Microphysical-Dynamical Interaction in Tropical Cyclone Intensification

In a real tropical cyclone (TC), microphysical processes interact directly with the convective updrafts/downdrafts and turbulent eddy circulations in clouds to generate the distribution of diabatic heating and cooling critical to TC intensification. In numerical simulations, such an interaction among microphysics, convection, and turbulence is subjected to how sub-grid scale processes are parameterized. A key microphysical process that depends strongly on microphysical-dynamical interaction is the hydrometeor sedimentation. In real clouds, hydrometeor particle fall velocity is determined by both particle properties (e.g., mass density, size, and shape) and the turbulent flow in which particles are embedded, and thus, the same hydrometeor particle will have different terminal velocity depending on where it resides: convective updraft or downdraft. In microphysical schemes, however, the particle fall velocities are often simply parameterized based on the empirical logarithmic fitting in terms of particle equivalent diameters. The impact of convective currents and turbulent mixing on fall velocity is neglected. In this study, we investigate the role of microphysical-dynamical interaction in regulating TC intensification and explore methods of incorporating dynamic effect in the parameterization of particle terminal velocity. We further test and demonstrate the improvement in TC intensity simulation by including microphysical-dynamical interaction in terminal velocity parameterization in two state-of-the-art modeling systems: the Weather Research and Forecasting (WRF) model and Hurricane WRF (HWRF) model.