Multi-stage Characteristics and Causes of a Severe Convective Process in Guangxi in Spring

Based on multi-source data, multi-stage characteristics and mechanisms involved in a severe convective process are analyzed which occurred in Guangxi on 4-5 April 2024. The process can be divided into 3 stages. The first stage is characterized by gales and hail, which occurs within the divergent background circulation on the left side of the upper jet exit area and in advance of the mid-level short-wave trough. The southerly air flows over mountains, causing warm and moist air to accumulate in the southern region of the Youjiang River Valley, which leads to strong convective instability. The frontal system moving southward, combined with the northward flow of cold air, creates stronger low-level frontogenesis in the valley. This interaction induces secondary circulation in the vertical direction due to topographic uplift, thereby triggering convection. The upper layer of the atmosphere is dry while the lower layer is wet, with sufficient unstable energy and large vertical wind shear in the western Guangxi, which promotes the development of convective cells to form supercell storms, resulting in gale and hail. The second stage is characterized by short-term heavy precipitation, influenced by the dynamic forces of the middle and upper-level systems. The upper-level jet and mid-level shortwave trough develop and propagate eastward. The divergence on the right side of the upper-level jet entrance area, combined with the synoptic scale uplift ahead of the mid-level short-wave trough and the shear line at 850 hPa, triggers elevated convection in the northeastern Guangxi. The starting height of convection is high, but the development height and echo intensity are lower than those of the first stage, and the inversion layer around 850 hPa is warmer that causes the hail particles easy to melt after falling, resulting in short-term heavy precipitation. The third stage is characterized by short-term heavy precipitation, with an intensity that surpasses that of the second stage. Both 850 hPa low-level jet and 925 hPa boundary jet develop at night, leading to significant deep convergence. The warm and humid transport significantly enhances thermal conditions in southeast Guangxi. The convective system develops in the exit region of 850 hPa low-level jet, at the junction of the south-north wind in 925 hPa boundary jet, and in areas of higher terrain. Due to high levels of precipitable water, the dense convective system structure with the low mass center and the convective system train effect, the weather is dominated by short-term heavy precipitation and stronger than that observed in the second stage.