Abstract:

By using methods of synoptic and physical quantity diagnosis, elevated thunderstorms which produce hail in the cold sector of stationary front from January to April in past thirty years in Guizhou are analyzed. The results show that elevated thunderstorm is one of the important types of thunderstorms which produce hail in Guizhou in the early spring. Half of the hail cases were caused by elevated thunderstorms. Most of the elevated thunderstorms which produced hail occurred in the Mid-Eastern Guizhou, different from the spatial distribution of hail occurring in Guizhou. Analyzing the 37 typical processes of elevated thunderstorms, we find that the stationary front between Guizhou and Yunnan is the most important weather system. At 200 hPa height the impacting weather system is the strong southwest flow before the tough. At 500 hPa height the impacting weather systems are the southwest flow, west flow and active tough. When the active tough moves to east it causes cold air moving to south area on low level. On low level 700 hPa and 850 hPa, the impacting weather system is the shear. With (due to) the moving of shear the wind changes fast. On the surface the stationary front always swings between the middle of Guizhou and the east of Yunnan, and commonly stays at the area between the two provinces. According to the changes of stationary front the weather situations can be classified into three types: 1) the stationary front is moving to the west, 2) the stationary front is static, and 3) the stationary front is retreating to the north. Comparative analysis of the 3 processes shows that: because of the stationary front, low level atmosphere is stable, but it lifts the uplifted height onto the top of the stationary front, then an inversion layer and CAPE occur and the vertical instability structure appears, presenting a graphic of “dry upper-level and wet lower-level”. The analysis of vapor conditions shows that vapor comes from the southern wind above the stationary front. The analysis of vertical wind shear shows that the strong vertical wind shear on the front is beneficial for the elevated thunderstorm. Because of the differences of geographical position and the changes of stationary front, the height of the stationary front inversion layer is different and the height of frontogenesis is different. When the stationary front moves to the west affected by the cold air from north, the frontogenesis function shows that the trigger mechanism is the θ_se density increase on the top of inversion layer, which is caused by the shear between the north wind and south wind. As the cold air is strong and deep, the layer of shear is higher. When the stationary front is static, the trigger mechanism is same as case one, but the cold air is not strong and deep, so the layer of shear is lower. When the stationary front moves to the north, the south wind is strong. The trigger mechanisms are F2 and F3, and the two factors make the θ_se increase in the south of Guizhou, which is different from the case one and case two.

Key words: hail, elevated-convection, stationary front, Guizhou