COMPRESSIBILITY AND WINDSPEED LIMITS IN TORNADOES

Numerical simulations, with a compressible model, show transient torna does with an axial windspeed that exceeds the speed of sound. In gener al, and even when the flow is subsonic, compressibility decreases the extremes in the windspeeds, as compared with a corresponding incompres sible numerical simulation. In theory, an isentropic axial flow will b ecome locally supersonic (with respect to the surface or a stationary shock wave) when the core pressure drops below 53% of the external pre ssure. When such supersonic flow develops in the numerical simulations , a shock wave occurs within the vortex breakdown, where the supersoni c axial jet terminates. The extreme low pressure and supersonic axial flow in these simulated suction vortices is not altogether unnatural. The associated radial inflow at the base of the suction vortex, just a bove the ground, is relatively tame by comparison, and in accord with observations of extreme tornado damage. In all compressible simulation s, the radial inflow was less than 1.26 times the thermodynamic speed limit, which, even for a convective available potential energy (CAPE) of 5000 m(2) s(-2), puts the radial inflow at less than about 126 m s( - 1).