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Abstract
The UCLA/GLA general circulation model has been endowed with new parameterizations of solar and terrestrial radiation, as well as new parameterized cloud optical properties. A simple representation of the cloud liquid water feedback is included. We have used the model and several observational datasets to analyze the effects of cloudiness on the Earth's radiation budget.
Analysis of January and July results obtained with the full model shows that the simulated Earth radiation budget is in reasonable agreement with Nimbus 7 data. The globally averaged planetary albedo and outgoing longwave radiation am both slightly less than observed. A tropical minimum of the outgoing longwave radiation is simulated, but is weaker than observed. Comparisons of the simulated cloudiness with observations from ISCCP and HIRS2/MSU show that the model overpredicts subtropical and midlatitude cloudiness.
The simulated cloud radiative forcings at the top of the atmosphere, at the Earth's surface, and across the atmosphere are discussed, and comparisons are made with the limited observations available. The simulated atmospheric cloud radiative forcing (ACRF) is comparable in magnitude to the latent heating. We have compared the clear-sky radiation fields obtained using Methods I and II of Cess and Potter; the results show significant differences between the two methods, primarily due to systematic variations of the cloudiness with time of day.
An important feature of the new terrestrial radiation parameterization is its incorporation (for the first time in this GCM) of the effects of the water vapor continuum. To determine the effects of this change on the model results, we performed a numerical experiment in which the effects of the water vapor continuum were neglected. The troposphere warmed dramatically, and shallow convection weakened, and the radiative effects of the clouds were significantly enhanced.
Abstract
The UCLA/GLA general circulation model has been endowed with new parameterizations of solar and terrestrial radiation, as well as new parameterized cloud optical properties. A simple representation of the cloud liquid water feedback is included. We have used the model and several observational datasets to analyze the effects of cloudiness on the Earth's radiation budget.
Analysis of January and July results obtained with the full model shows that the simulated Earth radiation budget is in reasonable agreement with Nimbus 7 data. The globally averaged planetary albedo and outgoing longwave radiation am both slightly less than observed. A tropical minimum of the outgoing longwave radiation is simulated, but is weaker than observed. Comparisons of the simulated cloudiness with observations from ISCCP and HIRS2/MSU show that the model overpredicts subtropical and midlatitude cloudiness.
The simulated cloud radiative forcings at the top of the atmosphere, at the Earth's surface, and across the atmosphere are discussed, and comparisons are made with the limited observations available. The simulated atmospheric cloud radiative forcing (ACRF) is comparable in magnitude to the latent heating. We have compared the clear-sky radiation fields obtained using Methods I and II of Cess and Potter; the results show significant differences between the two methods, primarily due to systematic variations of the cloudiness with time of day.
An important feature of the new terrestrial radiation parameterization is its incorporation (for the first time in this GCM) of the effects of the water vapor continuum. To determine the effects of this change on the model results, we performed a numerical experiment in which the effects of the water vapor continuum were neglected. The troposphere warmed dramatically, and shallow convection weakened, and the radiative effects of the clouds were significantly enhanced.
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.
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.
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.
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.
