On Modelling Tornadoes in isolation from the Parent Storm

Historically, tornadoes often have been modelled in open domains in which the boundary conditions on the top of the modelled vortex are imposed, specified by flow‐straightening, or extrapolated from upstream. In current three‐dimensional modelling efforts, reasons of computational efficiency still impose a need to model the tornado in isolation from 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‐resolution, axisymmetric storm model. The tornado in the nested model is compared with the tornado in the complete model. (The complete model by itself can also be thought of as containing a “fully interactive, two‐way nested model"). The use of extrapolated or straightened outflow boundary conditions on the nested model is shown to have a profound effect on the tornado solution. Maximum windspeeds in the nested model can differ by more than a factor of two from those in the corresponding region of the complete model. Only subcritical vortices are considered here.