一次浅薄低涡暴雨过程数值模拟及发展机制分析

摘要

The rainstorm with rainfall 226 mm in 6 h in the downstream of Changjiang River Valley on 27 July 2009 is simulated by WRF model.According to the model output,the diagnostic analysis is done for the shallow low vortex rainstorm event to investigate the development mechanism of the system.The re-sults show that the low vortex locates at forepart of westerlies trough,existing only in the low level of troposphere.The dynamic structure and temperature-moisture features all show that the vortex is a shal-low system.The low vortex rainfall mainly happens near the warm shear line of the east part of the vor-tex.Two moisture source regions are in the South China Sea and the East Sea,and the thick moisture con-vergence is up to 600 hPa.The low vortex has strong baroclinic structure features.The analysis of vortici-ty equation shows that the divergence and convergence of wind field contribute mainly to the positive vor-ticity change rate,and the 3D wind shear distribution has an important correlation with vorticity.The dy-namics of terrain obstruct makes the low vortex strengthened at leeward slope.The jets at high and low levels and the latent heat of condensation are helpful to the intensification of the low vortex and the torren-tial rainfall and the eastward motion.The wet isentropic surface perpendicular to the vortex surface means the low vortex is at mature stage,which predicts the rainfall is to weaken soon.When the developing con-dition of slantwise vorticity (Cd>0)keeps strengthening,it indicates that the combined result of barocli-nicity ,stability and wind vertical shear is also an important impact factor for the shallow low vortex devel-oping and moving.%运用WRF中尺度数值模式对2009年7月27日长江下游地区的一次6 h累计降水226 mm的暴雨过程进行数值模拟，利用模式输出资料，对引发此次暴雨过程的浅薄低涡移动演变和动力结构、温湿特征进行分析，探讨低涡发展东移机制。结果表明：低涡位于500 hPa西风槽前，仅出现在对流层低层，动力与温湿特征均表现其浅薄结构。低涡暴雨降水主要发生在低涡东侧暖式切变线附近。水汽来源于中国南海和东海两地，水汽辐合层深厚，达600 hPa。此次低涡具有较强的斜压结构特征。通过涡度方程分析，辐合辐散流场的维持和发展为正涡度变化率作出主要贡献，三维风场倾斜扭转项作用与正涡度变化存在重要正相关。地形的动力作用使得背风坡更有利于低涡绕山后加强。高低空急流耦合中心和水汽凝结潜热释放区均有利于引导低涡以及低涡暴雨加强东移。湿等熵面呈垂直陡立状，指示低涡已发展到成熟期，预示暴雨即将减弱。倾斜涡度发展条件犆犱＞0的连续增强，指示斜压性、稳定度和垂直风切变的综合作用也是这次低涡保持发展加强的重要原因。