Characteristics and causes of “3·31” extreme thunderstorm gale process in northern Jiangxi

Influenced by a strong squall line, an extreme thunderstorm gale process occurred in Nanchang City on the early morning of March 31, 2024, resulted in the death of three individuals from a residential building fall. Using conventional observations from regional automatic weather stations (AWS), Doppler weather radar data, and 5-minute ground-based observation data, the observation characteristics of the process were analyzed, the causes of disastrous winds were discussed in terms of its influenced system, environmental conditions, mesoscale system evolution and topographic effects, a conceptual model of such disastrous winds was obtained. The results are as follows. The wind event was widespread, intense, and highly destructive. The number of thunderstorms with winds above level 10 is the second most in Jiangxi Province since meteorological records began, and the maximum wind speed reached 45.5 m·s⁻¹. The key systems influencing this process include the high-altitude trough with a large radial extent, the deep low vortex, the strong low-level jet stream, the development of a powerful surface cyclone, and the high-altitude jet stream. The abundant water vapor in the lower atmosphere, moderate thermal vertical instability, and strong deep vertical wind shear provide extremely favorable conditions for the formation, and prolonged persistence of the squall line. Strong dry air entanglement (500 hPa T-T_d 26~40 ℃), the drag of short-term heavy precipitation (7-10 mm in 5 minutes) and strong vertical wind shear in the lower layer (0-1 km 12.7 m·s⁻¹) are conducive to the generation of extreme winds. The low temperature and high pressure at night in Poyang Lake, combined with the convergence of lake winds and environmental winds on its west side, is conducive to the generation of mesoscale low pressure and enhancement of extreme winds. This process was caused by a squall with a life time of about 7 h and a wind speed of more than 120 km·h⁻¹. The structure of the squall was clear, and the strongest pressure gradient between the thunderstorm high pressure and the pre-squall low pressure exceeded the historical extreme values. The rapid inflow behind the bow-shaped echo on the radar and the mesoscale vortex on the front side are important reasons for the disastrous winds in Nanchang. The mesoscale vortex embedded in the front of the squall line is the key early warning of this extreme winds, which occurred about 30 min earlier than the occurrence of disastrous winds.