Impact of parameterizing the turbulent orographic form drag on convection-permitting simulations of winds and precipitation over South China during the 2019 pre-summer rainy season

A turbulent orographic form drag (TOFD) parameterization, which accounts for the unresolved drag induced by the subgrid orographic variance, is implemented in the Weather Research and Forecasting (WRF) model with a horizontal grid spacing of 3 km. Its impact on the surface wind and precipitation forecasts over South China during a pre-summer rainy season (April-June 2019) is evaluated based on 3-hourly observations from more than 2500 stations, by comparing the results from two series of parallel simulation with the TOFD parameterization turned on or off, respectively. Results show that the seasonal mean root mean square error (RMSE) and bias of the surface wind have been significantly reduced by more than 7% and 5%, respectively. The low-level wind is also shown improved by comparing the two series of simulations with 12-hourly observations from 26 sounding stations. Consequently, the equitable threat score (ETS) is increased by the TOFD parameterization for most rainfall-intensity categories, and the six-category mean ETS shows an improvement of more than 3%. The mechanism of the TOFD impact on low-level wind and precipitation is investigated as well. The low-level southwesterlies from the South China Sea flow over the rough land in South China are found to be weakened, which leads to the low-level wind convergence and more precipitation over those regions. Moreover, this effect is more apparent during the daytime.