A numerical simulation of a tornado-spawning mini-supercell that occurred on 19 September 1990 over Kanto plain in Japan has been performed with ARPS Ver. 4.5.1 with horizontal resolution of 150m. Observed features of the mini-supercell including a tornado-like vortex were well reproduced by the simulation.

Vortex-lines that constitute the simulated tornado-like vortex consist of two major groups: One starts near the ground in the northeast, runs southeastward and tilts up at the east side of the vortex. The other originates from several hundred meters above the ground to the north, comes southward while descending, approaches from the west side of the vortex, and finally tilts up. These features of the vortex-lines are similar to those of the backward trajectories of air parcels that constitute the vortex.

In order to understand the mechanism that causes the complicated airflow structure in the simulated supercell, the pressure field was decomposed of buoyancy pressure and dynamic pressure. Then, the momentum budget along the trajectories is examined. It is found that, due to the interaction between the ambient vertical shear and the storm updraft (Rotunno and Klemp, 1982), a region of negative perturbation pressure develops on the north side of the updraft at the height of several hundred meters. This pressure gradient drives the air parcels traveling westward at the height of several hundred meters above the ground into the region north of the updraft and deflects their paths southward and downward. As they approach near the ground and near the tornado-like vortex, they are in turn driven into the vortex by the negative pressure perturbation associated with it. The contribution of the net buoyancy (buoyancy minus vertical gradient of the buoyancy pressure) to the downward motion along the trajectories is found to play a relatively minor role.