Abstract:

The regional cloudburst in Qingyang on July 21，2012 was the most severe rainfall event since the meteorological station of
Huanxian County was established in 1957，which resulted in a significant property loss and people casualty． Thus，based on the con-
ventional data from the automatic station and the historical similar cases of cloudburst，detailed analysis and investigation were carried
out in this paper． The results are as follows: accompanied with the surface cold front moving eastward，the speed of the upper － level
trough blocked by the west Pacific subtropical high was slowed down，and the cold air over Lake Baikal was abundantly accumulatedat
between two high pressures，which established the foundation for the severe rainstorm process． Before the typhoon“Vicente”landing，
due to the presence of the strong pressure gradient between typhoon depression and subtropical high，the southward low － level jet lead-
ing to Qingyang area was established and enhanced，which provided a plenty of water vapor for the cloudburst． Under the influence of
positive vorticity advection in front of the upper trough，there was obvious subtropical convergence in the middle and lower troposphere，
which consequently formed violent ascending motion． The ascending motion of warm and humid air from the low － level triggered the ex-
plosion of unsteady energy at the convection，and further formed local severe convection and heavy rain． At last，the mesoscale convec-
tive systems that caused the regional cloudburst in Qingyang originated from the development of lower － level vortex in Hetao． When the
mesoscale vortex moved to the east and northeast，its scale was distinctly enlarged and the lower － level of the whole system exhibited
obvious cyclonic shear and cyclonic vorticity． Consequently，the eastern moving and enhancement of the entire mesoscale convective
systems caused the regional cloudburst in Qingyang under suitable environment．

Key words:  cloudburst, low － level jet stream, meso － microscale systems, long distance influence of the typhoon, Qingyang

摘要：

庆阳市 2012 年 7 月 21 日区域性大暴雨创造了环县 1957 年建站以来的历史极值，导致了人民财产的重大损失。本文利用常规资料、自动站等探测资料以及历史相似个例，对 2012 年 7 月 21 日庆阳市出现的区域性大暴雨天气进行分析。结果表明:(1)高空低槽伴随地面冷锋东移，受西太平洋副热带高压的阻挡移动缓慢，副高、大陆高压不断加强，造成贝加尔湖冷空气在两高之间大量堆积，为这场大暴雨发生奠定了基础;(2)2012 年第 8 号台风“韦森特”登陆前，台风低压和副热带高压之间形成的强气压梯度导致通向庆阳市的南风低空急流建立并加强，为此次大暴雨的发生提供了充分的水汽条件;(3)在高空槽前正涡度平流的作用下，对流层中低层出现明显的辐合，产生强烈上升运动，低层暖湿气流抬升促使对流不稳定能量爆发，从而形成局地强对流和暴雨;(4)导致庆阳区域性大暴雨的中尺度对流系统起源于河套地区低层涡旋的发展，在中尺度涡旋向东北和向东扩展过程中，尺度明显增大，整个系统低层具有明显的气旋性切变和气旋性涡度，在有利的环境条件下整个中尺度对流系统的东移和发展壮大，最终导致庆阳市区域性大暴雨的发生。

关键词: 大暴雨, 低空急流, 中小尺度系统, 台风远距离影响, 庆阳市