Objective Identification And Cause Analysis Of Regional Heavy Rainfall Events During The Meiyu Period In The Middle And Lower Reaches Of The Yangtze River

The Objective Identification Technique for Regional Extreme Events(OITREE)and the daily precipitation data of 2374 stations in the middle and lower reaches of the Yangtze River(MLYR)were used to objectively identify the regional heavy rainfall events(RHREs)during the Meiyu period in the MLYR from 1961 to 2020.There are 257 RHR.Es in total in the MLYR.The RHREs during the Meiyu period is an important representative of summer precipitation in the MLYR:The RHREs in the MLYR mainly occur in summer,especially from June to July.The rainstorm precipitation during the Meiyu period accounts for about 45%of the total precipitation during the Meiyu period,and the accumulated precipitation of the RHREs during the Meiyu period accounts for about 56%of the rainstorm precipitation during the Meiyu period.These three indicators have increased significantly in the past 60 years(P<0.05).The duration of the RHREs in the MLYR is mainly 2 to 3 days,and the longest is 9 days.The cumulative intensity is mainly between 2000 and 4000 mm,and the cumulative area is about(20-50)× 104 km2.The RHREs in the MLYR mainly occurred in the east of Hubei,the south of Anhui,and the northeast of Jiangxi.The frequency and cumulative intensity of the RHREs have increased significantly in the past 60 years(P<0.05).The four indicators of extreme intensity,cumulative area,maximum area,and duration also showed a significant increasing trend(P<0.1).The frequency of RHREs and the five indicators all show obvious interdecadal changes.From the 1960s to the late 1980s,the RHREs was relatively rare.It showed an increasing trend in the 1990s and reached a peak at the end of the period,after a sharp decline at the beginning of 21 st century,there was a slow increase trend.There are obvious differences in the large-scale circulation of the same period(June to July)from the upper to the lower troposphere between the years with abnormally heavy rainfall during Meiyu period and the years with little precipitation during Meiyu period.In the years with abnormally heavy rainfall during the Meiyu period,the East Asian summer monsoon is prevalent in the lower troposphere,and the western Pacific subtropical high(WPSH)is westward and strong,and the South Asian high(SAH)is strong and easterly,and the westerly jet stream(WJS)is strong,providing abundant supply for the MLYR.The region provides abundant water vapor,which is conducive to the confluence of cold and warm air and the occurrence and maintenance of the Meiyu.However,the opposite is true for the years with little precipitation during Meiyu period.From 1961 to 2020,the top three RHREs during the Meiyu period of the comprehensive intensity in the MLYR occurred in 2020,1999,and 2016,respectively.In 2020,the WPSH was stronger,the East Asian WJS was also stronger,the double-blocking high was active,and the low-level circulation system transports abundant water vapor to the MLYR.In 1999,the westerly air flow from the Bay of Bengal in the lower troposphere was strong,the subtropical high in the middle troposphere was maintained in the western Pacific,and the high SAH was strong.In 2016,the WJS strengthened its northward movement,the WPSH was super strong,and the east Asian summer monsoon in the lower troposphere strengthened northward,which is conducive to the occurrence of the Meiyu.