Numerical simulations of supercell tornadogenesis associated with Typhoon Shanshan (2006)

On 17 September 2006, three tornadoes hit Kyusyu Island in western Japan during the passage of the outer rainband which accompanied Typhoon Shanshan (2006). Numerical simulations were conducted, in order to reveal the environmental field, tornado-producing storm and the generation processes of tornadoes. The simulated rainband on the right-front quadrant of the typhoon consists of a number of isolated active convective cells as the radar network observed. Some convective cells have a hook pattern and bounded weak region of hydrometeors, which are identical to the mini-supercell as shown in many previous studies. The simulation with a horizontal grid spacing of 50 m successfully reproduced a tornado spawned by a mini-supercell. The diameter of the vortex near the surface is about 500m. The vertical vorticity reached 0.7/s and surface pressure drop was about 12hPa. The model results indicate that the enhancement of low-level vertical vorticity is essential to the genesis of a tornado.