The thermodynamic speed limit and its violation in axisymmetric numerical simulations of tornado‐like vortices

Processes that regulate the central pressure and maximum wind speeds of tornado‐like vortices are explored with an axisymmetric numerical model. The model consists of a rotating cylinder of fluid enclosed within rigid boundaries. The momentum diffusivity is a fixed function of height. In the rotating reference frame, relative motion is induced by a buoyancy force in the vicinity of the rotation axis, leading to the formation of a central vortex. The work done by the central buoyancy force on a parcel rising along the axis defines theoretical and empirical wind speed bounds on both the updraft and the low‐level vortex. Certain processes are found that allow for the vortex to greatly exceed this wind speed bound, or the so‐called thermodynamic speed limit; however, in most of the parameter space the vortex wind speeds are close to the thermodynamic speed limit. The most effective limit‐breaking process involves a supercritical end‐wall vortex with an axial jet. In steady state, the supercritical vortex sustains wind speeds 2.0 times the speed limit. A transient end‐wall vortex, with the vortex breakdown travelling rapidly downwards toward the surface, is able to achieve wind speeds 5.0 times the speed limit. Warming of the subsiding vortex core past the vortex breakdown increases the maximum steady‐state azimuthal wind speed by about 20% from what it would be otherwise. Axial momentum diffusion is not found to significantly enhance the surface pressure deficit in any of the simulations.