Orographic Effects of the Tibetan Plateau on the Rainfall Variations Over Central China During the 1979 Summer1

Abstract

The nature of disturbance activity during the summer monsoon over East Asia was investigated using FGGE Level IIIb data from 1 May to 31 July 1979. This 92-day period was divided into two 46-day subperiods, i. e., Period I(1 May-15 June), and Period II (16 June31 July). To facilitate further study, rainfall variations over the Yangtze River basin were taken as a reference. The designation day "0" was used for days when rainfall peaks occurred over the Yangtze River basin. By this definition it is only natural to find a prominent cyclone located over central China on day 0, which is referred to as the "Period I China cyclone" and the "Period II China cyclone", for each subperiod, respectively.
The northeastern edge of the Tibetan Plateau is the birthplace of Period I China cyclone. A small-scale low-level cyclone trapped below 700mb is induced by the mechanical (not thermal) effect of the Tibetan Plateau on day -2. This edge cyclone then propagates south- ward along the eastern periphery, reaching the upper reaches of the Yangtze River by day 0. Between days +1 and +2, the low-level edge cyclone merges with an eastward propagating upper trough. Associated with this merged cyclone is widespread rainfall over central China and Japan.
The central Tibetan Plateau is the birthplace of the Period II China cyclone. At the initial stage on day -2, its character resembles the soalled "Tibetan vortex" with a vertical depth of 1km or less. It propagates eastward across the Tibetan Highland, eventually merging with a major mid-latitude cyclone in the vicinity of the Yellow River basin between days 0 and +1. At this time, the merged cyclone becomes coherent in the vertical and extends as high as 250mb. Thus, the Period II China cyclone experiences a dramatic change in its vertical structure from the incipient stage over the Tibetan Highland to the mature stage over the Yellow River basin.