A Numerical Study of the Interaction between the Large-Scale Monsoon Circulation and Orographic Precipitation over South and Southeast Asia

A regional climate model is used to simulate the summer monsoon onset in South and Southeast Asia during the year 2000 to explore the interaction between orographic precipitation and the large-scale monsoon circulation. In the control run, the model uses the U. S. Geological Survey topography data and simulates the observed monsoon onset reasonably well. In the sensitivity tests, mountains are removed within different regions south of the Tibetan Plateau. It is found that the Indochina Peninsula monsoon onset is closely related to the local wind-terrain-precipitation interaction, while the Indian monsoon onset is more controlled by the large-scale land-sea thermal contrast. The sensitivity tests suggest two opposite effects of high terrain on the monsoon circulation and precipitation. When the terrain height is below the lifted condensation level (LCL), the low-level westerlies and the orographic precipitation weaken with increasing terrain height due to the surface drag effect. When the terrain height is above the LCL, the positive feedback associated with the diabatic forcing of orographic precipitation is dominant, and a large mountain height leads to heavier orographic precipitation and stronger low-level westerlies. The sensitivity tests also show that the impact of orographic precipitation in the Indochina Peninsula extends up to 30 degrees longitude upstream and affects monsoon precipitation along the western coast of India.