Numerical simulations of tornado-producing supercell storm and tornado associated with Typhoon Shanshan (2006)

On 17 September 2006, three tornados hit Kyusyu Island, western Japan, during the passage of the outer rainband that accompanied Typhoon Shanshan (2006). The tornado which hit Nobeoka city in Kyusyu Island caused most severe damage and was assessed with F-2 scale. In order to reveal the environmental field and the tornado-producing storm, the numerical simulations were conducted. Moreover, the high-resolution simulation with the horizontal grid spacing of 50 m was attempted to reproduce the tornado explicitly.

The numerical model used in this study is the fully compressible nonhydrostatic model developed by Japan Meteorological Agency (JMA). We employed the bulk-type cloud microphysics scheme with six water species (water vapor, cloud water, rain, cloud ice, snow and graupel) and the turbulent closure scheme that predicts the turbulent kinetic energy. To conduct the high-resolution model integration, four telescoping one-way nested grids (Horizontal grid spacing 5 km; NHM5km, 1km; NHM1km, 250 m; NHM250m; 50 m; NHM50 m) were used. The initial and boundary conditions of NHM5km are provided from the operational regional analysis of JMA.

NHM1km reproduced the outer rainband about 300 km away from the typhoon center in the right-front quadrant of translating Shanshan. The wind hodograph around the rainband shows that the strong vertical shear with veering existed below 2 km AGL. The distribution of storm-relative helicity shows the peak with more than 750 m2 s-2 but that of CAPE is not related to the location of the rainband.

The rainband simulated by NHM250m consists of a number of isolated active convective cells as Radar observed. Some convective cells have the hook pattern and bounded weak region of hydrometeors at the southern tip of them. The maximum vertical vorticity is about 7 x 10-2 s-1 and upward motion is more than 30 m s-1 around 1 km and 3 km above AGL. The vertical and horizontal scale is small compared to the typical supercell storm over the Great Plains in the United States. These features are identical to the mini-supercell as showed in many previous studies. Another noteworthy feature is that the gust font near the surface boundary is distinguishable by the wind field and vertical vorticity, however, the horizontal gradient of temperature across it is weak. As this storm approached the land, the mesovortices at low-level intensified significantly.

The simulation by NHM50m successfully produced tornado spawned by the mini-supercell noted above. Note that this simulation includes full-physics processes, and free-slip surface condition is not used unlike the other previous studies. The tornado was generated on the gust front and moved with the rapid translation speed of about 100 km h-1 over the sea. The vertical vorticity reached 0.7 s-1 and surface pressure drop was about 12 hPa. The diameter of the vortex near the surface is about 500 m. The tornado exhibited the asymmetric structure with strong winds of more than 50 m s-1 only on the right side and tilted northwestward vertically.

Further analytical and numerical studies are being conducted to elucidate the tornadogenesis process.