A Numerical Study on the Wind-Terrain Interaction in the South and Southeast Asian monsoon region

Numerical simulations with a regional climate model are carried out to explore the roles of wind-terrain interaction in the circulation and precipitation in the South and Southeast Asian monsoon regions. In the control run, heavy precipitation is anchored on the windward side of the terrain, which is consistent with observations. In the sensitivity tests, topography is systematically removed, and precipitation spreads over a broad area, but maximum precipitation is reduced and the westerly monsoon flow is weakened. Analysis of the results suggests a positive interaction between the convection induced by the wind-terrain interaction and the westerly monsoon flow.

The control run simulates the gross features of the observed monsoon transition reasonably well. The summer monsoon onset first occurs in the Indo-China Peninsula in late April and early May. It then extends westward to the Bay of Bengal and eastward to the SCS in mid May, and the Indian monsoon onset occurs in late May. In the sensitivity tests, as topography is removed, the monsoon onset over the Indo-China peninsula is significantly delayed while the onset time of the Indian monsoon is nearly unchanged. This suggests that the monsoon onset over the Indo-China peninsula is triggered by the wind-terrain interaction while the Indian monsoon onset is mainly controlled by the land-sea thermal contrast.