2012年7月21日北京特大暴雨的多尺度特征

本文采用观测和NCEP分析场资料对2012年7月21日发生在北京地区的特大暴雨过程的天气形势、水汽来源和中尺度对流系统的特征进行了研究。结果如下:“7.21”北京暴雨过程是高低空与中低纬系统共同配合的结果,暴雨发生在“东高西低”的环流形势下,低涡、切变线、低槽冷锋和低空急流为此次过程的主要影响天气系统；孟加拉湾至西太平洋地区热带辐合带(ITCZ)活跃,其中热带气旋的活动有利于水汽向东亚大陆输送,此次暴雨过程中华北地区的水汽源地包括孟加拉湾和我国东部的渤海、黄海等,低层的水汽主要来自东部,中层的水汽主要来自孟加拉湾；北京的强降雨有两段,第1段降雨虽然发生在冷锋前,但有明显冷空气的侵入,并与地形和东风的作用有关,第2段降雨对流的组织和增强与冷锋强迫相关。在有利环境下,中尺度对流系统频繁发生发展,持续时间长,且稳定少动是此次特大暴雨形成的重要原因。

Multi-Scale Characteristics of Record Heavy Rainfall over Beijing Area on July 21, 2012

Observational and National Centers for Environmental Prediction (NCEP) analysis data are used to diagnose the characteristics of synoptic circulations, sources of water vapor, and mesoscale convective systems (MCS) related to the formation of heavy rainfall on July 21, 2012. Results indicate that the locations and intensities of upper- to lower-level synoptic circulations and mid- to low-latitude systems are favorable for the heavy rainfall formation over North China that occurred under typical weather patterns with high pressure to the east and low pressure to the west. Synoptic systems such as vortices, shear lines, troughs, cold fronts, and low-level jets affect the formation and intensity of heavy rainfall. Tropical cyclones in the Bay of Bengal and the western Pacific Ocean provide favorable conditions for the transportation of water vapor from the ocean to the mainland. The sources of water vapor to North China are the Bay of Bengal, the Bohai Sea, and the Yellow Sea. Water vapor in the lower troposphere is mainly transported from the latter two sources, and water vapor in the middle troposphere is mainly transported from the former. The entire heavy rainfall process involves two stages. During the first stage, the triggering of MCS is associated with cold air invasion, terrain, and easterly wind in the prefrontal area. During the second stage, the organization and intensification of MCS are forced by the cold front. Under favorable synoptic circulations, the frequent development of stable and persistent MCS is important for the formation of the heavy rainfall event examined in this study.