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.