Study of the three types of torrential rains of different formation mechanism during the meiyu period

Based on the formation mechanism and the analysis of observations, the torrential rainfalls during the meiyu-period from 1998 to 2007 could be assorted into three types, including the external forcing rainstorms, the self-organizing rainstorms and the non-organizing localized torrential rainfalls. The external forcing rainstorms are mainly related to a moving upper-level trough, or a moving-out Southwest vortex, or the weather pattern with a deep trough in north and a vortex in south. This kind of rainfall benefits from the lifting by the large scale circulation and the intrusion of the cold air from the middle troposphere. However, there are still differences among these rainstorm events, such as the interaction between low-level and upper-level jets as well as the way of impact of the cold trough on rainstorm. The self-organizing rainstorms usually persist for long life cycles due to the coherent regeneration processes, such as merging growth, up-stream and down-stream development, or new generation of mesoscale convective vortex. The low-level wind shear, moisture convergence zone and low-level jet are all favourable to this type of rainstorms. The non-organizing localized torrential rainfalls are usually found at noon over the mountain areas and the edge of sub-tropical high, as the direct results of convective instability. Since different formation mechanisms result in different types of meiyu period rainstorms, the corresponding difficulties to forecasting these rainstorms are not the same as well.