Characteristics Of Large-scale Circulation And Identification Of Moisture Sources For The Extreme Precipitation Events In The Southeastern Edge Of The Tibetan Plateau

Due to the complexity of regional topography and weather systems in the southeastern edge of the Tibetan Plateau（SETP）,the formation mechanism and the prediction of extreme precipitation events（EPEs）remains elusive.This study used gauged precipitation data,GPM-IMERG precipitation data and ERA5 reanalysis data.For methods,this paper used composite analysis,wave activity flux analysis,the Quasigeostrophic omega（QGω）analysis,K-means cluster analysis,and diagnosis of water vapor sources based on FLEXPART model.This paper discussed the large-scale circulation characteristics,significant signals and water vapor transport of EPEs in the SETP.The main conclusions are as follows:（1）The spatial distribution of gauged stations in the SETP was uneven.The GPM-IMERG precipitation is well consistent with that derived from precipitation observed by gauged stations,indicating that GPM-IMERG data could be used to estimate extreme precipitation in this area.There are large differences in the spatial distribution of precipitation in the SETP.Precipitation,extreme precipitation and heavy rainfall were mainly distributed in bands along the southeast boundary of the TP.The precipitation amount and frequency of precipitation,extreme precipitation and heavy precipitation(≥50 mm day^-1)have the common maximum value center located at the junction between the SETP and the Sichuan Basin（SCB）.The area around the maximum center is designated as a key area for this study.The precipitation amount and frequency of precipitation,extreme precipitation and heavy precipitation in the key area showed an increase trend.In the key area,the 100 EPEs were selected.EPEs occurred during the rainy season（May-September）in the SETP,and mostly occurred in July and August.EPEs are also showed an increase trend.（2）The large-scale characteristics of the 8 days before the EPEs in the SETP are mainly summarized as the two aspects in dynamic and thermal.In terms of dynamic characteristics,the upper-level jet maintained on the northern side of the TP ensures the continuous transmission of momentum,so that the center of South Asian high（SAH）moves eastward and the upper level divergence in the precipitation area is strengthened.The Western Pacific subtropical high（WPSH）extends westward from the Western Pacific Ocean（WP）to eastern China,which facilitates the transport of water vapor from the South China Sea（SCS）.In the lower level,the circulation in the south of the TP and SCB is continuously controlled by cyclonic anomaly,which caused the Bay of Bengal（BOB）to the SCS is controlled by westerly winds,and the SCS to the SCB is controlled by southerly winds,which facilitates the transport of water vapor from the BOB through the SCS and water vapor from the WP to the precipitation area.For the thermal characteristics,the vertical distribution of temperature anomaly shows a"top-cold and bottom-warm"structure around 40°N and moves south to the precipitation area and continues to increase in the 8 days before the EPEs.The enhancement of temperature anomaly dipole pattern leads to increased atmospheric instability,which is conducive to the development of convection.The above results indicate that there are synoptic-scale precursor signals for EPEs in SETP,some of which are strong and last for 8 days.Through the diagnosis of QGωequation for large-scale processes,the characteristics and functions of different precursor signals are further revealed.The absolute vorticity advection,which represents the background circulation,continues to affect the upward motion of the middle and low level in the 8 days before EPEs.The temperature advection associated with the eastward extension of the SAH and the western extension of the WPSH had a significant effect on the upward motion in the 3 days before EPEs.（3）We further classified the selected EPEs into three categories with K-means clustering method.Then,the anomalies of atmospheric circulation and its evolution with respect to different category of EPEs are explored.The results show that,during the period of precipitation,the geopotential height is characterized by a positive anomaly at the upper-level（200h Pa）and negative anomaly at the lower-level（850h Pa）,together with the enhanced vertical wind speed,a"top-cold and bottom-warm"atmospheric temperature anomaly structure,and the increased water vapor transport departing from the low latitude ocean area.However,their atmospheric circulations configuration,including the SAH,the WPSH,and the westerly jet in upper level,shows obvious difference and plays a dominant role in in shaping the precipitation formation of different types and associated water vapor transport:For category 1 EPEs,its associated water vapor transport mainly comes from the BOB and the SCS,which controlled both by the Indian monsoon and the East Asian monsoon.As to category 2ranked the highe frequency of occurrence,EPEs occurs with the strengthened WPSH,and its water vapor mainly comes from the SCS,while the water vapor transport from the BOB is restricted.When it comes to the category 3,its water vapor transport mainly controlled by the Indian monsoon.Prior to the occurrence of EPEs,it can be found that the Rossby wave action shows an increasing trend,the atmospheric vertical wind speed increase significantly,and the temperature anomaly was more significant.These features are conducive to the occurrence of extreme precipitation events.The results also indicate that the changes in WPSH position and intensity on synoptic time scales are better indicators for the prediction of heavy precipitation formation compared to SAH.（4）The moisture sources of EPEs in the SETP are mainly influenced by the East Asian summer monsoon,the South Asian summer monsoon,the westerlies,and local recycling processes.When extreme precipitation occurs,the moisture source of extreme precipitation presents significant abnormal characteristics,featured by an increase in the moisture contribution from the southwest of the TP and the Indochina Peninsula adjacent to the precipitation area with a transport time scale of 2-5 days,and an increase in the water vapor contribution from the BOB,the Indian Ocean,and the SCS regions from the low latitude ocean with a transport time scale of 6-10 days.However,the contribution of water vapor to regions such as the Indian Peninsula to the north and southwest of the TP has significantly decreased.The above results indicate that the occurrence of extreme precipitation is mainly related to the thermal conditions in the southwest of the TP,the enhancement of water vapor evaporation around the precipitation region（especially in the region adjacent to the southern part of the precipitation region）,and the enhancement of moisture contribution from the Indian and East Asian monsoons.The spatiotemporal distribution of extreme precipitation moisture sources in different weather types varies greatly.Compared with Type 1precipitation,Type 2 has an increased contribution of local recycling with 1-6 days transport,while the contribution of surrounding water vapor origin is relatively weak.Type 3 mainly shows an enhancement of the contribution of water vapor from the southwest of the precipitation region,while the contribution from the east and southeast of the precipitation region decreases,which could be attributed to the strengthening of the Indian monsoon transport and the weakening of the East Asian monsoon transport.