北京“7.21”暴雨的不稳定性及其触发机制分析  

Analysis on the Instability and Trigger Mechanism of Torrential Rainfall Event in Beijing on 21 July 2012

作　　者：刘璐[1,2] 冉令坤[1] 周玉淑[1] 高守亭[1] 

机构地区：[1]中国科学院大气物理研究所,北京100029 [2]中国科学院大学,北京100049

出　　处：《大气科学》2015年第3期583-595,共13页Chinese Journal of Atmospheric Sciences

基　　金：中国科学院重点部署项目KZZD-EW-05;国家重点基础研究发展计划(973计划)项目2013CB430105;国家自然科学基金项目41175060;41375052

摘　　要：本文利用WRF模拟的高分辨率资料对2012年7月21日北京特大暴雨过程的对流不稳定和条件对称不稳定性及其触发和维持机制进行了诊断分析.分析结果表明：（1）在临近暴雨发生时刻及暴雨初期,大气低层主要以对流不稳定为主,随后对流触发,不稳定性减弱,而低空急流和湿斜压性的增强,使得条件性对称不稳定加强,维持和加强了暴雨的不稳定性.（2）分析表明,在暴雨过程中主要由于较强的水平风的垂直切变造成湿位涡的斜压分量异常,从而导致条件性对称不稳定的产生.（3）本文分别对暴雨发生过程中的对流不稳定与条件对称不稳定的触发机制进行了分析,主要结论如下：暴雨初期对流性降水阶段,切变线上有利的垂直上升环境与地形的强迫抬升相互配合,触发了对流性降水.另外,北京上空的干冷空气入侵,也增强了大气的对流不稳定性,更易触发对流;对称不稳定导致的降水阶段,主要是由于北京上空冷暖空气的长期对峙,冷空气逐渐深入到暖湿空气下方,使得暖湿气团沿冷气团爬升,从而触发对称不稳定,造成持续性降水.此次暴雨过程中0900～1300 UTC时刻暴雨增幅的重要原因是0900 UTC北京风向突变,转为偏东风,且风速骤增,北京西北侧的喇叭口状的地形的强迫抬升作用,与上空750 hPa移来的切变线上的垂直运动相互叠加,形成中尺度涡旋,产生了强烈的上升运动,触发不稳定,产生大暴雨.We used simulated data of high spatio-temporal resolution to analyzing the convective instability, conditional symmetric instability, and triggering mechanisms of a torrential rainfall event occurring in Beijing on July 21, 2012. The results indicate that convective instability played the leading role at the initial time of the precipitation. This instability weakened with the occurrence of the heavy rainfall, and conditional symmetric instability was enhanced by the increase in moist baroclinicity and the low-level jet, which maintained and strengthened the subsequent precipitation. Moreover, during the process of precipitation, strong vertical wind shear caused the baroclinic component of the moist potential vorticity anomaly, thus leading to the generation of conditional symmetric instability. Further, during the initial rainfall of the convective instability stage, the terrain lifting force combined with the ascending air on the shear line to boost and stir up the convective instability. In addition, dry air invaded Beijing at the midlevel, which contributed to the convective precipitation. The conditional symmetric instable precipitation resulted in a long-term clash between the cold and warm air over Beijing, which gave rise to the persistent precipitation. The warm air was lifted up by the cold air, which triggered the conditional symmetric instability. Moreover, at 0900 UTC, the wind shifted to an east wind and was abruptly enhanced. The wind was then lifted by the terrain coupled with the ascending air on the shear line, causing intensive upward motion and agitated instability, which is the primary reason for precipitation enhancement during 0900-1300 UTC.

关 键 词：暴雨 对流不稳定 条件对称不稳定 地形 冷空气入侵 

分 类 号：P445[天文地球—大气科学及气象学]