Center for Monsoon System Research, Institute of Atmospheric Physics, Chinese Academy of Sciences
As for tropical cyclone formation, there are few studies focusing on the transition from a tropical disturbance to a tropical depression, which is accompanied by the formation, merger, and development of the mesoscale convective system (MCS). In this paper, FNL data and high-resolution numerical model are used to simulate the No. 1521 typhoon Dujuan three days before its formation, and the development and evolution characteristics of MCS in the process of Dujuan formation are preliminarily explored. During the formation of Dujuan, the tropical upper tropospheric trough (TUTT) was located farther east of it, and Dujuan migrated northwestward from the southeastern edge of the monsoon gyre (MG). The decreased vertical wind shear and MG-related lower-level convergence are conducive to the formation of Dujuan in the early stage of the formation of Dujuan, more MCSs generated favorable for the development of Dujuan from the depression disturbance. In the later stage of the formation, as the convection bursts, the MCSs merge to reduce the number of MCSs become more compact. The largest MCS shrinks towards to the core region of the tropical disturbance. As a result, the formation of Dujuan is accelerated, which is accompanied by a sudden increase in the vorticity at the low level, resulting in the deep convection in the tropical disturbance. The comparison of the characteristics of stratiform and convective precipitation in the MCSs indicates the coverage of stratiform precipitation is the largest, while the precipitation rate of convective precipitation is larger than that of stratiform precipitation and the variation of the coverage of convective precipitation is more significant. The increased percentage of stratiform precipitation is closely related to the intensification of Dujuan, and the increase in the convective precipitation rate is beneficial to the intensification of MCS. Their combined effect can promote the formation of Dujuan.