Lee vortex shedding in vertically non-uniform flows

Previous research on the formation of vortices in the lees of isolated oceanic mountains has assumed the existence of a vertically uniform fluid, whether in terms of a two-layer shallow water model or an infinitely-deep stratified fluid. Only the former explicitly accounts for the effects of an inversion, and neither fully incorporates the effects of a finite-width region of vertical wind shear.

A number of cases of vortex shedding identified from satellite imagery are analyzed to determine which atmospheric characteristics favor or discourage the existence of lee vortices. Vertical profiles of the atmosphere are obtained either from routine radiosonde observations or from numerical weather prediction model output. Quantities used to predict the appearance of shedding from two common models, the shallow water model and the constant stratification and wind speed model, are computed and the predictions checked against the satellite imagery to determine the performance of the models, and to discern which effects are empirically more likely to result in vortex shedding. The impact of inversion height and strength, and of wind shear and direction, are analyzed.

The effect of inversions and wind shear are also investigated through a series of numerical simulations motivated by the observed atmospheric structure in cases of vortex shedding. An idealized three-layer stability profile and a shallow layer of either forward or reverse unidirectional shear are employed to compare with the results of constant stability and wind speed runs. The resulting vortices are analyzed in terms of strength and behavior to determine how non-uniform winds and stability alter the creation of the vorticity which gives rise to the vortices.