On boundary layer separation in the lee of idealized topography

While boundary layer separation (BLS) has been the subject of intensive study in fluid mechanics and aerodynamics, the separation of the atmospheric boundary layer (ABL) from the ground surface, under the influence of gravity wave induced adverse pressure gradient, has received much less attention. The onset of BLS induced by gravity waves excited by stratified flow past idealized topography is examined in this study based on idealized simulations using the Navy's COAMPS® with a no-slip bottom boundary condition. The results indicate that the onset of wave-induced BLS is controlled by both the boundary layer and gravity wave characteristics. For BLS induced by trapped waves, the onset of BLS is closely related to the wave amplitude, therefore, the mountain height, instead of the downslope wind speed. Sensitivity tests indicate that the critical mountain height for the onset of BLS is relatively insensitive to the surface roughness, increases with decreasing Richardson number in the ABL, and increases with surface heat flux. The critical mountain height for the onset of BLS shows moderate sensitivity to terrain geometry such as the terrain horizontal length scale and three-dimensionality. BLS induced by propagating waves in uniformly stratified flow and by a hydraulic jump in the presence of an inversion is examined as well. The characteristics and dynamics associated with the three types of BLS are discussed. A linear ABL theory and a single layer viscous ABL theory are proposed, both of which provide a consistent description of the characteristics derived from the numerical simulations.