Tornado-Producing Mini Supercells Associated with Typhoon 9019

Abstract

On the night of 19 September 1990, nine mini-supercell storms were observed over the Kanto Plain of Japan in the northeast quadrant of Typhoon 9019. Three storms out of nine spawned a tornado. The characteristics of the storms and tornadoes were studied by means of a single Doppler radar, conventional radars, surface meteorological observations, and damage surveys. The mini-supercell storms exhibited characteristics similar to typical supercell storms over the Great Plains of the United States, such as hook echo, bounded weak echo region, slower movement relative to the mean wind, long lives, and rightward deviation of the storm motion relative to the mean wind shear was observed. They also displayed several differences from typical supercell storms with respect to the following points: 1) horizontal scale of the mesocyclone was smaller; and 2) vertical vorticity was confined to lower levels (less than 5 km above ground level). These similarities and differences correspond with those observed for miniature supercells in Hurricane Danny?s environment. The storm environment for these mini supercells was characterized by modest convective available potential energy (about 1600 J kg?1) and strong low-level wind shear with veering. The storm updrafts likely tilted the ambient horizontal vorticity associated with the strong low-level wind shear to generate mesocyclonic vertical vorticity. Two of the tornadoes were spawned by mini-supercell storms moving along a preexisting surface boundary that was accompanied by significant convergence, vertical vorticity, and horizontal gradients of temperature. An estimation of vertical vorticity near the ground in the boundary suggests a possibility that preexisting vertical vorticity contributed to supercell evolution resulting in tornadogenesis.