Causal Analysis Of Summer Extreme Precipitation Events In Dongting Lake Basin

In this work,based on daily precipitation data of 56 years(1961~2016)for 85 meteorological stations and daily reanalysis dataset from NCEP,a detailed analysis is performed to understand the causes of extreme rainfall events(ERT)during the summer(June~August)in Dongting Lake Basin.Firstly,the trend characteristics of extreme precipitation in summer are analyzed.Secondly,the driving force from extreme precipitation in summer is extracted by using the Slow Feature Analysis(SFA).Finally,Self-Organizing Map and k-means(SOM-k)is used to cluster the ERT,based on the foregoing work,the conclusion feature of different regions of ERT 3 days ago is explored.The conclusions are as follows:(1)The extreme precipitation indexes of Dongting Lake basin(R90p?Rx5day?R90n?R25mm?EPI?EPE)is basically on the rise during the summer,and the fluctuation is obvious,and the sudden change time of extreme precipitation events is mainly distributed in the 1960 s to the mid-1970 s and the 1990 s.The values of these six extreme precipitation indexes show the spatial distribution characteristics of decreasing from northwest region to southeast region,and the increase trend is most significant in the area near 27°N~28°N in the middle part.(2)The three indexes(R90n ? EPI ? EPE)better reflect the characteristics of extreme precipitation during the summer in Dongting Lake basin.The driving forcing from the R90n?EPI?EPE time series is extracted by using the SFA method,and to analyze the time-frequency characteristics of the driving forcing information and its influence on the extreme precipitation trend in Dongting Lake Basin with Morlet wavelet transform technology.The results show that the extreme precipitation in Dongting Lake basin is mainly affected by solar activity and the oscillation of the Western Pacific subtropical high(the change of position,shape and intensity of the Western Pacific subtropical high).The driving force of the three extreme precipitation indexes is different in different periods.During 1961~2016,solar activity had a greater impact on the extreme precipitation indexes in Dongting Lake basin than the Western Pacific subtropical high.(3)Affected by seasonal variation of the Western Pacific subtropical high,the three indexes(R90n ?EPI?EPE)were strongest in June,decreasing from the northwest to the southeast.A trend was most pronounced in July,with an upward trend in the area north of 27°N more pronounced than in the south.(4)Composite analysis of regional ERT shows that there are many reasons for the extreme rainfall: the enhancement of trough at 500 hPa,the development of low tropospheric cyclonic vorticity at 850 hPa,the central area of convergence upward flow is enhanced and moves northward,and the enhancement of southwest airflow around the Western Pacific subtropical high.(5)Although the anomalous moisture transport channels of regional ERT in various regions are not consistent,the source of anomalous moisture is basically the same,the excessive anomalous moisture transported from the South China Sea to Western Pacific.It indicates that the variation of Western Pacific subtropical high and circulation plays an important role in producing environmental conditions favorable for regional ERT.The seasonal variation of the Western Pacific subtropical high and the difference of synoptic condition together cause the spatial variability of the main extreme rainfall rain-band in Dongting Lake Basin: the class1 region is about 26°N~27°N,the class2 region is mainly in the southeast,the class3 region is about 27°N~28°N,the class4 region is about 26°N~28°N;the class1,class3 and class4 concentrated in June;the class2 concentrated in August.