Numerical Simulation of Tornadogenesis in an Outer-Rainband Minisupercell of Typhoon Shanshan on 17 September 2006

Abstract

On 17 September 2006, three tornadoes occurred along the east coast of Kyusyu Island in western Japan during the passage of an outer rainband in the right-front quadrant of Typhoon Shanshan. To clarify the structure of the tornado-producing storms and the mechanism of tornadogenesis, quadruply nested numerical simulations were performed using a nonhydrostatic model with an innermost horizontal grid spacing of 50 m. Several simulated convective storms in the outermost rainband exhibited characteristics of a minisupercell. One storm had a strong rotating updraft of more than 30 m s?1 and a large vertical vorticity exceeding 0.06 s?1. This storm spawned a tornado when the low-level mesocyclone intensified. The tornado was generated on the rear-flank gust front near the mesocyclone center when a secondary rear-flank downdraft (RFD) surge advanced cyclonically around the low-level mesocyclone and overtook the rear-flank gust front at its left-front edge. Backward trajectories and vorticity budget analysis along the trajectories indicate that the secondary RFD surge played a key role in tornadogenesis by barotropically transporting the large streamwise vorticity associated with the environmental low-level veering shear toward the surface. When the secondary RFD outflow surge boundary reached the rear-flank gust front, the horizontal convergence was enhanced, contributing to the rapid amplification of the vertically tilted streamwise vorticity. The diagnostics of the vertical momentum equation and several sensitivity experiments demonstrated that precipitation loading in the area of a hook-shaped precipitation pattern was crucial to the behavior of the RFD and the subsequent tornadogenesis.