Sub-vortices Within a Numerically Simulated Tornado: The Role of Unstable Vortex Rossby Waves

Multiple sub-vortices corresponding to suction vortices in observations are obtained within a simulated tornado for the EF4 tornado case of Funing, China on June 23, 2016. Within the simulation, the tornado evolves from a one-cell structure with vorticity maximum at its center to a two-cell structure with a ring of vorticity maximum. Five well-defined sub-vortices develop along the ring. The radial profile of tangential wind across the vorticity ring satisfies the necessary condition of barotropic instability associated with phase-locked, counter-propagating vortex Rossby waves (VRWs) along the ring edges. The phased-locked waves revolve around the parent vortex at a speed less than the maximum azimuthal-mean tangential velocity, agreeing with theoretically predicted VRW phase speed. The radii within which the wave activities are confined to are also correctly predicted by the VRW theory where radial group velocity approaches zero. Several other characteristics related to the simulated sub-vortices agree with VRW theories also. The most unstable azimuthal wavenumber depends on the width and the relative magnitude of vorticity of the vortex ring. Their values estimated from the simulation prior to sub-vortex formation correctly predict wavenumber five as the most unstable. The largest contribution to wave kinetic energy is diagnosed to be from the radial shear of azimuthal wind term, consistent with barotropic instability. Vorticity diagnostics show that vertical vorticity stretching is the primary vorticity source for the intensification and maintenance of the simulated sub-vortices.