How are supercells with extreme flash heavy rainfall different from other severe supercells in the Yangtze-Huai River Region?

Using radar observation data together with ERA5 reanalysis data, this study exposes the mesocyclone characteristics of supercells associated with extreme flash heavy rainfall (FHR, hourly rainfall intensity ≥50 mm) in Jiangsu Province of China during 2020–2021, and reveals their differences from other severe supercells associated with convective hazards such as hail, gales and tornadoes. The results show that most FHR supercells occur in July and August, seasonally later than hail supercells or gale supercells in this region. During the warm season, FHR supercells generally move eastward led by westerly flow since most of them appear along the north edge of the western Pacific subtropical high. Half of FHR supercell mesocyclones sustain only about 2–3 radar volume scans (12–18 min). Most extreme FHR supercells are embedded in mesoscale convective systems and are triggered at more favorable locations where more abundant moisture converges and stronger low-level vertical wind shear is produced. In comparison, the developing extreme FHR supercell generally needs more abundant moisture but not as much higher convective available potential energy or stronger vertical wind shear as hail or gale supercells. In terms of mesocyclone characteristics, tornadic supercells exhibit the strongest rotation intensity, followed by gale and hail supercells, and FHR supercells possess the weakest rotation intensity. Hail supercells have the largest storm diameter, deepest mesocyclone, and highest elevation of strongest rotation, followed by gale supercells, FHR supercells and tornadic supercells. These differences are possibly related to the distinct formation mechanisms of the various severe convective phenomena induced by supercell: hail grows in lower ambient temperature and therefore corresponds to the highest vertical extension of supercell; the majority of convective windstorms in the Yangtze-Huai River region is associated with mid-upper-level dry air entraining into the storm and triggering a strong evaporative effect; and the definition of a tornadic supercell implies that the least diameter and the strongest rotation must approach ground. In contrast, most FHR events in this region are linked to warm-rain processes.