Study On The Precipitation Characteristics And Formation Mechanism Of Linear Convection Along The Coast Of South China

?/?-meso linear-shaped convection events(LCEs)often occured over coastal Southern China with a frontal rain band over northern South China or Yangtze River Basin,causing local and unexpected torrential rainfall,accumulated precipitaition 300 mm or more in 24 hours,which is difficult to forecast.To clarify the LCEs,based on multiple obsearvation data and ERA-Interim reanalysis data,LCEs temporal and spatial distribution,precipitation characteristics and its relationship to cold front have been analyzed firstly,and then we examined LCEs synoptic scales and mesoscale conditions occuring over Pearl River Delta(PRD),Yangjiang(YJ)and Shanwei(SW)regions,and several environmtal key parameters have been summarized.Three LCEs schematic diagrams are generalized after detailed analysis including convection initiation(CI),propagation and linear-shaped evolution processes.LCE over coastal Shanwei with extreme rainfall is examined as a typical case,and a dynamic effect can be accounted for sustained rainfall.The results are as follows:(1)LCEs always occured over YJ,SW and PRD,at coastline of Southern China,with distance 200~600km front of cold front during pre-summer season and active South China Sea summer monsoon stage.High convective precipitation proportion of LCEs showed double peaks or multiple peaks,and discrepancy in high-value shortduration heavy rainfall centers over YJ,SW and PRD resulted from complex terrain and meso-small scale river valleys over coastline.Precipitation caused by LCEs is characterized by unexpected,local,long-duration,torrential and convective.(2)LCEs are a class of ?-meso convective events under weak synoptic scale forcing.We offered an effective operational criteria after a large number of statistis analyzed,provided that environmental conditions over coastal Southern China satisfied parameters including KI>34?,SI<0?,LI<-2?,PWAT>50mm,LCEs would happen more probably,which can improve its forecasting accuracy based on existing numerical models.(3)Mutual reason that CI over YJ,SW and PRD is complex terrain interacted with sinuous coastline.Bay effect triggering smallscale convengence played a crutial role in formation of quasi-stationary mesoscale echo traning over YJ.Urban heat-island over inland reigion of PRD caused sea breeze strengthening in the moring,and producing wind convergence line over coastline,CI finally.Mountain-valley breeze interacted with sea-land breeze when they changed asynchronously that dominanly forced local CI during sunsise over SW.(4)Vapor transport team and vapor vertical gradient team are responsible for the nature of precipitation over LCEs and frontal rain band.Southwesterly jet enhanced vertical wind velocity shear at midnight in boundary,and then the horizontal vorticity partly converts into vertical vorticity,overlying the vertical vorticity originated from horizontal shear,accounting for long duration at midnight in aspect of dynamic mechanism.In addition,updraft vertical velocity caused by convective latent heat accounted for 38.5%~75.1% of total updraft vertical velocity,indicating convective latent heat release is also an important factor for LCEs.