Axisymmetric Analysis of Tornado-Like Vortices in Simulated Supercells

Abstract Present-day simulations of supercell thunderstorms have high resolution and incorporate the physical processes known to be conducive to simulating tornado-like vortices (TLVs). In such supercell simulations, TLVs are identified by the strength, duration, and location of the vertical vorticity in the simulated supercell. To bring the analysis of these supercell-produced TLVs a step closer to observations and theory, the TLV in an advanced supercell simulation is identified as the tornado center, and the Cartesian model velocities are transformed to cylindrical coordinates and azimuthally averaged. The azimuthally averaged TLV exhibits many of the observed and theoretically modeled features of tornadoes, including an end-wall vortex with strong maximum vertical and azimuthal velocities ( w max and υ max ) close to the ground with transition to the weaker core velocities ( w c and υ c ) aloft through vortex breakdown. A theory for the rotating-flow boundary layer with radial and azimuthal inflow velocities modeled on the axisymmetric analysis is shown to produce good qualitative agreement with the analyzed axisymmetric TLVs; however, the theoretical w max and υ max are too large as the theory does not account for vortex breakdown. Estimation of the corner flow swirl ratio suggests a limit of υ max / υ c slightly greater than unity; since υ c is a feature of the mesocyclone, there is a limit on the extent to which the amplified velocities of the end-wall velocities can be realized. To augment the diversity of cases, the present analysis is applied to a simplified set of supercell simulations; the present theory explains several features of the axisymmetric vortices.