The Genesis Mechanism Of Downstream Rainstorms Affected By The Convection Structure Of The Tibetan Plateau And Water Vapor Transport Feature Based On The Elevated Topography

China is a large country with complex terrains forming three staircases.The Tibetan Plateau(TP)is the first staircase with its main body above 3000 m,regarded as “the world water tower”,“the third pole” and “the ridge of the world”.On the one hand,the elevated TPacts as a barrier to force water vapor from tropical oceans to flow along the eastern and southern flanks of the TP,providing favorable conditions for downstream rainstorms in summer.On the other hand,convective systems over the TP would move out of the TP and then trigger cyclonic vortexes to cause summer downstream rainstorms in combination with water vapor transport.Water vapor transport serves a vital role in the process of the elevated topography and convective source of the TPinfluencing downstream rainstorms in summer.The goal of this study is to investigate the genesis mechanism of downstream rainstorms affected by the elevated topography and convection of the TP from the water vapor transport perspective.This study would provide possible explanations for downstream rainstorms,which may benefit extreme precipitation forecasting and floodhazard mitigationover China.Based on the maximum C-band frequency-modulated continuous-wave(C-FMCW)radar echo intensity at Naqu collected from the Third Tibetan Plateau Atmospheric Scientific Experiment,hourly/daily rain-gauge station data over China mainland,and multiple reanalysis datasets,the effects of the large topography and convective source of the TP on downstream rainstorms are investigated through various statistical methods.The Weather Research and Forecasting(WRF)model and FLEXible PARTicle(FLEXPART)Lagrangian dispersion model are also employed to confirm the results acquired from observational and reanalysis data.The mainresults areconcludedas follows:(1)The maximum echoes received by the C-FMCW radar in Naqu can well reflect both convection over the central TP and regional heat source structures.The dipole water vapor flux vortex(WVFV)in the middle layer of the atmosphere,as well as a three-dimensional WVFV coupled structure with high-level divergence and low-level convergence are the important mechanismsfacilitatingconvection development insitu.(2)In summer,the TP is a huge heat source.The frequent convective motions over the central TP under the dynamic and thermal driving effects of the TP provide a significant precursory strong signal for the genesis and development of summer rainstorms over the Yangtze River Basin(YRB).According to the reanalysis data and WRF model simulation with high temporal resolution,this deep convection over the central TP has been diagnosed to impactrainstorms downstream of the YRB via a three-dimensional WVFV coupled structure with high-level divergence and low-level convergence.The eastward propagation of the WVFV structure would enhance convection and thereby develop rainstorms downstream of the YRB.A FLEXPART Lagrangian dispersion model tracked the trajectory of air masses originating from the central TP toward the middle–lower reaches of the YRB,which supported the robustness of thediagnosticresults.(3)Undertheorographicforcing oftheTPand itssecond staircaseterrain(i.e.,theLoess Plateau,Yungui Plateau,and QilianMountain et al.),twoexisting watervaportransport channels providefavorablebackgrounds forsummerrainstormsover North China.Onechannelflows alongthe northernedge oftheTPviawesterlies.Theotherchannelflowsalongthe eastern edges oftheTPandits secondstaircaseterrain accompanied by southerlymonsoon airflow,with water vaporfromtheBayofBengal,SouthChinaSea,and westernPacific.Thesetwo watervapor transportchannels eventuallyconvergeoverNorth China.Theabundant watervaportransported bytwo channelsnotonly offers a favorablemoisturebackground forsummerrainstormsover NorthChina,but alsoresults inthe development ofWVFVthat suppliesdynamicconvergence conditionsforrainstormsinsitu.Atlow levels,watervaportransport mainlyflows alongthe easternedge oftheTPsecondstaircaseterrain towards North China.(4)Quantitativeanalysisalso finds thatthe convergencecomponent(79.1%)appears to havea somewhatcrucialroleinstrengtheningwater vaporfluxconvergencerelativetothe advectionpart(20.9%)at lowerlevel.Theenhancedwatervaporflux convergencealongthe eastern edge oftheTPsecondstaircaseterrain favors summerrainstorms overNorth China.Additionally,the resultsfromtheWRF modelsimulation ofa randomlyselected rainstorm case originatingfromtheTPto NorthChinaconfirm thecrucial effectoftheTPsecond staircase terrainon summerrainstormsover NorthChina accompaniedby watervaportransport.