Mechanisms for Torrential Rain Associated with the Mei-Yu Development during SCSMEX 1998

Abstract

A case of torrential precipitation associated with the mei-yu front, an Asian summer monsoon system east of the Tibetan Plateau, is studied using the coupled fifth-generation Pennsylvania State University?NCAR Mesoscale Model (MM5) and the NASA Goddard Space Flight Center Parameterization for Land?Atmosphere?Cloud Exchange model. The impact of both remote and local sources of water vapor on the location and intensity of mei-yu precipitation are studied by numerical experiments. The results demonstrate that the main source of water vapor for this heavy precipitation event over the Yangtze River Valley is the Bay of Bengal. Moisture is transported by a southwesterly low-level jet (LLJ) southeast of the Tibetan Plateau. Although the LLJ is largely manipulated by large-scale forcing, the mesoscale circulation that results from mei-yu condensational heating acts to increase the maximum wind speed of the jet. The condensation-induced local circulation strengthens the moisture transport in the LLJ, providing a positive feedback that sustains the mei-yu precipitation system. Precipitation recycling increases the total precipitation in the Yangtze River Valley only slightly, but it tends to shift the maximum rainfall center toward the warmer side of the mei-yu rain belt. This shift is due to the pronounced increase in atmospheric moisture and decrease in surface temperature over the warm side of the rain belt.