江苏一次飑线过程的数值模拟及其形成机制分析

Numerical Simulation on a Squall Line Process in Jiangsu Province and Analyses of Its Formation Mechanism

摘要: 利用WRF中尺度数值模式对2012年5月16日江苏北部一次飑线过程进行了数值模拟，采用三重嵌套，模式水平分辨率最高为5 km。利用模式输出的高分辨率资料，分析了地面和中层的飑线结构及飑线发展机制，结果表明，模拟的地面散度场存在一个“散度环”、三条辐合线，雷暴高压带形成一条辐散线，其前部和后部各存在一条辐合线，中层500 hPa上升运动中心南侧和北侧分别存在正涡度和负涡度中心。飑线的气流与经典模式的顺切变和逆切变气流不同，上升气流和下沉气流基本处在同一垂直气柱中，并在中层分离。中高空产生的降水物质掉入下半部分的下沉气流中，不会对上升气流形成拖曳作用，下降的降水物质在下沉气流中蒸发降温，在地面形成雷暴高压和冷池。利用涡度方程，分析了中层涡度偶形成的机制，垂直风切变和强垂直运动梯度相互作用在上升运动中心北侧形成负涡度，南侧形成正涡度。雷暴高压中的冷池和垂直风切变相互作用的结果使得雷暴在冷池前缘新生，而在冷池后部消亡，飑线自激发展。

Abstract: A squall line process in northern Jiangsu Province on May 16, 2012 was simulated by using a mesoscale numerical model WRF. The maximum horizontal resolution of the model was 5 km with triple nesting. Based on the high resolution model output data, the squall line structures in the surface and middle levels and the squall line development mechanism are analyzed. The results show that there exists a divergence ring and three convergence lines in the simulated surface field. A divergence line formed along the thunderstorm high zone, a convergence line formed in the front and another back of the squall line. There are positive and negative vorticity centers to the south and north sides of the vertical velocity center at 500 hPa, respectively. The airflow in squall line is different from the typical model which contains an upshear updraft and a downshear downdraft. The rear inflow at middle level separates into an updraft and a downdraft which are basically in the same column. The liquid or ice particles produced in the middle and upper levels fall into the downdrafts not dragging the updraft. When they evaporate in the dry downdrafts, they cool the air and then trigger the formation of surface thunderstorm high and cold pool. Based on the vorticity equation, the formation mechanism of mid-level vorticity couple is analyzed. The interaction between vertical wind shear and strong gradient of updraft induces the negative vorticity to the north side of the vertical velocity center and positive vorticity to the south side. The interaction between cold pool in thunderstorm high and vertical wind shear induces thunderstorm regeneration at the front of cold pool, weakening at the back of cold pool, and squall line self-excitation development.