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

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

Search for other papers by Ming-Dean Cheng in
Current site
Google Scholar
PubMed
Close
Restricted access

Abstract

The diagnostic cumulus ensemble model presented in Part I of this paper is applied to the data taken during Phase III of the GARP Tropical Atlantic Experiment (GATE) to examine the effects of convective-scale downdrafts on the large-scale heat and moisture budgets. The model is used to diagnose the mass fluxes of cumulus updrafts and downdrafts using the observed values of Q1Q2QR, where Q1, Q2 and QR are the apparent heat source, the apparent moisture sink and radiative heating rate, respectively. Using the diagnosed mass fluxes, we calculate the cumulus contributions to the residuals of heat and moisture budgets separately. We demonstrate that the inclusion of convective-scale downdrafts is essential to obtain accurately the cumulus contributions to the budget residuals. In the lower troposphere the contributions of downdrafts to Q1QR and Q2 are comparable to the contributions of updrafts. Convective-scale updrafts tend to warm and dry the cumulus environment, while convective-scale downdrafts tend to cool and moisten the lower cloud layer. We find that significant downdraft activity is usually associated with organized convection. There is almost no downdraft activity within the analyzed domain when only scattered convection is observed. The downdraft effects should be properly included in cumulus parameterization schemes.

Abstract

The diagnostic cumulus ensemble model presented in Part I of this paper is applied to the data taken during Phase III of the GARP Tropical Atlantic Experiment (GATE) to examine the effects of convective-scale downdrafts on the large-scale heat and moisture budgets. The model is used to diagnose the mass fluxes of cumulus updrafts and downdrafts using the observed values of Q1Q2QR, where Q1, Q2 and QR are the apparent heat source, the apparent moisture sink and radiative heating rate, respectively. Using the diagnosed mass fluxes, we calculate the cumulus contributions to the residuals of heat and moisture budgets separately. We demonstrate that the inclusion of convective-scale downdrafts is essential to obtain accurately the cumulus contributions to the budget residuals. In the lower troposphere the contributions of downdrafts to Q1QR and Q2 are comparable to the contributions of updrafts. Convective-scale updrafts tend to warm and dry the cumulus environment, while convective-scale downdrafts tend to cool and moisten the lower cloud layer. We find that significant downdraft activity is usually associated with organized convection. There is almost no downdraft activity within the analyzed domain when only scattered convection is observed. The downdraft effects should be properly included in cumulus parameterization schemes.

Save