Tornado intensification near the ground: suction vortices

A key property of the asymptotic converging flow in the boundary layer under a potential vortex was not exploited by Lilly (1969). If the property had been recognized, that seminal unpublished work may have successfully branched into the theory of suction vortices. This property is that the viscosity in the boundary layer removes angular momentum, but, remarkably, in the asymptotic converging flow, the pressure head has not been reduced. Using this asymptotic property allows for relatively simple analytical solutions for the vortex the erupts from the converging boundary layer. The vortex has a strong axial flow that could be called a "suction vortex". The suction vortex is supercritical in the sense that the downward propogation of centrifugal waves, which enforce the axial pressure gradient toward hydrostatic balance, is unable to propagate downward past the vortex breakdown event capping the suction vortex.

The analytical solutions for suction vortices allows for diagnosis of the enhancement of pressure deficit that can be sustained in the supercritical flow. This enhancement is consistent with the deduction of Fujita (1971), who estimated that a suction vortex would have a wind speed twice that of the parent vortex. This magnitude of enhancement is observed in numerical simulations with a resolved viscous boundary layer, most notably those of Fiedler (1998), which analyzed simulations of suction vortices in the three-dimensional, multiple-vortex regime. Those simulations are updated here, at higher resolutions and with better visualizations. The updated simulations provided opportunity to more widely explore the properties of suction vortices, those dynamical creatures that are all the more amazing when one bears in mind that friction is the key ingredient to their intensity.