Numerical simulation of tornadogenesis in a supercell storm

A nested grid nonhydrostatic model (PSU/NCAR MM5 version 3.6.3) was used to simulate tornadoes, which occurred in Saga Prefecture, southwest of Fukuoka, Japan, on 27 June 2004. The model was initialized with objective analysis data. The model succeeded in simulating several weak tornadoes and their parent storm. One supercell storm and one supercell-like storm were generated in the frontal zone and those cells merged into a large single supercell. After the merger, the supercell intensified and generated some tornadoes. The storm exhibited features below; the presence of a persistent mesocyclone (vertical vorticity > 0.01 /s) within the updraft region and the hook-shaped or S-shaped structures. We focused on the strongest developed tornado. The significantly developed vertical vortex advected the condensate cyclonically from the main precipitating area extending northwest of the tornado vortex. This feature of the simulated tornado corresponded closely with a fine structure of tornadoes observed by mobile Doppler radar in the United States (Wurman and Gill 2000, Bluestein et al. 2003). The maxima of horizontal wind speed and vertical vorticity of the vortex near the surface were 37 m/s and 0.58 /s, respectively. These values exceed the threshold of the F1 tornado. The tornadogenesis occurred when the mesocyclone associated with the supercell-like storm was colocated with a low-level vortex. We found that there were two different generating processes for the low-level vortex. One is the tilting of the horizontal vortex, and the other is the horizontal shear instability. The low-level vortex was subsequently stretched by the strong updraft at low-levels, and then developed into the tornado vortex.