Lee vortices in evolving large-scale flows

Simulations of lee vortices in different types of slowly evolving large-scale flows are performed with a focus on the morphology of the vortices and the wake. Towards this end, a large-scale evolving flow consisting of a translating localized barotropic jet is used, in which the horizontally nondivergent wind field contains regions of difluence and confluence. The vortices generated by this evolving large-scale flow are compared with those occurring in steady large-scale flows to compare the effects strictly due to flow evolution with those arising from confluence and difluence.

Even when the flow evolves on a two-day timescale, the wake and lee vortex structure for a flow with uniform static stability over a mountain of fixed geometry is not simply determined by the instantaneous large-scale wind speed at the mountain crest. Instead it is also necessary to account for the acceleration or deceleration of the large-scale flow. Furthermore, even when the large-scale flow is steady, moderate amounts of large-scale confluence or difluence can significantly modify the wake and the behavior of any lee vortices.

Three types of vortex behavior developed in these simulations: periodic vortex shedding, which is visually similar to the well-known Von Karman vortex street; dipole shedding, in which a single pair of vortices sheds downstream followed by no others; and attached vortices, which do not shed. Large-scale flow deceleration and large-scale flow difluence are both unfavorable for periodic vortex shedding. Large-scale flow deceleration is more likely to lead to attached vortices due to upstream Doppler shifting, and large-scale flow difluence is more often associated with dipole shedding.