Impacts of Terminal Velocity and Drop Size Distribution Shape on the Numerical Simulation of Precipitation

The accuracy of numerical weather prediction is mainly affected by the description of physical processes in cloud microphysics schemes. In current microphysical parameterizations the fall speed and diameter-concentration distribution of hydrometeors are described using the mass-weighted terminal velocity and drop size distribution shape parameter. Therefore, the description in different schemes directly influences numerical weather prediction. In this study, the Weather Research and Forecasting (WRF) model (version 3.5.1), coupled with a new bulk two-moment microphysics scheme, was used to simulate a severe precipitation event that occurred in South China on 8 May 2013. Two descriptions of terminal velocity, two descriptions of the size distribution parameter, and four combinations of each, were evaluated and analyzed. The results were as follows: (1) Changes in snowfall terminal velocity had certain impacts on precipitation intensity; (2) Changes in the size distribution shape parameter generated more obvious impacts in terms of both the intensity and development of precipitation; (3) Combining the Ferrier mass-weighted terminal velocity and the improved size distribution shape parameter, using long-term observations in East Asia, showed clear advantages compared with three other sensitivity runs.