A Synoptic Overview of a Heavy Rain Event in Southern China

Abstract

The meteorological events surrounding the heavy rains of 23?25 June 1983 in the Yangtze River Valley of China are investigated. The rains developed along a persistent quasi-stationary frontal boundary that separated warm, moist tropical air from slightly cooler continental air. The frontal zone was characterized by a thermally direct circulation driven by horizontal confluence in the lower troposphere. Aloft, the flow was broadly anticyclonic. A mobile 500 mb short-wave trough moved eastward in the westerlies north of the Quinghai-Xizang Plateau. In its wake, cooler and drier Siberian air was funneled southward, east of the Plateau, to reinforce the baroclinic zone. A very weak short-wave trough also moved eastward in the much weaker 500 mb monsoon westerlies south of the Plateau. The arrival of the cooler air from the north and the weak trough from the southwest invigorated a preexisting cyclonic circulation over the Sichuan Basin. A massive mesoscale convective system erupted in a complex interaction of these features with the local topography. The generation of a midlevel cyclonic vorticity maximum by the mesoscale convective complex appeared to be comparable to what has been seen in similar cases over North America. Warm air advection in the lower troposphere appeared to help trigger the rainfall through weak synoptic scale ascent of 1?2 cm s?1. An analysis of the water vapor flux revealed the South China Sea as a moisture source in the surface-to-850 mb layer, and the Indian subcontinent-Bay of Bengal region as the moisture source in the 850?500 mb layer.