一次多弓状中尺度雨带的成因机理及其与水平涡度的关系

用WRF中尺度数值模式、NCEP/NCAR再分析资料、多普勒雷达观测资料等,对2018年5月5日发生在我国华中地区的一次多弓状雨带降水过程的形成机理及其与水平涡度的关系进行分析发现:雨带发生在切变线南侧的西南气流中,多弓状雨带出现前,大尺度高低层气旋式曲率的水平涡度和对流有效位能为降水提供了有利于上升运动的背景场。弓状雨带最初形成在对流不稳定和低层气流辐合条件下,局地强降水引发的下沉运动使中低层大风出现,大风中心南侧反气旋式的环流与背景场中的西南气流汇合构成了短波槽,尾部雨带出现在短波槽中,弓状头部生成于北侧的气旋式风场切变中,大风中心相较南北两侧更快的移速使雨带中部向前侧凸起。流场上的短波槽发生在700 hPa以下,在西南气流的背景场下,该槽向前后两侧的双向传播是多弓状雨带形成的重要触发因子。中尺度弓状雨带附近低层的水平涡度在强盛期、减弱期和消散期有着明显不同的结构特征;而在高层,雨带发展时和强盛期都为气旋式水平涡度控制,减弱期多为反气旋式水平涡度控制。

THE RELATIONSHIP BETWEEN THE FORMATION MECHANISM OF A MULTI-MESOSCALE-BOW-RAINBAND AND HORIZONTAL VORTICITY

A multi-bow-rainband process in central China on May 5, 2018 was simulated using WRF mesoscale numerical model, NCEP/NCAR reanalysis data and Doppler radar observation data. The relationship between the formation of multi-bow-rainband and horizontal vorticity was studied. It is found that these rainbands occurred in the southwest airflow on the south of the shear line. Before the multi-bow-rainband emerged, the large-scale clockwise-rotating horizontal vorticity in the high and low level and convective effective potential energy provided a favorable background for precipitation. The bow-rainband was originally formed under the condition of convective instability and low-level airflow convergence. The gale in the middle and low level appeared as a result of downward motion caused by local heavy precipitation. The anticyclonic circulation on the south side of low-level gale center merged with southwest airflow in the background field, forming a short-wave trough where the tail of rainband emerged. The head of the bow-rainband generated in the cyclonic circulation on the north side of gale center. The gale center moved faster than the north and south side of the gale, inducing the middle of the rainband convex to move forward. The short-wave trough of airflow field occurred below 700hPa in the background of south-west airflow, and the two-way propagation (forward and backward) of this trough was an important trigger for the formation of multi-bow-rainband. The horizontal vorticity in the low level near the mesoscale-bow-rainband had different structural characteristics during the strong, weak and dissipative periods of the mesoscale-bow-rainband. In the high level, horizontal vorticity was cyclonic in the developing and strong periods of rainbands, whereas it was anticyclonic in the weak period of rainbands.