Moist orographic gravity wave drag parameterization reduces the bias of summer rainfall over the Tibetan Plateau

Summer precipitation over the Tibetan Plateau (TP) significantly affects the regional and global climate. In this paper, the orographic gravity wave drag (OGWD) parameterization is revised by incorporating the moisture effect of saturated air into the Weather Research and Forecasting model. Seasonal simulations are performed for the summer rainfall over the TP during 2003–2023 employing the original and modified OGWD schemes. The results indicate that the rainfall and thus the wet biases over the southeastern TP are reduced when the moist OGWD scheme is employed. The moisture effect first enhances the lower-tropospheric OGWD over the Indian Peninsula, thereby weakening the low-level winds. The decrease in the horizontal wind speed leads to a reduction of the surface gravity wave momentum flux and thus the OGWD, which in turn strengthens the low-level flow. The latter process initiates intensification of the monsoon circulation over the Bay of Bengal (BOB). During mid-to-late summer, the monsoon circulation is continuously intensified through the circulation–precipitation interaction. The enhanced cyclonic circulation over the BOB induces an easterly flow to the south of the TP, which is opposed to the climatological southwesterly monsoon circulation. Therefore, the water vapor transport toward the TP is decreased.