一次梅雨期苏北大暴雨的时空结构及其非线性亚临界对称不稳定

利用WRF V2.2模式对2006年6月30日00时—7月1日12时（世界时）江苏宝应地区的大暴雨过程进行数值模拟,并利用从模拟结果提取出的环境风场和中尺度扰动风场对此次大暴雨过程的时空结构,尤其是风场的垂直切变和非线性亚临界对称稳定性进行分析,结果表明：整层大气平均风垂直切变矢量转为西南偏西风后其南风分量随时间增大到最大值,对暴雨的发生有提前5-4 h的指示意义,相应时刻绝对水平螺旋度亦达到极值;暴雨发生前3 h首次出现强烈的整层垂直上升气流,预示大暴雨发生;在暴雨发生前9 h,高层200 hPa涡度、散度两波呈π/2位相差,对应强烈地转不平衡重力波,随时间向下传播,至暴雨发生前3 h下传到800 hPa;中尺度扰动风场的加强与降水的加强关系密切,在暴雨发生前3 h,出现中尺度非线性亚临界对称不稳定,激发暴雨发生,并对大暴雨的发生有重要的指示意义。

Analysis of Spatial-Temporal Structures and Nonlinear Subcritical Symmetric Instability in a Heavy Rainstorm in North Jiangsu

By utilizing the WRF（weather research and forecasting） model V2.2,a heavy rainfall process during 0000 UTC 30 June—1200 UTC 1 July 2006 in the region of Baoying located north of Jiangsu province was simulated;based on the extracted data of environmental wind field and mesoscale disturbance wind field from the model output,the spatial-temporal structures of the rainstorm,especially the perpendicular shear of wind field and nonlinear subcritical symmetric instability were analyzed.The results show that after the direction turning of vertically averaged wind shear from WNW to WSW,the maximum of south wind component indicates the precipitation peak which emerges after 5—4 hours as well as the peak of absolute horizontal helicity.The first emergence of strong updrafts though the total column of the atmosphere indicates the torrential rain 3 hours later.The π/2 phase difference between divergence and vorticity at 200 hPa in 9 h before the rainstorm,corresponding to intensely non-geostrophic gravity waves,propagates downward to 800 hPa within 3 hours before the rainstorm.The increase of mesoscale disturbance wind field was closely related to the increase of the precipitation.Nonlinear subcritical symmetric instability appears in 2—1 h before the rainstorm and simulates the heavy rainfall,which has the indicative meaning to the rainstorm.