Large-Scale Forcing and Cloud–Radiation Interaction in the Tropical Deep Convective Regime

Xiaofan Li Space Applications Corporation, Vienna, Virginia

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C-H. Sui NASA/Goddard Space Flight Center, Greenbelt, Maryland

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K-M. Lau NASA/Goddard Space Flight Center, Greenbelt, Maryland

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M-D. Chou NASA/Goddard Space Flight Center, Greenbelt, Maryland

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Abstract

The simulations of tropical convection and thermodynamic states in response to different imposed large-scale forcing are carried out by using a cloud-resolving model and are evaluated with the Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment observation. The model is forced either with imposed large-scale vertical velocity and horizontal temperature and moisture advections (model 1) or with imposed total temperature and moisture advections (model 2). The comparison of simulations with observations shows that bias in temperature and moisture simulations by model 1 is smaller than that by model 2. This indicates that the adjustment of the mean thermodynamic stability distribution by vertical advection in model 1 is responsible for better simulations.

Model 1 is used to examine effects of different parameterized solar radiative and cloud microphysical processes. A revised parameterization scheme for cloud single scattering properties in solar radiation calculations is found to generate more solar heating in the upper troposphere and less heating in the middle and lower troposphere. The change in the vertical heating distribution is suggested to stabilize the environment and to cause less stratiform cloud that further induces stabilization through cloud–IR interaction. The revised scheme also causes a drier middle and lower troposphere by weakening vertical moisture flux convergence. Also tested is the effect of a revised parameterization scheme for cloud microphysical processes that tends to generate more ice clouds. The cloud-induced thermal effect in which less ice cloud leads to less infrared cooling at cloud top and more heating below cloud top is similar to the effect of no cloud–radiation interaction shown in a sensitivity experiment. However, the exclusion of cloud–radiation interaction causes drying by enhancing condensation, and the reduction of ice clouds by the microphysics scheme induces moistening by suppressing condensation.

Corresponding author address: Dr. C.-H. Sui, Code 913, NASA/GSFC, Greenbelt, MD 20771. E-mail: sui@climate.gsfc.nasa.gov

Abstract

The simulations of tropical convection and thermodynamic states in response to different imposed large-scale forcing are carried out by using a cloud-resolving model and are evaluated with the Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment observation. The model is forced either with imposed large-scale vertical velocity and horizontal temperature and moisture advections (model 1) or with imposed total temperature and moisture advections (model 2). The comparison of simulations with observations shows that bias in temperature and moisture simulations by model 1 is smaller than that by model 2. This indicates that the adjustment of the mean thermodynamic stability distribution by vertical advection in model 1 is responsible for better simulations.

Model 1 is used to examine effects of different parameterized solar radiative and cloud microphysical processes. A revised parameterization scheme for cloud single scattering properties in solar radiation calculations is found to generate more solar heating in the upper troposphere and less heating in the middle and lower troposphere. The change in the vertical heating distribution is suggested to stabilize the environment and to cause less stratiform cloud that further induces stabilization through cloud–IR interaction. The revised scheme also causes a drier middle and lower troposphere by weakening vertical moisture flux convergence. Also tested is the effect of a revised parameterization scheme for cloud microphysical processes that tends to generate more ice clouds. The cloud-induced thermal effect in which less ice cloud leads to less infrared cooling at cloud top and more heating below cloud top is similar to the effect of no cloud–radiation interaction shown in a sensitivity experiment. However, the exclusion of cloud–radiation interaction causes drying by enhancing condensation, and the reduction of ice clouds by the microphysics scheme induces moistening by suppressing condensation.

Corresponding author address: Dr. C.-H. Sui, Code 913, NASA/GSFC, Greenbelt, MD 20771. E-mail: sui@climate.gsfc.nasa.gov

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