一次飑线过程的数值模拟及诊断分析

A case study on squall line:Numerical simulation and diagnosis analysis

2003年04月12日江西、福建交界处发生一次飑线过程,该飑线过程造成了雷暴、冰雹和大风等强烈天气.本文使用NCEP1°×1°逐6 h分析资料及卫星、雷达观测资料对这次飑线过程发生的大尺度背景进行了分析,之后本文使用Advanced Research WRF(ARW)2.2对该过程进行了模拟,并利用模拟结果对飑线系统的演变过程、近地层结构、对流层结构进行了分析,结果表明:1)本次飑线发生于大尺度鞍型场中,它是在冷锋、切变线、低空西南急流、高空西风急流以及中尺度对流复合体等系统共同作用下生成,从位涡角度看,它是高空正的湿位涡移至低层斜压不稳定的冷锋上所诱生的一次强对流过程;2)位于江西、福建交接处的武夷山地形对本次飑线的形成、移动和发展有较大影响,敏感试验表明若将武夷山地形高度减至一半则无法生成飑线系统,但对流单体依然能够生成;3)近地层风场表明本次飑线过程主要为南北向气流的辐合,东西向气流的辐合较小,温度场和海平面气压场上则可以清晰的看出冷空气丘和中高(低)压结构,冷空气丘的存在对风场有明显的加速作用,风速在飑锋处达到最大,且飑锋出流与飑线系统的移速和方向较为匹配,这是本次飑线过程维持和发展的主要因子;4)对流层结构的分析则显示,本次飑线介于前部层云降水飑线(LS)和平行层云降水飑线(PS)之间,其移动方向上(纬向)的垂直结构类似于Front-Fed LS(FFLS)结构,垂直于其移动方向(经向)的垂直结构则与典型的尾部层云降水飑线(TS)结构一致.

On April 12^th, 2003, a squall line occurred near the border area between Jiangxi and Fujian provinces. The squall line caused thunderstorms, hailstorms and gale. In this paper, we simulate this event with Advanced Research Weather Research Forecasting Model ver2.2 （ARW 2.2）. The simulated results were applied to analyze squall line system development, and near surface and troposphere structures. These results indicate： 1） The squall line developed in saddle pattern field, and was initiated by the interactions among cold fronts, shear lines, southwestern low level jet, western upper-level jet and mesoscale convective complex system. From the viewpoint of potential vorticities, it was a strong convection caused by the movement of upper-level virtual potential vorticity to the low level unstable baroclinic front~ 2） The topography near the border between Jiangxi and Fujian provinces has a significant effect on the initiation, movement and development of the squall line. Sensitivity experiments indicate if the height of Wuyi Mountains were cut down to the half, the convection cell would have been initiated too; 3） Near surface wind field shows this squall line is a convergence of N-S airflow, while the E-W airflow convergence is small. In temperature and sea surface pressure field, the cold pool and upper-level low pressure structure are obvious. The existence of cold pool accelerated the wind field significantly. Wind speed reached its maximum at the peak of the squall line, and the outflow of the squall line was peak matched well with the movement speed and direction of the squall line system, which is the main reason why the squall line was maintained and developed. 4） After analyzing the troposphere structure, we found that the squall line is between leading and parallel stratiform precipitations（LS and PS）, and its E-W vertical structure is similar to Front-Fed LS（FFI.S structure）, while its N- S vertical structure is consistent to the representative trailing stratiform precipitation （TS）.