Numerical Simulation of Tornadoes in a Mini-Supercell Associated with Typhoon Tapah on 22 September 2019

Abstract

A tornado hit Nobeoka city on the southeast coast of Kyushu Island, Japan on 22 September 2019 when Typhoon Tapah was located about 500 km to the southwest of Kyushu Island and moving northeastward. Triply-nested numerical simulations are performed to reproduce the typhoon, a parent storm, and associated tornadoes. The simulation with the coarsest resolution reasonably reproduces Typhoon Tapah and associated outer rainbands at several 500 km east of its center, where the environment around the rainband is found to be favorable for mini-supercells. The simulation with the finest resolution reproduces a train of mini-supercells and associated tornadoes. The mini-supercells have typical structures that cause tornadoes associated with tropical cyclones. The minimum central pressure of the strongest tornado is 945 hPa. The time evolution of the simulated tornadoes is very fast: significant transitions of vortex structure occur within 1 minute before the tornado attains its peak strength. Most of the circulation of the tornado is derived from rear-flank downdrafts. Three tornadoes occur sequentially in association with non-occluding mesocyclogenesis, where a new tornado develops in the northwest of the old one.