Circulation Patterns Of Summer Precipitation In The Southeastern Qinghai-Tibet Plateau Based On Self-organizing Map Metho

The Tibetan Plateau(TP),widely recognized as the "Water Tower of Asia," experiences a pronounced concentration of precipitation in its southeastern part(SETP),where summer rainfall contributes to more than 60% of the annual total.To investigate the daily atmospheric circulation over the SETP from 1981 to 2020,this study combines three sets of reanalysis data(ERA5,CPC,and GPM)with daily precipitation data obtained from ground-based observations across the TP.A multivariate self-organizing mapping(SOM)method is employed for classification,utilizing variables such as 500 h Pa geopotential height,400 h Pa vertical velocity,and whole-atmosphere precipitation.Through this method,nine distinct weather regimes(WRs)are derived,denoted as SOM1,SOM2,and so forth,which exert influence on summer precipitation over the SETP.Correspondingly,the classification of these WRs aligns with observed patterns of summer rainfall,including SOM9.Furthermore,an exploration of the factors influencing precipitation variations is undertaken from the perspective of water vapor transport and the water balance.Building upon the identification of these nine WRs,the study applies the K-means method to establish four distinct circulation patterns that impact extreme summer precipitation over the SETP.Subsequently,these patterns are analyzed in relation to occurrences of extreme precipitation.The main conclusions are listed below.(1)The summer precipitation on the plateau is affected by various circulation patterns,which can be classified into nine categories based on their unique large-scale circulation and water vapor transport features.While some of these patterns,like SOM5 and SOM8,promote precipitation by receiving water vapor from the western and southern boundaries and primarily from the southern boundary,respectively,others like SOM1,SOM2,and SOM3 do not facilitate precipitation,and water vapor is released from the southern boundary.(2)The occurrence of drought and flood over the SETP is caused by the interannual variability of different WRs.Specifically,the high frequency of SOM5 and SOM8 days leads to droughts and floods in the southeast of the plateau,while the increasing frequency of SOM1,SOM2,and SOM3 results in droughts over the SETP.The frequency of different WRs provides insights into the daily perspective of drought and flood over the SETP.(3)ENSO primarily regulates precipitation over the SETP by controlling the frequency of SOM4 and SOM9,although these WRs are not the main contributors to droughts and floods over the SETP.Therefore,while ENSO significantly affects precipitation,it is not the primary factor affecting precipitation over the SETP.(4)The amount of extreme summer precipitation over the SETP varies from 50 to 100 mm,with an extreme precipitation threshold of 10 to 20 mm.Extreme precipitation events contribute between 18% to 30% of the total summer precipitation.Except for isolated stations,extreme precipitation events demonstrate a substantial upward trend.These results suggest that extreme precipitation events are becoming more frequent over the SETP.(5)The K-means method was used to re-cluster the WRs that impact summer precipitation over the SETP into four distinct types,each with a frequency of occurrence exceeding 20%.The spatial distribution of the types reveals that the first and second types have an approximately opposite distribution.Moreover,the types can distinguish between areas with high values of daily average extreme precipitation and those with high-frequency extreme precipitation.