Differences in Tropical Cyclone Tornado Activities and Key Tornadic Environments between China and the U.S

Tornadoes may have different features in different climate regimes and/or over different topography. Substantial distinctions have been observed between tornadoes in China and the U.S. The yearly midlatitude tornado occurrence in China is only 5-10% of that in the U. S., due to lower vertical wind shear and the consequent lower significant tornado parameter (STP) in the tornado incidence area during tornado season. Similar to mid-latitude tornadoes, tropical cyclone (TC) tornadoes are also quite different between China and the U. S. Even though tornado-producing TC in China has a similar frequency as in the U. S., with an average of 4.4 per year, TC tornadoes are much less than those in the U. S. with an average of 8.5 versus 51.6 per year. This study aims to reveal the differences in the characteristics of TC tornadoes as well as their parent TCs, and the key tornadic environments behind the large discrepancy of TC tornado frequency between the two countries, using tornado datasets as well as the fifth-generation ECMWF reanalysis (ERA5) dataset from 2007 to 2021.

Results show that TC tornadoes in China are mostly located along the coastline while TC tornadoes in the U. S. are located both along the coastline and inland far away from the coastline. The average latitude of TC tornadoes in China is more to the south than that in the U. S. Moreover, TC tornadoes are mostly distributed in the northeast quadrant of TC in the U. S., but in both the northeast and northwest quadrants of TC in China. The number of TC tornadoes in China peaks in September but in August in the U. S., with a similar peak hour at 15:00-18:00 local time.

The reasons for the huge difference in TC tornado number between China and the U.S. are three folds. Firstly, the regions with the most beneficial thermodynamic and kinematic conditions overlap in the northeast quadrant of parent TCs in the U. S., while the two beneficial regions situated in two different quadrants of TC in China. The maximum value of entraining convective available potential energy (E-CAPE) is located in the southwest quadrant while the maximum 0-1km storm relative helicity (SRH1) and 0-6km vertical wind shear (SHR6) were located in the northeast quadrant. This non-overlapping juxtaposition between favorable thermodynamic and kinematic regions results in a lower composite parameter such as entrainment-STP (E-STP) and entrainment-EHI (E-EHI), compared with those in the U. S. In addition to this misplacement of favorable thermodynamic and kinematic conditions, tornadic TC in China also have smaller E-CAPE and 0-6km vertical wind shear. Moreover, compared with the U. S., tornadic TCs in China have generally shorter track, which may have provided less opportunity for tornado genesis. The more-to-the-south average latitude of TC center when producing tornadoes in China than in the U. S. suggests less baroclinicity and thus also less favorable environment for tornado genesis.