary layer and free troposphere are realistic. The observed geographical and seasonal variations of stratocumulus cloudiness are fairly well simulated. The simulation of the stratocumulus clouds associated with wintertime cold-air outbreaks is particularly realistic. Examples are given of individual events. The positions of the subtropical marine stratocumulus regimes are realistically simulated, although their observed frequency of occurrence is seriously underpredicted. The observed summertime abundance of Arctic stratus clouds is also underpredicted.

In the GCM results, the layer cloud instability appears to limit the extent of the marine subtropical stratocumulus regimes. The instability also frequently occurs in association with cumulus convection over land.

Cumulus convection acts as a very significant sink of PBL mass throughout the tropics, and over the midlatitude continents in summer.

Three experiments have been performed to investigate the sensitivity of the GCM results to aspects of the PBL and stratocumulus parameterizations. For all three experiments, the model was started from 1 June conditions of the second year of the three-year run, and the July-mean results of each experiment were compared with the three-year composite simulated July, as well as with observations.

In the first experiment, the direct interaction of the stratocumulus clouds with the boundary-layer turbulence was disabled. The results show a significant and unrealistic increase in both stratocumulus cloudiness and total cloudiness; a 22 W m−2 reduction in both the globally averaged net radiation flux at the top of the atmosphere and the total surface energy flux; and substantial changes in the relative magnitudes of the components of the surface energy flux. The primary cause of these changes is the absence of cloud-top entrainment instability in the experiment.

In the second experiment, the PBL depth was fixed at a prescribed, globally uniform value. The results show a pronounced and unrealistic increase in cumulus precipitation, particularly over land; an unrealistic tendency for stratocumulus cloudiness to occur preferentially over land; and a marked shift in the surface energy balance, accompanied by stronger PBL wind speeds.

In the third experiment, the diurnal cycle of solar insulation was replaced by a daily-mean “torroidal sun.” The results show a 3% increase in the global albedo, even though the global cloudiness decreases slightly. The PBL depth increases dramatically over land and especially over desert regions. The precipitation rate sharply increases over the continents, which become sink regions for atmospheric moisture.

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David A. Randall, Harshvardhan, Donald A. Dazlich, and Thomas G. Corsetti

Abstract

We have analyzed the effects of radiatively active clouds on the climate simulated by the UCLA/GLA GCM, with particular attention to the effects of the upper tropospheric stratiform clouds associated with deep cumulus convection, and the interactions of these clouds with convection and the large-scale circulation.

Several numerical experiments have been performed to investigate the mechanisms through which the clouds influence the large-scale circulation. In the “NODETLQ” experiment, no liquid water or ice was detrained from cumulus clouds into the environment; all of the condensate was rained out. Upper level supersaturation cloudiness was drastically reduced, the atmosphere dried, and tropical outgoing longwave radiation increased. In the “NOANVIL” experiment, the radiative effects of the optically thich upper-level cloud sheets associated with deep cumulus convection were neglected. The land surface received more solar radiation in regions of convection, leading to enhanced surface fluxes and a dramatic increase in precipitation. In the “NOCRF” experiment, the longwave atmospheric cloud radiative forcing (ACRF) was omitted, paralleling the recent experiment of Slingo and Slingo. The results suggest that the ACRF enhances deep penetrative convection and precipitation, while suppressing shallow convection. They also indicate that the ACRF warms and moistens the tropical troposphere. The results of this experiment are somewhat ambiguous, however; for example, the ACRF suppresses precipitation in some parts of the tropics, and enhances it in others.

To isolate the effects of the ACRF in a simpler setting, we have analyzed the climate of an ocean-covered Earth, which we call Seaworld. The key simplicities of Seaworld are the fixed boundary temperature with no land points, the lack of mountains, and the zonal uniformity of the boundary conditions. Results are presented from two Seaworld simulations. The first includes a full suite of physical parameterizations, while the second omits all radiative effects of the clouds. The differences between the two runs are, therefore, entirely due to the direct and indirect and indirect effects of the ACRF. Results show that the ACRF in the cloudy run accurately represents the radiative heating perturbation relative to the cloud-free run. The cloudy run is warmer in the middle troposphere, contains much more precipitable water, and has about 15% more globally averaged precipitation. There is a double tropical rain band in the cloud-free run, and a single, more intense tropical rain band in the cloudy run. The cloud-free run produces relatively weak but frequent cumulus convection, while the cloudy run produces relatively intense but infrequent convection. The mean meridional circulation transport nearly twice as much mass in the cloudy run. The increased tropical rising motion in the cloudy run leads to a deeper boundary layer and also to more moisture in the troposphere above the boundary layer. This accounts for the increased precipitable water content of the atmosphere. The clouds lead to an increase in the intensity of the tropical easterlies, and cause the midlatitude westerly jets to shift equatorward.

