Numerical simulation of a tornadogenesis in a mini-supercell associated with Typhoon Shanshan on 17 September 2006

On 17 September 2006, three tornadoes occurred on the east coast of Kyusyu Island in western Japan during the passage of an outer rainband of Typhoon Shanshan. In order to clarify the structure of the tornado-producing storms and generation processes of the tornadoes, we performed numerical simulations in which fully compressible nonhydrostatic models (JMANHM; Saito et al., 2006) having horizontal grid spacings of 5km, 1km, 250 m and 50 m, respectively are nested into the operational regional analysis of the Japan Meteorological Agency (JMA).

The simulation well reproduced the outer rainband on the right-front quadrant of the typhoon. The rainband consisted of a number of isolated convective cells with a hook-shaped precipitation pattern and a bounded weak precipitation region. The horizontal and vertical dimensions of the storms were both only about 5 km, and a near-surface temperature difference across the gust front was very small (about 1 K). These characteristics demonstrate that the storms were mini-supercells.

The innermost simulation with a horizontal grid spacing of 50 m successfully reproduced a tornado spawned by the mini-supercell storm. The diameter of the tornado vortex near the surface was about 500 m, and the vertical vorticity reached 1.0 /s. The tornado was generated on the left-front tip of the rear-flank downdraft (RFD) when the RFD wrapped around the low-level mesocyclone and reached the rear-flank gust front. Backward trajectories originating from the tornado and a vorticity budget analysis along them elucidate that the RFD played a key role in the tornadogenesis. The diagnostics of the vertical momentum equation and several sensitivity experiments show that the precipitation loading in the area of hook-shaped precipitation pattern is crucial to the formation of the RFD and the subsequent tornadogenesis.