Microphysics affect the sensitivities of rainfall to different horizontal-resolution simulations: Evidence from a case study of the Weather Research and Forecasting model runs

Abstract

The Weather Research and Forecasting (WRF) model was used to examine the sensitivities of Typhoon Fitow (2013) to the variations in horizontal mesh spacing ranging from 9 km to 1 km and to different microphysics schemes. The minimum sea level pressure decreased by 5 hPa and the maximum wind speeds increased by 20 m s−1 near the typhoon center as the horizontal grid spacing decreased from 9 km, 3 km, and 1 km in both Purdue-Lin and National Severe Storms Laboratory (NSSL) microphysics. The strengthening of the tropical cyclone may have been due to similar physical processes in the Purdue-Lin and NSSL simulations. Fine-resolution simulations produced localized and intense rainfall, in correspondence to localized and intense upward motions. Moreover, as the horizontal mesh spacing reduced from 9 km to 1 km, the distributions of the upward and downward motions broadened and the radar reflectivity bins with contour frequencies >10% increased. The simulated rainfall in the NSSL with a horizontal grid spacing below 3 km and in Purdue-Lin with a 1 km grid spacing reproduced spatial and temporal distributions similar to the actual observations. This suggested that the elaborate microphysics may compensate for the lack of horizontal resolution, to some extent. The precipitation budget analysis further suggested that the diminishment of rainfall was attributed to the reduced net condensation and hydrometeor convergence respectively in the Purdue-Lin and NSSL scheme as the mesh spacing reduced, which was further attributable to the decreased condensation or deposition.