Mountain waves and mountain wakes in stratified airflows past three-dimensional obstacles

The thesis concerns the dynamics of formation and stability of mountain wakes in airflows past isolated mesoscale obstacles.;The structure of a mountain wake is inferred from airborne observations downwind of the Island of Hawaii. It is shown that mountain wakes can assume a form different from von Karman vortex streets. Hawaii's wake is generated in the east-northeasterly trade-wind flow and extends about two hundred kilometers downwind from the island. It consists of two elongated quasi-steady counterrotating eddies that give rise to a wide region of strong reversed flow along the wake axis. The moderate unsteadiness takes a form of side-to-side movement of the wake perpendicular to the mean flow direction.;The effect of surface friction on mountain wakes forming in a mixed planetary-boundary layer is investigated using the shallow-water equations with topography. The effective Reynolds number of bottom friction is proportional to the fluid depth and inversely proportional to the surface roughness and the horizontal size of an obstacle, resulting in flows past larger obstacles being more "viscous." For frictional strength characteristic of atmospheric flows, the wake vorticity continues to be primarily generated by a pseudo-inviscid process in hydraulic jumps, but the hydraulic stability properties of steady wakes are strongly controlled by bottom friction. Through a comparison of linear stability analysis and nonlinear numerical simulations it is shown that the frictional stabilization of wakes is related to the elimination of absolute instability in the wake region. The simulation of the flow past Hawaii produces a wake which is consistent with the observations.;The role of wave-breaking aloft in a transition from mountain-wave flows to flows with mountain wakes is investigated for a continuously-stratified atmosphere in which mean wind reverses direction aloft, producing a critical level for stationary waves. For a fixed Richardson number at the critical level, the response of the flow past an axisymmetric obstacle was found to depend on the non-dimensional height of the critical level and the mountain. Two distinct responses have been identified in the numerical experiments: a weak response, where the critical level acts as an almost perfect absorber for the entire spectrum of the vertically propagating waves (described well by the linear theory); and an intense response, with the wave-breaking occurring at or below the critical level. In the latter cases, a continuously-stratified analog of the 2-D hydraulic flow develops below the wave-breaking region, with strongly decelerated flow at low levels and lee-eddies appearing downstream of a lee "jump."...