A Simulation Study of the Mesoscale Convective Systems Associated with a Meiyu Frontal Heavy Rain Event

In this study, evolution of the mesoscale convective systems (MCSs) within a Meiyu front during a particularly heavy rainfall event on 22 June 1999 in East China was simulated by using a nonhydrostatic numerical model ARPS (Advanced Regional Prediction System). Investigations were conducted with emphasis on the impact of the interaction among multi-scale weather systems (MWSs) on the development of MCSs in the Meiyu frontal environment. For this case, the development of MCSs experienced three di?erent stages. (1) The convections associ- ated with MCSs were firstly triggered by the eastward-moving Southwest Vortex (SWV) from the Sichuan Basin, accompanying the intensification of the upper-level jet (ULJ) and the low-level jet (LLJ) that were approaching the Meiyu front. (2) Next, a low-level shear line (LSL) formed, which strengthened and organized the MCSs after the SWV decayed. Meanwhile, the ULJ and LLJ enhanced and produced favorable conditions for the MCSs development. (3) Finally, as the MCSs got intensified, a mesoscale convective vortex (MCV), a mesoscale LLJ and a mesoscale ULJ were established. Then a coupled-development of MWSs was achieved through the vertical frontal circulations, which further enhanced the MCV and resulted in the heavy rainfall. This is a new physical mechanism for the formation of Meiyu heavy rainfall related to the SWV during the warm season in East China. In the three stages of the heavy rainfall, the vertical frontal circulations exhibited distinguished structures and played a dynamic role, and they enhanced the interaction among the MWSs. A further examination on the formation and evolution of the MCV showed that the MCV was mainly caused by the latent heat release of the MCSs, and the positive feedback between the MCSs and MCV was a key characteristic of the scale interaction in this case.