Taken together, our results show that upper tropospheric clouds associated with moist convection, whose importance has recently been emphasized in observational studies, play a very complex and powerful role in determining the model results. This points to a need to develop more realistic parameterizations of these clouds.

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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

Abstract

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

Abstract

Precipitation variability is analyzed in two versions of the Community Atmospheric Model (CAM), the standard model, CAM, and a “multiscale modeling framework” (MMF), in which the cumulus parameterization has been replaced with a cloud-resolving model. Probability distribution functions (PDFs) of daily mean rainfall in three geographic locations [the Amazon Basin and western Pacific in December–February (DJF) and the North American Great Plains in June–August (JJA)] indicate that the CAM produces too much light–moderate rainfall (10 ∼ 20 mm day−1), and not enough heavy rainfall, compared to observations. The MMF underestimates rain contributions from the lightest rainfall rates but correctly simulates more intense rainfall events. These differences are not always apparent in seasonal mean rainfall totals.

Analysis of 3–6-hourly rainfall and sounding data in the same locations reveals that the CAM produces moderately intense rainfall as soon as the boundary layer energizes. Precipitation is also concurrent with tropospheric relative humidity and lifted parcel buoyancy increases. In contrast, the MMF and observations are characterized by a lag of several hours between boundary layer energy buildup and precipitation, and a gradual increase in the depth of low-level relative humidity maximum prior to rainfall.

The environmental entrainment rate selection in the CAM cumulus parameterization influences CAM precipitation timing and intensity, and may contribute to the midlevel dry bias in that model. The resulting low-intensity rainfall in the CAM leads to rainfall–canopy vegetation interactions that are different from those simulated by the MMF. The authors present evidence suggesting that this interaction may artificially inflate North American Great Plains summertime rainfall totals in the CAM.

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Chin-Hoh Moeng, Don H. Lenschow, and David A. Randall

Abstract

When the surface buoyancy flux is small and the shear is weak, turbulence circulations within a stratus-topped boundary layer are driven by two buoyancy-generating processes at cloud top: radiative cooling and evaporative cooling. These two processes respond very differently to entrainment, however. When the entrainment rate increases, the effectiveness of radiative cooling in driving circulations decreases (a negative feedback) but the effectiveness of evaporative cooling can increase (a positive feedback). The roles of these two competing feedbacks in determining the entrainment rate, and hence in determining cloud breakup, are examined in this paper through large eddy simulations.

Three stratus cases (with a small surface buoyancy flux) are simulated: one is stable with respect to the Lilly–Randall–Deardorff cloud-top entrainment instability criterion, and the other two are unstable. Only one of the two cloud decks in the unstable regime dissipates totally; the other remains nearly solid. A method is proposed to separate the cloud-top radiative and evaporative cooling contributions to downdraft acceleration, which drives the boundary-layer circulations. Analysis of these three flow fields shows that cloud dissipates totally only in the case that the evaporative feedback dominates. When the radiative feedback dominates, as in one of the unstable cases, the cloud remains nearly solid even though the Lilly–Randall–Deardorff criterion is satisfied.

To confirm the key role of cloud-top evaporative cooling in this positive feedback loop, two controlled experiments have been conducted—one with evaporative cooling turned off and the other with radiative cooling turned off—after the cloud was brought into the unstable regime with respect to the Lilly–Randall–Deardorff criterion. The cloud without evaporative cooling (for which boundary-layer circulations are driven only by cloud-top radiative cooling) remains solid, while that without radiative cooling (in which circulations are driven only by evaporative cooling) dissipates rapidly.

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Takanobu Yamaguchi, David A. Randall, and Marat F. Khairoutdinov

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Numerical diffusion can be minimized using fine grid spacing and/or higher-order numerical schemes. In this study, the authors focus on higher-order scalar advection schemes and their effects on simulated cloud fields. A monotonic multidimensional odd-order conservative advection scheme has been implemented, following the approach of Leonard. It has been tested in simulations of idealized scalar fields advected by simple prescribed motion, as well as turbulence fields; large-eddy simulations of turbulent stratocumulus clouds; and simulations of deep convective clouds. New third-, fifth-, and seventh-order schemes are compared with the second-order scheme originally used in the model. For the deep cumulus case, a high-resolution large-eddy simulation with the same domain size is used as a benchmark.

The fifth-order scheme shows much less numerical diffusion than the lower-order scheme. The additional improvement with the seventh-order scheme is minor. The higher-order scheme generally produces simulated cloud fields similar to those obtained with a lower-order scheme with a finer grid spacing. This effect is especially noticeable for the updraft-core statistics of the deep cumulus simulation, as compared with the benchmark simulation. The fifth-order scheme with coarse horizontal resolution produces results close to those of the benchmark simulation. Compared to a high-resolution simulation with the low-order scheme, the numerical cost of the fifth-order simulation is smaller than a factor of 10.

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