Based on thunderstorm gales data in recent 40 years and the conventional and new monitoring data in recent 18 years in Shanxi Province, the spatial and temporal distribution of thunderstorm gales in Shanxi Province are analyzed firstly, and then the weather types, characteristic physical quantity thresholds and conceptual models of thunderstorm gales are studied by using the methods of cluster analysis and mesoscale weather analysis. The results show that the average number of thunderstorm gale days in Shanxi had the regional distribution characteristics with more in the north and less in the south, more in mountainous areas and less in plain, and more in western mountainous areas and less in eastern mountainous areas. The extreme thunderstorm gales mainly occurred in the north of Shanxi and Lüliang mountainous regions. The annual thunderstorm gales days in the western mountainous area showed an increasing trend, while there were no changes or decreasing trends in other areas. The thunderstorm gales mainly occurred from May to August, accounting for 75% of the total days of thunderstorm gales in the whole year, and thunderstorm gales occurred most frequently at 16:00 and 21:00 (Beijing Time) in a day. The flow patterns of thunderstorm gales in Shanxi are mainly divided into six types, which are forward-tilting trough, backward-tilting trough, transverse trough, subtropical high and low-level warm shear line, subtropical high and low-level cold shear line, and northwest air flow. The K index threshold of all patterns from April to May was significantly lower than that from June to September, while the temperature difference between 850 hPa and 500 hPa was obviously higher than that from June to September. When each pattern meets the characteristic physical quantity thresholds of each month, it can trigger the strong thunderstorm gale weather in Shanxi, while the flow pattern configuration of forward-tilting trough has the highest percentage of triggering extreme thunderstorm gales. Over the same period, the K indices of the patterns of backward-tilting trough, subtropical high and low-level warm shear line, subtropical high and low-level cold shear line are significantly higher than those of the patterns of forward-tilting trough and northwest air flow, while the Si index of the forward-tilting trough is obviously higher than that of other patterns, which indicated that the dynamic instability condition of the forward-tilting trough triggering thunderstorm gale is better than the thermal instability condition. The CAPE and 0 ℃ layer height thresholds of the patterns of subtropical high and low-level warm and cold shear lines are significantly higher and the thresholds of T-T_d and cloud top black body temperature in the lower layer are significantly lower than those of the other four patterns. Whether hail is accompanied by thunderstorm gale process can be accurately judged by 0 ℃ layer height threshold of each month.

Based on the infrared and water vapor cloud images of Himawari-8 satellite, the visible cloud images of FY-2E satellite, the composite reflectivity factor mosaic maps of Doppler radar, the observation data of conventional weather stations and automatic weather stations and radionsonde data, the cloud image characteristics and maintaining mechanism of the squall line process in Shanxi Province on 21 September 2017 were analyzed. The results are as follows: (1) The squall line weather process was caused by the large-scale Mongolia cold vortex weather system. The surface cold front moved eastward to the unstable potential area, which triggered the formation of the squall line system. The transformation of system configuration structure in high and low level of troposphere and the formation of mesoscale cyclone and convergence line from the outflow cold air of surface mesoscale high pressure and environmental wind field were the development and maintenance mechanisms of the squall line. The convective cloud clusters merged and developed between the surface cold front and 850 hPa shear line, and the development of surface mesoscale high and low pressure increased the pressure gradient, which led to the enhancement of the squall line, and further caused surface gale during the passage of the squall line. (2) At the initial stage, the squall line was formed in the large gradient area behind the low cloud-top brightness temperature region, the rough texture area of cloud-top, the wet side of the dry and wet boundary, and the position of cold cloud cover slightly moved in front of the squall line. At the development stage, the echo of the squall line strengthened in low cloud-top brightness temperature region, and it moved along the moving direction of the low value center of brightness temperature. At the mature stage, the radar echo of the squall line coincided with the low value region of cloud-top brightness temperature. (3) The arcus cloud line, the up rushing cloud-top and the shadows on one side of convective cloud belt were the early signs of the development and enhancement of convective cloud clusters.