A Comparison between the only Two Tornado Outbreak Events in China: Tropical Cyclone vs. Extratropical Cyclone Environments

Tornado outbreak is defined when ten or more tornadoes occur within a particular synoptic-scale system. Tornado outbreaks are usually associated with Tropical cyclones (TCs) and Extratropical cyclones (ECs). For instance, the tornado outbreak with 118 tornadoes from 15^th to 18^th in September 2004 was associated with the TC Ivan, and the tornado outbreak with 361 tornadoes from 3^rd to 11^th May 2003 was associated with an EC. However, unlike the U.S., tornado outbreaks seldom occur in China. Up to now, there are only two tornado outbreaks in China. One was associated with the TC YAGI on 12 August 2018, producing 11 tornadoes, and the other one was from an extratropical cyclone on 10 July 2021 (referred to as EC 711 hereafter), producing 13 tornadoes. This study attempts to reveal the differences in the features of tornadoes, their parent storms and the key tornadic environments of these only two tornado outbreaks in China, using Doppler radar data as well as the fifth-generation ECMWF reanalysis (ERA5) data.

The locations of tornadoes with respect to the center of TC or EC and their parent storms are quite different between TC YAGI and EC 711. Most of tornadoes in TC YAGI were generated in the northeast quadrant of the TC, while most of tornadoes in EC 711 were produced in the southeast quadrant of the EC. A majority of tornadoes in EC 711 were produced from supercells imbedded in QLCSs or cloud clusters, whereas most of tornadoes in TC YAGI were spawned from discrete mini-supercells. The rotational velocity of mesocyclones and the strength of tornado vortex signatures (TVSs) in EC 711 were stronger than those in TC YAGI, possibly due to their stronger vertical wind shear.

Concerning the environmental features conducive to tornado genesis, apparent mid-level dry intrusion was observed during the most concentrated tornado-genesis periods in both EC 711 and TC YAGI. However, the key thermodynamic and kinematic parameters that are favorable for tornado genesis during the concentrated tornado-genesis periods are quite distinct between TC YAGI and EC 711. Compared with TC YAGI, EC 711 featured higher maximum values of key environmental parameters highly associated with the development of convection and tornadoes, such as entraining convective available potential energy (E-CAPE), 0-1-km storm-relative helicity (SRH1), 0-6-km vertical wind shear (SHR6), entraining energy-helicity index (E-EHI) and entraining significant tornado parameter (E-STP).