Impact Of East Asian Extratropical Cyclones On Heavy Precipitation Event

In this paper,the influence of extratropical cyclones on precipitation in East Asia during1979-2018 is systematically analyzed,including the relationship between cyclone intensity and precipitation intensity,spatial distribution characteristics of cyclone and precipitation,and cyclone classification combined with precipitation.Based on the classification results,the circulation characteristics and influencing factors of heavy precipitation events related to heavy cyclones are analyzed.Since subsynoptic-scale cyclones occur most frequently in the Jiang-Huai region in spring,this paper analyzes the reason of precipitation increase in the Jiang-Huai region due to the occurrence of subsynoptic-scale cyclones.The following results are obtained:（1）Compared with the average cyclone depth,central wind and other intensity indexes,the maximum product of standardized wind speed and specific humidity(vq_max)in the two-dimensional cyclone area is the strongest correlation between cyclone comprehensive intensity index and precipitation intensity（r=0.74）.In order to simplify the definition of cyclone intensity index,vq_max within 300km from the cyclone center is selected to characterize the overall cyclone intensity,and based on this index,the East Asian extratropical cyclones are divided into four grades.With the increase of cyclone grade,its corresponding precipitation intensity also increases significantly.In particular,32.8%of heavy rainfall events in East Asia in summer are associated with category 4 severe cyclones.（2）Based on the cyclone classification results,the circulation characteristics and influencing factors of heavy precipitation events related to quaternary severe cyclones are analyzed.It is found that there are great differences in the horizontal scales of the quaternary severe cyclones that cause heavy precipitation,which are divided into sub-synoptic scales and synoptic scales.There are more sub-synoptic-scale cyclones（68%）among the four severe cyclones related to heavy precipitation,but on average,synoptic-scale cyclones have stronger intensity and longer life.Among them,the major precipitation areas associated with the sub-synoptic scale（synoptic scale）4 strong cyclones are mainly located in the southwest and northeast of Jianghuai（eastern coastal areas）,and the major precipitation areas in the upper and lower troposphere show convergence at low level and divergence at high level,which provide water vapor and dynamic support for the occurrence of heavy precipitation events.In addition,the source and moving speed of Rossby wave train corresponding to the four strong cyclones of different scales are different,and the influence degree of potential vortex anomalies at different levels of troposphere caused by different reasons on each stage of cyclone development is also significantly different.（3）Subsynoptic-scale cyclone activity contributes more than 25%to spring precipitation over the Jianghuai region,and cyclone activity correspondes to significant cyclonic vorticity and convergence movement in the lower troposphere,which provides favorable dynamic conditions for local precipitation intensification.The interannual variation of subsynoptic-scale cyclone intensity is positively correlated with the precipitation anomaly and the frequency of heavy precipitation in the southeast of Jianghuai area in spring.When the sub-synoptic-scale cyclones intensify over the Jianghuai region,an abnormal inverted trough circulation appears from the Tibetan Plateau to the middle and lower reaches of the Yangtze River in China,which is conducive to the formation of cyclonic circulation anomalies over the Jianghuai region.At the same time,the barotropism of lower troposphere is enhanced with the enhancement of warm and humid air flow in southwest China,which promotes the intensification of sub-synoptic-scale cyclone activity in Jianghuai region.The abnormal heating of the air column over the Qinghai-Tibet Plateau in spring resultes in the decrease of the lower tropospheric geopotential height,and the southwesterly winds are strengthened,which lead to the intensification of sub-synoptic-scale cyclone activities over the Jianghuai region.