Impacts of the Thermal Effects of Sub-grid Orography on the Heavy Rainfall Events Along the Yangtze River Valley in 1991

A P-σregional climate model using a parameterization scheme to account for the thermal effects of the sub-grid scale orography was used to simulate the three heavy rainfall events that occurred within the Yangtze River Valley during the mei-yu period of 1991.The simulation results showed that by considering the sub-grid scale topography scheme,one can significantly improve the performance of the model for simulating the rainfall distribution and intensity during these three heavy rainfall events,most especially the second and third.It was also discovered that the rainfall was mainly due to convective precipitation.The comparison between experiments,either with and without the sub-grid scale topography scheme,showed that the model using the scheme reproduced the convergence intensity and distribution at the 850 hPa level and the ascending motion and moisture convergence center located at 500 hPa over the Yangtze River valley. However,some deviations still exist in the simulation of the atmospheric moisture content,the convergence distribution and the moisture transportation route,which mainly result in lower simulated precipitation levels.Further analysis of the simulation results demonstrated that the sub-grid topography scheme modified the distribution of the surface energy budget components,especially at the south and southwest edges of the Tibetan Plateau,leading to the development and eastward propagation of the negative geopotential height difference and positive temperature-lapse rate difference at 700 hPa,which possibly led to an improved precipitation simulation over eastern China.