Comparison of the characteristics and formation mechanisms of Grade 12 gale winds in Yichun, Jiangxi during two successive Derecho events

On April 21, 2025, two successive Derecho events, separated by approximately 4 hours, occurred sequentially along a corridor extending from northern Hunan through northern Jiangxi to Zhejiang. Both derechos intensified notably over Yichun, Jiangxi, where record-breaking Grade 12 gales were observed at four stations, resulting in severe damage. To improve understanding of the formation mechanisms of Derecho events and extreme gales, this study compares the characteristics, causes and intensification mechanisms of the two Grade 12 gale episodes in Yichun using upper-air and surface observations, dual-polarization radar data and ERA5 reanalysis data. The main findings are as follows. (1) The extreme weather was triggered by two sequential Derecho events processes. Favorable environmental conditions for Derecho events occurrence include low-level warm advection, mid-level dry layers, deep 0–8 km vertical wind shear of 30.7 m·s⁻¹, and a mean entrainment-layer wind speed of 19.2 m·s⁻¹. (2) From 15:30 to 15:50 Beijing Time, the first Grade 12 gale in Yichun was jointly induced by multiple successive downbursts and density currents associated with cold pools. The cold pools produced a 1-hour temperature decrease of 10 °C following the merger and intensification of supercells and small convective cells. (3) The second Grade 12 gale in Yichun during 19:00–19:30 Beijing Time was generated by combined effects of downbursts and storm-scale momentum downward transport. Such momentum transport was characterized by a temporally continuous high-wind core of 27 m·s⁻¹ that rapidly descended from an altitude of 4–7 km to the ground. (4) For spring nowcasting services over the region south of the Yangtze River, particular attention should be paid to the rapid intensification of local gales when downbursts, downward momentum transport, and intense cold pools occur simultaneously or successively at a given location. Under the environmental conditions characterized by strong winds within the entrainment layer, multi-cell storms can also produce surface Grade 12 gales through pronounced downward momentum transport.