1521号台风“杜鹃”生成阶段中尺度对流系统和降水演变特征

在热带气旋的生成过程中，从热带扰动向热带低压增强的过程研究较少，而这一过程必定伴随着中尺度对流系统（MCS）的生成、合并和发展。本文利用FNL分析资料和高分辨率模式对1521号台风“杜鹃”生成前3天进行数值模拟，着重探讨了“杜鹃”生成过程中MCS的发展演变特征。在“杜鹃”生成过程中，对流层上部槽（TUTT）位置偏东，“杜鹃”主要从季风涡旋（MG）的东南侧边缘向西北方向移动，因此，减弱的大尺度垂直风切和MG东南侧的低层辐合都为“杜鹃”的生成提供了有利的动力条件。在“杜鹃”生成前期阶段，新生成更多的MCS有利于在热带扰动阶段“杜鹃”的增强；“杜鹃”生成的中后期阶段，对流爆发，MCS发生合并使得MCS个数减少，MCS结构变得更加紧凑，面积最大的MCS逐渐向热带扰动中心区域收缩，“杜鹃”生成速度加快。对比MCS中的层云降水和对流云降水的特征发现，MCS中层云降水覆盖面积更大；对流云降水的降雨率比层云的降雨率更大，同时变化幅度更明显。层云降水百分比的增加与“杜鹃”增强的过程关系密切，但对流云降水的降雨率增加有利于MCS增强，层云和对流云的共同作用促进了“杜鹃”生成。

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.