飑线, 环境条件, 自组织结构, 组织化过程, 极端大风

The occurrence, development mechanism of complex terrain squall line and the warning index of severe weather have been

the focus of the short time approaching forecast of severe convective weather in Shanxi Province. On July 25, 2022, a rare squall line with a maximum instantaneous wind of magnitude 12 occurred in southern Shanxi Province. Based on multi-source observations and reanalysis data, the triggering, evolution and organization of the squall line process were analyzed in this paper. The results are as follows: (1) Radar observations showed that the life history of the squall line could be divided into three stages: Firstly, the upstream discrete convective cells moving eastward to form a linear convective system, and then the development of new thunderstorms in mountainous areas formed a multi-cell storm, thereafter, the linear convective system merged into the multi-cell storm and organized into a squall line. At the former two stages, the vertical wind shear was weak, but the favorable environment such as high energy, high humidity and unstable stratification made the convection mainly dominated by multi-cell storms with a low degree of organization. At the third stage, the forward-tilting structure became more obvious, the stratification instability increased significantly, the local vertical wind shear improved significantly due to near low-level disturbance, and the convection quickly organized into a strong squall line. (2) The development and maintenance of surface mesoscale convergence line, dew point front and mesoscale vortex are the main mechanisms of system triggering. The self-organized structure of forward inflow and rear outflow co-existed, the convergence at the low level and divergence in the upper level co-existed in the vertical direction, and the coexistence of environmental inflow and system outflow in the horizontal direction made the squall line maintain and develop. (3) From the perspective of thermal structure, the strong cold pool formed by merging and strengthening of surface cold pool at the third stage is the key cause of extreme thunderstorm wind. (4) The surface convergence line and mesoscale vortex occur more than 20 min earlier than the convective organization and strengthening, and the high value area of radial velocity in the low layer and the radial convergence in the middle layer appear 10-25 min earlier than the surface gale, which

has certain indication significance for monitoring and warning.

squall line, environment condition, self-organization structure, organization, extreme thunderstorm gale