A 15-m resolution numerical simulation of a tornado in South China: Structure and vorticity budget of the suction vortices

Abstract

An EF3 tornado occurred in the spiral cloud belt surrounding Typhoon Rainbow on October 4, 2015. We performed six nested numerical simulations with horizontal resolutions of 4050, 1350, 450, 150, 50 and 15 m to analyze the structure and mechanisms of formation of the tornado. The typical structure of the tornadic supercell was simulated at a resolution of 444 m. The tornado-like vortex system developed until the grid spacing was ≤148 m. The suction vortex inside the tornado were captured at a resolution of 15 m. These suction vortices developed and rotated rapidly before being organized into the main vortex of the tornado. We used the vorticity equation to analyze the vorticity budget of the suction vortex. The main contribution to the vorticity was the solenoidal term. The horizontal convergence term mainly influenced the high-level outflow, whereas the tilting term affected the mid-level updraft. The frictional term mainly affected the lower levels where the suction vortex developed. We propose a conceptual model based on our results. Influenced by the frictional force, the suction vortex generated, developed and merged into the tornado vortex near the surface. The vertical circulation of the strong spiral region was influenced by convergence at higher levels and inflow at the lower rear of the tornado. Tilting of the vorticity tube and merging of the suction vortex caused the hydrometeors to aggregate and fall at the location where the tornado was generated.