Classification And Cause Analysis Of Banded MCSs Over The Tibet Plateau In Summer

Based on TBB data during summer from2007to2011, Banded MCSs (BMs) over the Tibet Plateau with stable characteristics are screened out and Classified, cause analysis of BMs are discussed by type with NCEP data and HYSPLIT model. Results show that37cases of BMs with stable characteristics fall into three types according to shape:North Convex type (NCT), South Boundary type (SBT) and Zonal type (ZT), the maximum incidence rate is for NCT BMs and minimum for ZT BMs. These BM occur in30%of the whole summer, especially in nearly50%of July. The main cause of BM is that there’s South Asia high and jet stream on upper level, shear line with convergence on500hPa, and air with high temperature and humidity in the south of shear line. On500hPa, BMs of ZT commonly occur between westerly airflow in north side of plateau and eastern airflow in south side. NCT are in the synoptic situation that there’s lank westerly airflow in northern plateau, strong trough in Bay of Bengal, influence of southwest by south flow on eastern and southern plateau. In the process of SBT, there’s northwest airflow over the plateau and strong cyclone in Bay of Bengal.Our analysis shows that there are four vapor divergence regions around the plateau, vapor in convective zone is mainly derived from south side of southwest edge and southeastern plateau.These statistical conclusion apply to typical example of BMs of NCT, the numerical simulation experiments have good results in case of precipitation and circulation background fields, but in case of TBB and potential temperature,the results are not ideal.By using WRF model on June19,2011, typical BMs of NCT has been carried on the numerical simulation process, the results of precipitation and circulation background field are available. Data analysis of WRF shows that the mesoscale precipitation is mainly under the condition with air of divergence aloft and mesoscale vortex in the evolution process of elimination at low level. After the development for the low vortex with shear line and weakened into a horizontal slot, rain belt is located on the north of shear line at first, and then along the axis of the low vortex distribution, then coincided with the location of the low vortex trip, finally from north to south of the trough. Low-level convergence and high-level divergence is the main power structure, the divergence enhances at first and then weakens in the process of precipitation enhancement, and the sinking of divergence center has a good corresponding with the precipitation enhancement in time; convective instability weakens when the Precipitation strengthens, south movement of the front may be beneficial to the release of unstable energy, strong moist baroclinic atmosphere is conducive to the development of vertical vorticity, high value area of equivalent potential temperature at middle level and precipitation area is corresponded on position.