THE THERMODYNAMIC SPEED LIMIT AND ITS VIOLATION IN AXISYMMETRICAL NUMERICAL SIMULATIONS OF TORNADO-LIKE VORTICES

Processes that regulate the central pressure and maximum wind speeds o f tornado-like vortices are explored with an axisymmetric numerical mo del. The model consists of a rotating cylinder of fluid enclosed withi n rigid boundaries. The momentum diffusivity is a fixed function of he ight. In the rotating reference frame, relative motion is induced by a buoyancy force in the vicinity of the rotation axis, leading to the f ormation of a central vortex. The work done by the central buoyancy fo rce on a parcel rising along the axis defines theoretical and empirica l wind speed bounds on both the updraft and the low-level vortex. Cert ain processes are found that allow for the vortex to greatly exceed th is wind speed bound, or the so-called thermodynamic speed limit, howev er, in most of the parameter space the vortex wind speeds are close to the thermodynamic speed limit. The most effective limit-breaking proc ess involves a supercritical end-wall vortex with an axial jet. In ste ady state, the supercritical vortex sustains wind speeds 2. 0 times th e speed limit. A transient end-wall vortex, with the vortex breakdown travelling rapidly downwards toward the surface, is able to achieve wi nd speeds 5.0 times the speed limit. Warming of the subsiding vortex c ore past the vortex breakdown increases the maximum steady-state azimu thal wind speed by about 20% from what it would be otherwise. Axial mo mentum diffusion is not found to significantly enhance the surface pre ssure deficit in any of the simulations.