Effects of artificial local compensation of convective mass flux in the cumulus parameterization

Abstract

In this study, a hybrid mass flux cumulus scheme (HYMACS) is developed for the Weather Research and Forecasting Model (WRF). Idealized experiments are performed to evaluate its effects on tropical cyclone simulations. Classical cumulus schemes assume artificial local compensation of convective mass flux. In contrast, HYMACS treats subgrid-scale mass flux convergence or divergence as parameterized mass sources or sinks. When the mass sources or sinks are introduced to the mass continuity equation in a nonhydrostatic fully compressible model, the model dynamics would resolve the mass-compensating motion, i.e., dynamic compensation of convective mass flux. A hierarchy of experiments is conducted to demonstrate the effects of the artificial local compensation. The results of the mass compensation experiment show that the amplitude of the column mass change with the artificial local compensation is more sensitive to the change of the horizontal resolution between 3 and 27 km than the dynamic compensation. The results of the piggybacking tropical cyclone simulations at 9 km resolution suggest that the artificial local compensation in the Kain-Fritsch scheme (KF) concentrates vertical exchange of dry static energy and moisture and induces secondary circulation, which could lead to sea level pressure decrease and enhanced precipitation. These results indicate that the artificial local compensation at the gray-zone resolution could cause significant effects on tropical cyclone dynamics, so it is important to avoid the artificial local compensation for cumulus parameterization at such resolution.