“7.21”北京特大暴雨的湿位涡诊断分析

Diagnostic Analysis of Moist Potential Vorticity for‘7.21'Beijing Rainstorm

为了探讨湿位涡场的特征与北京"7.21"暴雨关系,利用NCEP/NCAR再分析资料集,对"7.21"北京特大暴雨过程进行湿位涡诊断分析。结果表明:对流层中高层的MPV1高值中心的冷空气叠加在对流层中低层负的MPV1之上,对北京上空位势不稳定能量的储存和释放十分有利,为北京暴雨的发展提供了有利的条件。北京强降水与MPV1的负值中心和MPV1的密集带密切相关。北京强降水期间与MPV2正值区相对应,其降水区的移动方向与MPV2正值中心的移动方向一致,而负MPV2的向下伸展程度,可能与北京降水强度也密切相关。MPV2的负值区走向还与锋面走向非常一致。暴雨区大气不稳定形成的机制是高低空系统的耦合配置下,暖湿空气的上升运动触发了对流层低层暴雨区对流不稳定能量释放,使得上升运动得到加强,虽在对流层中高层为大气对流稳定层结,却满足条件对称不稳定层结,从而使气流继续倾斜上升到对流层上层,维持了北京强降水的发生。

In order to explore the relationship between moist potential vorticity field characteristics and Beijing ‘7.21＇heavy rainstorm, using NCEP/NCAR reanalysis data, the author conducted a diagnostic analysis of moist potential vorticity for the‘7.21＇torrential rain process in Beijing. The results showed that： the favorable configuration that the cold air in high MPV1 value center of the middle and high level troposphere superimposed on the negative MPV1 of the middle and low level troposphere, which was very beneficial for the potential instability energy storage and release over Beijing region, and the development of the heavy rainfall in Beijing. The strong precipitation was closely related to the negative center of MPV1 and intensive band of MPV1. The intense precipitation in Beijing correspond with the positive area of MPV2, and the direction of movement of its precipitation zone coincided with the direction of movement of the center of positive MPV2. While the negative MPV2 downward extension degree, might be closely related to the Beijing precipitation intensity. Negative MPV2 region was also consistent with frontal direction. The mechanism of the formation of atmospheric instability over heavy rain area was that the high and low level systems coupled configuration, the warm air vertical motion triggered the convective instability energy release, and the ascending motion had been strengthened. The atmospheric convective stability in middle and high level troposphere, while the conditional symmetric instability stratification in the high and low level troposphere. So that the airflow tilted up to upper level troposphere, and maintained the strong precipitation in Beijing.