Environmental analysis on the first documented tornado outbreak in China

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.