Evolution Characteristics of Mesoscale Convective System during the Formation of Tropical Cyclone Dujuan (1521)

With regard to tropical cyclone formation, few studies focus 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 study, the Final Operational Global Analysis (FNL) data and high-resolution numerical model are used to simulate typhoon Dujuan (No. 1521) three days before its formation, and the development and evolution characteristics of MCS in the formation process of Dujuan 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 low-level convergence are conducive to the formation of Dujuan. In the early stage of the formation of Dujuan, more MCSs are generated, which is favorable for the development of Dujuan from tropical disturbance to a tropical depression. In the later stage of the formation of Dujuan, with the occurrence of convective bursts, the MCSs merge, reducing its number and becoming more compact and the largest MCS shrinks towards the core region of the tropical disturbance. As a result, the formation of Dujuan is accelerated. The comparison of the characteristics of stratiform and convective precipitation in the MCSs indicates that the coverage of stratiform precipitation is the larger than that of convective precipitation, whereas 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 than that of stratiform precipitation. 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 the MCS. Their combined effect promotes the formation of Dujuan.