Microphysics Modification Based on Simplied Constrained Gamma Distribution

Microphysics parameterization plays an important role in Numerical Weather Prediction (NWP). Most NWP models, including the Weather Research Forecast (WRF) model, use single-moment (bulk water) schemes derived based on an assumption of the exponential distribution with a fixed intercept parameter for raindrop size distributions (DSDs) such as the Marshall-Palmer (M-P) DSD model (Marshall and Palmer 1948). However, recent observation studies show rain DSDs are better characterized by a constrained gamma distribution model and can be retrieved from polarimetric radar measurements. The constrained gamma model was further simplified to a single parameter for the parameterization of warm rain microphysical processes (Zhang et al. 2006). In this study, the simplified constrained gamma (SCG) rain DSD model is implemented on WRF to improve the WSM-6 mixed phase scheme (Hong and Lim 2006). The modifications to rain evaporation and accretion, and rain interactions with snow and graupel are accounted. The impact on numerical forecasts for an ideal case is studied.