Environmental analysis on the first documented tornado outbreak in China

Abstract

This study presents an analysis on the storm environmental features during the first recorded tornado outbreak in the modern history of China on August 13–14, 2018. Eleven reported tornadoes were spawned within a 20‐hr period by Typhoon Yagi (1814) during its dissipating stage after its landfall. They primarily formed when Yagi was at a relatively weak intensity. Six tornadoes were generated after that Yagi had lost its intensity as a tropical depression. These tornadoes were mainly located in the northeast quadrant with respect to the typhoon center. Specifically, they were concentrated in the overlapping region of the large storm relative helicity (SRH) and large entraining convective available potential energy (E‐CAPE) that considers the effects of the environmental air entrainment. The intrusion of midlevel dry air was responsible for the spatial pattern of large‐value E‐CAPE. Special focus was further put into the dynamical diagnosis of the cause that the maxima in SRH were distributed in Yagi's northeast sector. On cylindrical coordinates, the northeast quadrant was characterized by the strongest upward increase in the radial velocity in a layer of 950–700 hPa, which resulted from the interaction between the typhoon circulation and the subtropical high and a midlevel trough. The resultant in‐up‐out radial wind profile contributed to the large helicity in the northeast quadrant. In the lowest 50 hPa layer, the helicity was mainly owed to the upward increase in the tangential velocity associated with typhoon vortex. The diagnoses of the atmospheric environment for the tornado outbreak in this case may provide additional benefit for assessing the tornado risk within landfalling tropical cyclones in China.

Environmental ingredients that lead to tornado outbreak and tornado failure: A comparison between two similar recurving tropical cyclones

“…In addition to encountering the midlevel westerlies, Yagi started to recurve towards the northeast along with the steering flows when it was located near the western-most edge of the subtropical high (refer to the 5880-gpm isopleth in Figure 1a) at 1200 UTC on 13 August. Shortly after the recurvature process, Yagi spawned 6 tornadoes in its northeast quadrant from 0200 to 0530 UTC on 14 August, during which Yagi was at a relatively low intensity of a tropical depression (Bai, 2021). Typhoon In-Fa ( 2021) made its first landfall on the coast of Zhejiang Province at an intensity of a typhoon at 0430 UTC on 25 July 2021 (Figure 1b).…”

“…According to Verbout et al (2007), 48% of the TCs that made landfall on the southern coasts of the United States and then recurved towards the northeast underwent tornado outbreaks. In central eastern China, the only reported outbreak of tornado occurred when Typhoon Yagi (2018, hereinafter referred to as Yagi) recurved towards the northeast (Bai, 2021). Ten tornadoes were generated shortly after the recurvature of Yagi.…”

“…The north‐eastward steering flows forced by the aforementioned large‐scale systems tend to produce in‐up‐out radial wind profiles in the northeast quadrant relative to the TC centre. Such an upward increase structure in radial velocity (on cylindrical coordinates) at low levels contributes to the enhanced ambient helicity in the northeast quadrant during TC recurvature (Bai, 2021; Molinari & Vollaro, 2008).…”