Sensitivity of a Simulated Deep Convective Storm to WRF Microphysical Schemes and Horizontal Resolution

Bulk microphysical (MP) parameterizations describe the processes by which water and ice particles grow and precipitate within a cloud. These schemes allow models to represent cloud processes that occur on the microscale and cannot be properly resolved. Uncertainties in climate simulations and operational forecasts remain due to the choice of MP scheme. To analyze the effect different parameterizations have on cloud processes and storm development, this study simulates a single, isolated deep convective storm occurring in a low wind shear environment using the Weather Research and Forecasting (WRF) model in idealized mode at “convection-permitting” scale (i.e., without a deep convection parameterization). WRF was initialized using an atmospheric vertical profile from the National Center of Atmospheric Research Community Climate System Model (CCSM3) averaged for the summer months (June, July, August) for 1970-1999 over Jasper, IN, USA. Four MP parameterizations available in WRF (WSM6, Thompson, Milbrandt-Yau, and Morrison) were compared at three different horizontal resolutions (2 km, 1 km, and 250 m). An analysis of storm evolution and structure is performed, including precipitation efficiency and storm dynamics. Results showed large increases in surface precipitation with increasing resolution regardless of the MP parameterization, especially between 1 km and 250 m grid spacing. This occurs because of greater condensation associated with increased updraft mass flux at higher resolution. Overall differences between schemes were consistent at the various resolutions tested. Out of the four schemes, WSM6 produced the lowest precipitation amount, which results from a relatively high rain evaporation rate. Further research will involve running the simulation at higher resolution (e.g. 125 m) and initializing storms with different soundings, including those simulated by CCSM3 for future climate scenarios.