竜巻様渦の遷移に関する数値実験

Abstract

The transition of tornado-like vortices is numerically investigated with the use of nonhydrostatic axisymmetric and three-dimensional models. Tornado-like vortices are generated by imposing a specified external force near the center axis of the computational domains that are rotating at a constant angular velocity. When the vortex transition from one to two-celled vortex occurs, there is an overshoot of the radial inflow near the surface. The overshoot flow turns upward, rebounding outward to a larger radius. At the same time, downdraft penetrates to the surface at the rotating axis. The overshoot is generated by coupled effects of inward advection and inward pressure gradient force. The downdraft is caused by downward pressure gradient. Thereafter, the two celled vortex transform to a multiple-vortex structure because the horizontal shear and Kelvin-Helmholtz instabilities occur in the non-axisymmetric flow.