四川盆地极端暴雨过程基本特征分析

Characteristics Analyses of Extreme Rainstorm Events in Sichuan Basin

本文利用实况观测资料和NCEP再分析资料,选取1981-2015年四川盆地出现的23次极端暴雨天气个例,分析了其基本气候特征、主要环流形势、影响系统及中尺度对流环境条件,结果表明:(1)大多数极端暴雨都出现在持续性暴雨过程中,且极端暴雨出现前至少12 h已开始出现暴雨,暴雨中心主要出现在盆地西北部和西南部。(2)极端暴雨过程主要出现在500 hPa为"东高西低"型和"两高切变"型这两种环流背景形势下,"东高西低"型过程前24 h内副热带高压将西伸北抬,过程中仍保持稳定甚至会继续西伸北抬,而"两高切变型"过程前24 h内和暴雨过程中,副热带高压动态均无明显规律。(3)有3次极端暴雨过程有登陆台风,其外围环流形成的强水汽输送对暴雨有直接影响,6次过程有远距离海上台风向西或向北移动,对盆地内降水系统东移有一定的阻挡作用,利于强降雨维持。(4)"东高西低"型暴雨主要触发系统是西南低涡和高原低涡,"两高切变"型暴雨主要触发系统是切变线,且700 hPa有冷平流入侵,两种类型暴雨在200 hPa均为南亚高压东北侧的分流辐散区,暴雨中心均位于低层高比湿区和辐合中心,其中"东高西低"型暴雨低层偏南气流更强,暴雨中心主要位于盆地西北部,而"两高切变"型暴雨低层偏南气流更弱,暴雨中心位于盆地西南部的频次更高。(5)极端暴雨过程具有低层高比湿、整层相对湿度大、暖云层厚、CAPE呈狭长形态、垂直风切变小等特征,因此降水效率高,同时850 hPa比湿和假相当位温具有显著正距平,过程结束后850 hPa假相当位温明显下降。并据此建立了四川盆地极端暴雨概念模型,可供今后极端性过程的预报参考。

An analysis of the climatic characteristics, circulation situation, impact system and mesoscale convective environmental conditions in 23 extreme rainstorm events in Sichuan Basin during 1981--2015 is performed by using conventional data and NCEP/NCAR reanalysis data. It is found that- （1） most ex- treme rainstorm events occur in continuous rainstorm processes, and rainstorms start ahead of the extreme storms, with rainstorm centers mainly in the northwest and southwest of Sichuan Basin. （2） The extreme rainstorm events occur mainly under the 500 hPa circulation background of ＂east-high pressure and west- low-pressure＂ and ＂shear between two high-pressure＂. Subtropical high would stretch toward northwest in 24 h during the rainstorm process in the ＂east-high-pressure and west-low-pressure＂ pattern. But subtropi- cal high acts irregularly during the rainstorm process in the type of the ＂shear between two high-pres- sure＂. （3） Three extreme rainstorm events occur with landing typhoon, and the strong water vapor trans- portation formed by the outer circulation of typhoon has a direct impact on the heavy rain. Six extreme rainstorm events occur because of the blocking effect of the long-distance sea typhoon moving to west or north, and it is conducive to the maintenance of the heavy rain. （4） The main trigger system of the ＂east- high-pressure and west-low-pressure＂ is the southwest vortex and the plateau vortex while the main trigger system of the ＂shear between two high-pressure＂ is the shear line, and the 700 hPa level has cold advection invasion. Two types of rainstorms occur on the divergence zone of the northeastern part of South Asia high pressure at 200 hPa, and the rainstorm center is located in the low-level high specific humidity area and convergence center. The stronger southerly airflow in the type of ＂east-high pressure and west-low-pres- sure＂ makes the rainstorm center located in the northwest of the basin, and the weaker southerly airflow in the type of ＂shear between two high-pressure＂ causes the rainstorm center to stay in the southwest of the basin. （5） The characteristics of high specific humidity in the low level, high relative humidity in the whole layer, thick warm clouds, long and narrow CAPE, small vertical wind shear, high efficiency precip- itation, and positive anomalies of 850 hPa humidity and pseudo-equivalent temperature appear during the extreme rainstorm processes. And the 850 hPa pseudo-equivalent potential temperature significantly de- creases after the rainstorm. Based on the above, the conceptual model of extreme rainstorm in Sichuan Ba- sin has been built, and it would benefit the operation of prediction.