Mechanisms of an Elevated Thunderstorm Process over Southern China in the Early Spring of 2009

An elevated thunderstorm process in southern China on 3 March 2009 is analyzed by using conventional observation,6. 7 μm satellite vapor images and TBB,lighting location and NCEP / NCAR reanalysis. The main synoptic systems include trough at middle-low levels,vortex and shear line,low-level jet,upper-level southern and northern western jet. The storm is triggered by low-level jet warm water vapor transportation and invading of high-level cold air. The boundary is dynamically stable for the existence of a thick cold pad. Convection instability appears betw een low-level warm and humid southwest jet and the cold and dry air at middle and high levels,and strengthened by the descending motion of the upper jet. A strong baro-clinic frontal zone is formed by the meeting of southwest jet and the cold and dry air northw ard at low-middle levels. The confluence of the southwest air descending branch and the cold and dry air from northern area causes northeast backflow which couples with the southwest winds above the boundary and forms a secondary front circulation,the ascending air at middle-upper levels couples with descending air of northern westerly jet causes another secondary positive circulation,the two northw ard titling secondary circulations are very favorable for tilting ascending movement. The warm advection and water vapor flux convergence of low-level jet,the strong divergence at the right entrance of upper-level western jet and the convergence along the southern upper-level jet are also contributable for meso-scale ascending movement. Thunders and hails appear usually at the dense band of θse,low-level jet speeds,water vapor flux and 300 h Pa divergence. In addition,the environmental conditions as the thin humid layer,strong wind shear,low-level temperature inversion,ascending motion at-20 ~ 0℃ super cooled water layer are very advantageous to thunderstorms.