Idealized simulations of wave-induced boundary-layer separation in the lee of mesoscale topography

The present contribution focuses on the application of large-eddy simulation (LES) to the study of wave-induced boundary-layer separation (BLS) in the lee of mesoscale topography. A boundary layer forms at the interface between a fluid and an external body, but may detach from it. In stratified flows, the adverse pressure gradients causing BLS can be determined by internal waves propagating in the fluid. Wave-induced BLS in the atmosphere is often associated to hydraulic jumps or to the onset of atmospheric rotors.

Atmospheric soundings observed on the windward side of the Medicine Bow Range (Wyoming) during a leeside BLS event are used as the upstream forcing for several idealized simulations. Results show that LES can capture the essential features of BLS as known from in-situ and remote observations. The most distinctive elements of the flow field are a breaking hydrostatic wave at upper levels and strong, pulsating katabatic flow near the surface. The latter detaches from the ground upstream of a recirculation region, where highly non-stationary rotor and sub-rotor vortices occur. Results show the sensitivity of the rotor dynamics to increasing surface friction, which enhances flow deceleration on the lee of the obstacle and shifts the separation point upstream. The amplitude of wavelike motions downstream of the ridge also decreases with increasing friction, leading to less intense rotor circulations.