The Bulk Effects of Cumulus Momentum Transports in Tropical Cyclones

Abstract

The net influence of subgrid scale processes on the tangential momentum field in tropical cyclones is calculated as a residual of the grid-scale momentum budget for eight rawinsonde composite data sets. Results show that developing cyclones of the northwest Pacific and northwest Atlantic have similar residual profiles in the vertical: positive cyclonic acceleration at upper and lower levels and negative cyclonic acceleration at between levels. Stronger systems have a greater magnitude of the subgrid scale effect. These results are believed to represent primarily the cumulus momentum transport (defined as cumulus friction). A simple single cloud model is developed to diagnose vertical momentum rearrangement through mass recycling. Model results agree well with the residual calculations from the composites. The total convective effect (TCE) on the tangential momentum includes the vertical transport by all the upward and downward motions which can be decomposed into the mean vertical motion and the local mall recycling. This TCE is found to reduce bath the low-level cyclonic and the upper-level anticyclonic winds. These results imply that the net effect of the convective transport is down-gradient and acts to reduce the vertical wind shear of the cyclone. This mutual interaction between the cumulus convection and the larger scale flow features and their possible influence on tropical cyclone development is discussed.