Cause analysis of a disastrous gale event induced by squall line in the mature stage in northern Sichuan

Based on intensive radiosonde data, visible cloud images from Himawari-8 satellite, radar data and observations from regional automatic weather stations, we have made the comprehensive analysis of a squall line event in northern Sichuan on 4 June 2016, with focus on the falling area characteristics of ground-disastrous gale induced by squall line in the mature stage. Then combining with high resolution data simulated by WRF, we have analyzed the relationship between the gale area and rear inflow jet with the squall line system, and explored the formation and maintenance mechanism of rear inflow jet. The results show that the gale center in the ground is located in the vicinity of the strong convective cloud clusters at the top end of the central axis of the rear inflow jet at the mature stage of the squall line. A surface meso scale convergence line is observed at the front side of matured squall line, and has a high indicative significance for the development of squall line, with indicative time being ahead about one hour. The rear inflow jet is formed under the accelerated effect of the horizontal vorticity produced in the rear boundary of convective cell on the rear inflow of squall line in the middle level due to the unbalanced temperature distribution of the squall system. A positive feedback effect between the rear inflow jet and the horizontal perturbation temperature gradient is an important physical mechanism for the formation of the gale at the mature stage and bow-shaped echo. Convective Available Potential Energy and dry and cold advection intensity in the mid-and upper-level are main factors for forecasting surface maximum gale induced by squall line. Water vapor conditions in the mid- and low-level play major roles for the maintenance and development of squall line.