Characteristic Analysis Of Typhoon Precipitation And Drought-flood Abrupt Alternation In Flood Season In Anhui Province

Typhoon is an important component of precipitation during flood season in monsoon area.The amount of typhoon precipitation often directly determines the drought and flood situation in the area.Anhui Province,located in East China,is a typhoon affecting region.Typhoon-related disasters are its major natural disasters.5concentrated periods of precipitation are divided based on the characteristics of precipitation distribution and atmospheric circulation in flood season,namely,the pre-rainy season,the plum rains period and the Huang-Huai rainy season,the drought period and the post-rainy season.Based on the identification of typhoon precipitation events in Anhui Province,the temporal and spatial distribution of typhoon precipitation characteristics such as typhoon precipitation and frequency in Anhui Province are analyzed.The influencing factors affecting typhoon precipitation in Anhui Province are determined through correlation analysis,and the prediction model of typhoon precipitation in flood season based on back propagation is established.Using the improved Period Drought-Flood Abrupt Alternation Index(PDFAI),the drought-flood abrupt alternation in Anhui Province during 1957-2019 is identified.The historical evolution and the relationship between typhoon activities and drought-flood abrupt alternation are discussed.The results show that:(1)The typhoon precipitation in flood season in Anhui Province has spatial heterogeneity which showing a decreasing trend from south to north latitude.The annual average typhoon precipitations in three geographical regions of Jiangnan,Jianghuai and Huaibei in flood season are 181 mm,137mm and 113 mm,respectively.The inter-annual and inter-decadal differences of typhoon precipitation are significant.The typhoon precipitation in flood season reached its maximum in the 1980 s,and decreased significantly after 2000 s.The interdecadal variation trends of typhoon precipitation in flood season in Jiangnan,Jianghuai and Huaibei regions are roughly the same,and the typhoon precipitation increases first,then decreases and then increases.During 1957-2019,the average typhoons precipitation of 5 concentrated periods in Anhui Province are 34,62,65,73 and 56 mm,respectively.The number of typhoon precipitation events in the pre-rainy season is the least,and the number of years except typhoon-affecting-precipitation years is 55-57.The number of typhoon precipitation events in the post-rainy season is the most,which is 18-23.(2)The accuracy of the BP-based typhoon event prediction model in the simulation period from 1957 to 2009 is 96%,and its accuracy in the validation period from 2010 to2019 is 90%.The reason for the errors may be that the precipitation of typhoons in these years is mainly affected by short-term weather changes,and the precipitation mechanism is different from that of typhoons in other years with similar values of influencing factors.In addition,the absolute value of the mean relative error predicted by the typhoon precipitation amount model for the simulation period is 21%.A significant error can be found in 2000 as the mechanism of typhoon precipitation in this year is different from that of other normal years.The error in 2000 is probably caused by the impact of vertical shear anomalies over the western Pacific which hinders the development of typhoon embryos.Besides,a significant error also can be found in 2009,probably due to the influence of the asymmetry in the typhoon cloud system.Additionally,the absolute value of the mean relative error predicted by the typhoon precipitation amount model in the validation period is 17%.It may be that the anticyclone circulation caused by the double typhoon effect prevents the typhoon from going up north and makes the typhoon center wander in Anhui.The precipitation range in 2019 caused by typhoon landing is different from that of perennial years.(3)The drought-flood abrupt alternation in Anhui Province is selected by the improved PFDAI index.During 1957-2019,there are 21 years of drought-flood events and 17 years of flood-drought events.Drought-flood abrupt change events occurred 12 times from pre-flood season plum rains period,and the frequency of drought-flood events is twice as many as flood-drought events.Besides,there are 13 drought-flood abrupt alternation from plum rains period to Huanghuai rainy season.Among that,there are 6 drought-flood events and 7 flood-flood abrupt events.What' more,the number of drought-flood abrupt alternation in the other periods is 9,the frequency of drought-flood events is twice as many as flood-drought events.Typhoon activity has a certain precursory effect on the occurrence of drought-flood abrupt alternation.In the drought-flood events,52 % of the events have typhoon activity in the flood period.In the flood-drought events,29 % of the events have typhoon activity in the flood period.