The Characteristics And Formation Mechanism Of Snowfall On The Tibetan Plateau Affected By Tcs Over The Bay Of Bengal

Tropical cyclones(TCs)generated over the Bay of Bengal(Bo B)often cause heavy snowfall on the Tibetan Plateau(TP)after landfall.Snowfall accumulation in high altitudes is prone to snow hazards,which seriously affect the lives of local people and the development of animal husbandry.In this study,the characteristics of Bo B TCs causing the TP snowfall and the temporal and spatial characteristics of snowfall over the TP under the influence of TCs are statistically analyzed,based on the best track data provided by JTWC from 1981 to 2020,snow-related ground observations and satellite TBB data.This is done by partitioning the TC-influenced snowfall on the TP through tracing cloud clusters.Then,combined with the fifth generation of the European Centre for Medium-Range Weather Forecast reanalysis data for the global climate and weather(ERA5),the spatial pattern of snowfall on TC-influenced days is examined and the objective circulation classification is conducted.Meanwile,the relative importance of the factors in affecting the spatial pattern and intensity of snowfall is quantified.Finally,the development of an extreme precipitation event(EPE)with a sizeable portion of snowfall along the southeastern margin of the TP during the approach of TC Rashmi(2008)from the Bo B is examined.This work will i)reveal what important aspects of the large-scale flows that accounted for the northward movement of the Rashmi vortex,while keeping its TC identity,until reaching the TP,ii)explores how certain mesoscale processes led to the generation of the EPE at the Cona and Bomi regions,and iii)summarizes the synoptic conceptual model of Rashmi affecting extreme snow and rain processes in the southeast of the TP.Major results are given as follows.During the 40-year period of 1981-2020,141 Bo B TCs were formed,of which around 35%(50)impacted 83% of the snowfall meteorological stations on the TP during their northward or westward movements,and the average distance between the TC center and the snowfall stations is 1277 km.Snowfall-related TCs have a 10 a first major recurrence,and its number proportion shows a significant decreasing trend.The frequency and intensity of TC-influenced snowfall on the TP during 1981-2020 show little decreasing trend,and the monthly distribution of both exhibits a bimodal characteristic consistent with the frequency distribution of Bo B TCs.The peak snow depth affected by Bo B TCs occurs in October-December,with the precipitation and snow depth on a snowfall day in December accounting for more than 75% and 55% of the monthly total,respectively.TC-influenced snowfall is significantly increased in the eastern and southern edges of Tibet,western Sichuan,and the southern margin of Qinghai,and locations at higher elevations are more likely to receive TC-related snowfall.The spatial pattern of precipitation amount(PA)on snowfall days could be objectively categorized into western(24%)and southeastern(76%)types.The moisture transport of Bo B TCs by an southerly warm-moist airstream at 500 h Pa formed between the southern branch trough(SBT)and the Western Pacific Subtropical High(WPSH),the front formed by the convergence of cold and warm air,and the change in position of the upper South Asian high are significant factors causing the different spatial distribution of snowfall on the TP.There were large differences in the spatial distribution of moisture trasnsport,the distance from TC center,surface temperature,the topographically-induced upward motion,the WPSH’s western-most position,the SBT intensity,and the atmospheric stability,but the first three factors are the most important at most stations.There are obvious spatial differences in the response of PA to different impact factors.The westward position of the WPSH is not conducive to snowfall all over the TP.The greater the intensity of the southern trough,the more favorable the PA in most areas.The sensitivity of PA to a single factor of the WPSH or the southern trough is weak,which may depend more on the coordination of both.In most areas of the TP,including the southern TP,the closer the TC center is,the more favorable the PA is,while a small part of the region,represented by Qinghai,is less affected by the storm.Only in a small part of the southern edge of Tibet,the PA tends to be small when the surface temperature is high.In other snowfall areas,the surface temperature of warm and wet advection brought by the northward TC along the southwesterly flow is easy to be larger,and PA has a positive response to the surface temperature.For nearly half of the stations,the stronger the topographically forced uplift,the more stable the atmospheric stratification,the more PA on snowfall days.However,the spatial distribution of its response to the two factors is very uneven.Diagnostic analysis of the EPE shows the importance of an anomalous southerly flow,resulting from the merging of Rashmi into a meridionally deep trough at the western periphery of WPSH,in steering the storm and transporting tropical warm-moist air,thereby supplying necessary moisture for precipitation production over the TP.A mesoscale data analysis reveals that(i)the Rashmi vortex maintained its TC identity during its northward movement in the warm sector with weak-gradient flows;(ii)the extreme precipitation event occurred under potentially stable conditions;(iii)topographical uplifting of the southerly warm-moist air,enhanced by the approaching vortex with some degree of slantwise instability,led to the development of heavy to extreme precipitation along the southeastern margin of the TP;and(iv)the most influential uplifting of the intense vortex flows carrying ample moisture over steep topography favored the generation of the record-breaking daily snowfall of 98 mm(in water depth),and daily precipitation of 87 mm with rain-snow-rain changeovers at two high-elevated stations,respectively.The extreme precipitation and phase changeovers could be uncovered by an unusual upper-air sounding that shows a profound saturated layer from the surface to upper troposphere with a moist-adiabatic upper 100-h Pa layer and a bottom 100-h Pa melting layer.Based on observational statistics and diagnosis analysis of the typical case,this paper reveals the characteristics and formation mechanism of TP snowfall under the influence of Bo B TCs,which is conducive to the prediction and impact assessment of snowfall events on the TP,and has important significance for improving the mechanism understanding and forecast of heavy snowfall in plateau.