Impact of Sub‐Grid Turbulent Orographic Form Drag on the Summer Monsoon Precipitation Simulation Over China

Abstract Terrain has multi‐scale dynamic impacts on the climate. The Regional Climate Model version 4 (RegCM4) has simulation uncertainty partly due to insufficient description of sub‐grid orographic drag (SOD). In this study, the sub‐grid turbulent orographic form drag (TOFD) scheme has been adopted into the RegCM4 to better describe SOD. Results show that the overestimated near surface wind over rugged areas leads to non‐negligible biases of the summer monsoon precipitation simulation over China in the original RegCM4, whose skill in simulating the summer precipitation over the monsoonal regions of China can be remarkably improved by adopting the sub‐grid TOFD scheme. Compared to the original RegCM4, the Taylor scores (root mean square error) of the precipitation simulated by the RegCM4 with the sub‐grid TOFD scheme are increased (decreased) by 4%, 13%, 11%, and 7% (8%, 5%, 30%, and 16%) over the southeastern China, the mid‐lower Yangtze River Basin, the Huang‐Huai Basin, and the northeastern China, respectively. The improved precipitation simulation mainly comes from the atmospheric circulation simulation improved by adopting the sub‐grid TOFD scheme, which leads to cyclonic (anti‐cyclonic) wind differences over the southeastern (northeastern) China from the boundary layer to the mid‐troposphere and thereafter vertical motion differences in the middle troposphere, further resulting in the variation of the simulated precipitation intensity (heavy to torrential) and the convective precipitation and consequential improvement of summer monsoon precipitation simulation. This study indicates that the dynamic impact of micro‐scale terrain plays an important role in the summer monsoon precipitation simulation.