Formation Environment of the Deadliest Tornado (EF4) in the Past 40 Years in China on 23 June 2016

An EF4 tornado with a damage swath 34.5 km long and the maximum width of 4.1 km, hit Yancheng, Jiangsu Province, China during ~1410–1500 local standard time on 23 June 2016, claiming 99 fatalities and 846 injuries. It was the deadliest tornado in the past 40 years in China, the 3rd deadliest, and one of only several EF4 tornadoes officially recorded in Chinese history. This work documents the evolution of mesocyclone and tornado vortex signature (TVS), and the real-time operational severe convective weather forecasting and warning service during this event. The formation environment of the tornadic supercell was also examined using WRF simulations.

The tornado was generated in a strong mesocyclone (with the maximum rotational velocity of 42.2 m s-1) of a supercell associated with a strong TVS (with the maximum gate-to-gate radial velocity difference of 84.5 m s-1) in front of a trough that extended southwestward from the northeast cold vortex, a typical quasi-stationary upper-level vortex with cold center that usually causes series of severe convective weather in China. Automatic weather stations at 1-min interval revealed detailed variations of gust winds, temperature and humidity near the edge of the tornado swath. The AWS observations, which have a horizontal resolution of 5-10 km, clearly showed the evolution of the mesocyclone at the surface along the tornado track.

Observational analyses showed that the tornadic supercell was spawned at the exit of a low-level jet (LLJ) at 850 hPa with no obvious surface boundaries in front of an extensive northeast-southwest-oriented cold front. The environment was quite unstable with a surface-based CAPE of 3831.4 J/kg, and a 850-hPa-based CAPE of 1417.0J/kg. Convection-permitting numerical simulation well captured the timing and the location of the CI and its pre-convective environment at different scales. It was likely the deceleration of the LLJ at 850 hPa that resulted in sufficient moisture accumulation and convergence, and thus the convection initiation above the PBL.

Severe convective weather warnings were issued about one hour before the tornadogenesis by Jiangsu Meteorological Observatory in real time. The first tornado warning in Chinese history was issued in this case when no tornado reports were received and the tornado indeed happened, despite the warning was issued 20 minutes after the tornadogenesis but still 30 minutes before the tornado demise. Stimulated by the increasing impacts of tornadoes, China Meteorological Administration is starting from this year a quasi-operational testing of making tornado forecasting and warning in National Meteoorlogical Center, Liangsu, Anhui，Hubei，Guangdong, Zhejiang provinces.