Impacts of Subgrid Orographic Drag on the Summer Monsoon Circulation and Precipitation in East Asia

This work is an investigation of the impacts of three different components of parameterized subgrid orographic drag (SOD) on the summer monsoon circulation and precipitation in East Asia, that is, flow-blocking drag (FBD), low-level gravity wave drag (GWD), and upper-level GWD. Using the global Weather Research and Forecasting model, a series of numerical experiments are conducted with different configurations of SOD parameterization. In the case of no SOD, the simulated lower tropospheric winds are too strong in an elongated region extending from East China to the east of Japan, accompanied with excessive rainfall. The intensity of the western Pacific subtropical high is overestimated, thus suppressing the precipitation over the subtropical ocean. With the parameterization of SOD, the circulation and precipitation biases are alleviated, depending on the geographical and vertical distribution of SOD. The FBD is very weak and only contributes to the circulation in eastern East Asia. Contrastingly, there is remarkable low-level GWD in the lower latitudes which helps reduce the low-level wind biases in South China. The upper-level GWD only occurs in the midlatitudes, preferentially above the subtropical jet. It can not only decelerate the winds in the midlatitudes but also reduce the southwesterlies in East China through the interaction with the upper-level jet. Alleviation of precipitation bias over the land of East Asia is primarily due to the low-level GWD which weakens the water vapor flux. In contrast, the upper-level GWD has the greatest contribution to the enhancement of precipitation over the subtropical ocean by increasing the low-level lifting.