Climatology of Tropical Cyclone Tornadoes in China from 2006 to 2018

Landfalling tropical cyclones (TCs) often spawn tornadoes (hereinafter referred to as TC tornadoes), which pose a threat to both human life and property in coastal regions. Although TC tornadoes are generally less intense than their midlatitude counterparts, they cause about 10% of the fatalities and about 0.5% of the damage to property attributable to their parent landfalling TCs. In the U.S., there are about 3 landfall tropical storms and hurricanes annually, which produce ~ 70 TC tornadoes. In China, there are about 8 landfall TCs each year, however, a systematic analysis of historical TC tornadoes in contiguous China has been missing. Previous studies on TC tornadoes in China have mainly focused on individual case reviews or a regional climate of TC tornadoes. With the increasing online exposure of TC tornadoes, both the general public and meteorologists in China have become more aware of TC tornadoes. This study aims to formulate a basic climatology of TC tornadoes in China from 2006 to 2018 and to examine the spatial distributions of some key environmental parameters that have been found to be closely related with TC tornadoes. In particular, the TC tornadoes that occurred in 2018 and their environmental features were discussed given their high societal impact.

Results showed there were 5 TC-tornadoes per year on average in China, which are much less than those in the U.S. About one-third of landfall TCs in China were tornadic. TC-tornadoes preferentially formed in the afternoon shortly before and within about 36 h after TC landfall. Most of the TC-tornadoes were spawned within 500 km of TC center. Two notable characteristics were found: (1) TC-tornadoes in China mainly occurred in the northeast quadrant (Earth-relative coordinates) rather than the right-front quadrant (TC motion-relative coordinates) of the parent TC circulation; and (2) most tornadoes were produced by TCs that made landfall with a relatively weak intensity, in contrast with the U.S., where most tornadoes are associated with stronger TCs. Further analyses showed that TC-tornadoes in China tend to be spawned in an environment with a large low-level storm-relative helicity and a large convective available potential energy, taking entrainment effects into account. TC-tornadoes were particularly active in 2018, with 25 reported tornadoes accounting for 37% of the total surveyed samples. The first recorded tornado outbreak in the modern history of China occurred in the envelope of TC Yaji (2018). The outbreak event, with 11 reported tornadoes, probably resulted from the significant mid-level intrusion of dry air and the recurvature of Yaji due to the interaction of Yaji with a mid-latitude trough.