Effects of Downdrafts and Mesoscale Convective Organization on the Heat and Moisture Budgets of Tropical Cloud Clusters. Part III: Effects of Mesoscale Convective Organization

Ming-Dean Cheng Department of Atmospheric Sciences, University of California, Los Angeles, California

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Michio Yanai Department of Atmospheric Sciences, University of California, Los Angeles, California

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Abstract

The effects of mesoscale convective organization on the large-scale heat and moisture budgets of tropical cloud clusters are studied using the GATE Phase III data. Two basic questions concerning the mesoscale effects are addressed: 1) the interpretation of the mesoscale processes contributing to observed heat and moisture budget residuals, and 2) the coupling between dynamic and thermodynamic fields of the environment of organized cumulus convection.

The contributions from mesoscale circulations to the observed heat and moisture budget residuals are identified by eliminating the effects of the subsidence compensating the cumulus mass flux from these quantities. Then, the mesoscale contributions are objectively isolated from the observed budget residuals with the aid of a diagnostic cumulus ensemble model developed in Part I of this paper. The isolated mesoscale contributions clearly show warming and drying in the upper troposphere and cooling and moistening in the lower troposphere, indicating the condensation within the anvil cloud and evaporation of rainwater beneath the anvil.

Large values of the thermodynamically diagnosed tilting angle of deep cumulus clouds appear well before the mesoscale convective organization is detected by radar, suggesting that there are thermodynamically preferred regions for the formation of organized cumulus convection. The tilting angle further increases as squall clusters develop. Because the mesoscale organization and the updraft tilting angle are also related to the vertical wind shear, there must be a strong coupling between the dynamic and thermodynamic fields of the environment of organized cumulus convection. Remarkable time correlations between the updraft tilting angle and the cloud work function of deep clouds, and the low-level wind shear are shown to illustrate this coupling.

Abstract

The effects of mesoscale convective organization on the large-scale heat and moisture budgets of tropical cloud clusters are studied using the GATE Phase III data. Two basic questions concerning the mesoscale effects are addressed: 1) the interpretation of the mesoscale processes contributing to observed heat and moisture budget residuals, and 2) the coupling between dynamic and thermodynamic fields of the environment of organized cumulus convection.

The contributions from mesoscale circulations to the observed heat and moisture budget residuals are identified by eliminating the effects of the subsidence compensating the cumulus mass flux from these quantities. Then, the mesoscale contributions are objectively isolated from the observed budget residuals with the aid of a diagnostic cumulus ensemble model developed in Part I of this paper. The isolated mesoscale contributions clearly show warming and drying in the upper troposphere and cooling and moistening in the lower troposphere, indicating the condensation within the anvil cloud and evaporation of rainwater beneath the anvil.

Large values of the thermodynamically diagnosed tilting angle of deep cumulus clouds appear well before the mesoscale convective organization is detected by radar, suggesting that there are thermodynamically preferred regions for the formation of organized cumulus convection. The tilting angle further increases as squall clusters develop. Because the mesoscale organization and the updraft tilting angle are also related to the vertical wind shear, there must be a strong coupling between the dynamic and thermodynamic fields of the environment of organized cumulus convection. Remarkable time correlations between the updraft tilting angle and the cloud work function of deep clouds, and the low-level wind shear are shown to illustrate this coupling.

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