ON MODELING TORNADOES IN ISOLATION FROM THE PARENT STORM

Historically, tornadoes often have been modelled in open domains in wh ich the boundary conditions on the top of the modelled vortex are impo sed, specified by flow-straightening, or extrapolated from upstream. I n current three-dimensional modelling efforts, reasons of computationa l efficiency still impose a need to model the tornado in isolation fro m the storm using what could be called a ''one-way nested model''. The effect of such isolation on a tornado simulation is investigated here with a redundant one-way nested model embedded within a high-resoluti on, axisymmetric storm model. The tornado in the nested model is compa red with the tornado in the complete model. (The complete model by its elf can also be thought of as containing a ''fully interactive, two-wa y nested model''). The use of extrapolated or straightened outflow bou ndary conditions on the nested model is shown to have a profound effec t on the tornado solution. Maximum windspeeds in the nested model can differ by more than a factor of two from those in the corresponding re gion of the complete model. Only subcritical vortices are considered h ere.