Genesis and development of spring rainstorms in Northern Southeast Asia: Southwest China-Northern Indochina and the Northern South China Sea

During May and June, the monsoon rainfall in northern Southeast Asia is primarily produced by rainstorms. At the mature stage, these storms, coupled with a midtropospheric subsynoptic-scale trough, produce rainfall ≥50 mm (6 h)^-1 and exhibit a cyclonic surface vortex. With a scale ~O(10²) km, rainstorms during the period of 1979-2016 are identified with station and satellite observations, along with assimilation data. Several dynamic processes of rainstorm geneses are disclosed by an extensive analysis. 1) Maximum occurrence of rainstorm geneses is located in the midtroposphere of two regions (northern Vietnam-southwestern China and the northern South China Sea), but eventually penetrates downward to the surface. 2) The environment favorable for rainstorm genesis is a southwest-northeast-oriented narrow trough formed by the confluence of the midtropospheric northeasterly around the eastern Tibetan Plateau and the lower-tropospheric monsoon southwesterlies. Because the criterion for Charney-Stern instability is met by the shear flow of this narrow trough, rainstorms are likely initiated by this instability. 3) The majority of rainstorm geneses occurs during the evening over the land and the morning at sea. This timing preference is caused by the modulation of the clockwise rotation of the East Asia continent circulation in response to the diurnal variation of the land-sea thermal contrast. These new findings from this study offer not only a new perspective for the genesis mechanism of the late spring-early summer rainstorms in northern Southeast Asia, but also a new initiative to develop medium-range forecasts for these rainstorms.