Mountain waves produced by a stratified shear flow with a boundary layer. Part III: Trapped lee waves and horizontal momentum transport

International audienceThe boundary layer theory for non-hydrostatic mountain waves presented in Part II is extended to include upward propagating gravity waves and trapped lee waves. To do so, the background wind with constant shear used in Part II is smoothly curved and become constant above a "boundary-layer" height which is much larger than the inner layer scale. As in Part II, the pressure drag stays well predicted by a gravity wave drag when the surface Richardson number > 1 and by a form drag when 1) but situations characterized by positive and negative Reynolds stress now combine when ∼ 1. In the latter case, and even when dissipation produces positive stress in the lower part of the inner layer, a property we associated with form drag in Part II, negative stresses are quite systematically found aloft. These negative stresses are due to upward propagating waves and trapped lee waves, the first being associated with negative vertical flux of pseudo-momentum aloft the inner layer, the second to negative horizontal flux of pseudo-momentum downstream the obstacle. These results suggest that the significance of mountain waves for the large scale flow is more substantial than expected and when compared to the enhancement of the boundary layer form drag by the mountain