On The Moving Characteristics Of MCC Over Eastern Tibetan Plateau In Rainy Season Based On ERA_Interim Reanalysis Data

Based on the TBB data from Fengyun-2 stationary meteorological satellite coupled with the reanalysis data ERA_Interim from ECMWF and CloudSat satellite data from May to September between 2012 and 2016,we conduct a contrastive analysis on the moving characteristics of mesoscale convective complex(MCC)over Tibetan Plateau(TP)in rainy season.The results show that MCC over the TP can be divided into three types according to its moving characteristics,which are MCC that move out of the TP in northeast direction(NE-MCC)and in east direction(E-MCC)and decay locally over the TP(L-MCC).Generally,MCC that can move out of the TP are not that strong due to its location and lifespan,while MCC that with moderate convective intensity and longer lifespan shows obvious eastward or north-eastward moving feature.The original birthplace of moving MCC is in the south of 33°N and between 90°E-95°E.It takes 12?18 hours for the system to move out of the TP and NE-MCC takes longer time than E-MCC.Compared with NE-MCC and E-MCC,L-MCC is with the smallest cloud cover,the strongest TBB and the maximum divergence difference between two levels that can lead to intense updrafts to enhance the development of L-MCC and at the same time forming a positive feedback mechanism.Meanwhile,IWC and LWC of L-MCC can be twice as much as E-MCC.Different moving direction of NE-MCC and E-MCC mainly affected by steering flow at 500hPa.It is beneficial for the development of MCC if the environmental configuration field has been low level convergence and positive vorticity coupled with high level divergence and negative vorticity or its gradient.The eastward moving of MCC system is usually accompanied by the eastward movement of the low vortexes in eastern TP.However,L-MCC cannot move out of the TP due to its short lifespan.A comparison between NE-MCC case and E-MCC case that both can result in downstream rainstorm shows that circulation configuration combined with wind velocity at 500hPa decide the moving speed of MCC,which means E-MCC case with high wind velocity but low moving speed is the result of shortwave troughs at 500hPa.The divergence gradient direction of the vertical section in the MCC center represents the moving trend of MCC,so that the divergence gradient direction of NE-MCC case is north-east while E-MCC case is east.Besides,the location of MCC's mature stage have considerable influence of its later development.The mature stage of NE-MCC case has already move out of the TP,which can lead to greater possibility of strengthening the system,while the mature stage of E-MCC case still inside the margin of the TP that make the system becoming weaker.A comparison between E-MCC cases is conducted to find out what is the factor of downstream rainfall.The results show that local moisture content,updraft velocity and synoptic system are the factors.Casel that can lead to heavy rainfall due to cold air from the surface convergence line triggering off convective instability energy.However,case2 cannot lead to precipitation result from no suitable system to stimulate the further development of convection.In addition,the higher the altitude,the smaller the effective particle size of ice particles and the larger the number of ice particles.Cold convective clouds at high altitude will extend downwards after moving out of the plateau.