Large‐amplitude internal wave generation in the lee of step‐shaped topography

Abstract

[1] Laboratory experiments show that as uniformly stratified fluid flows over a smoothly-varying step, two wave-like phenomena result: boundary-trapped lee waves, characterized by an undular shear layer downstream of the base of the step, and internal waves, which propagate vertically away from the step. The frequency of both types of waves is an approximately constant fraction of the buoyancy frequency. The frequency of internal waves is moderately smaller than lee waves if the step size is small but, if the step size is large, the frequencies match more closely. Thus nonlinear dynamics act so that internal waves are generated not by topography alone but also by flow over the boundary-trapped lee waves that act as ''fluidic'' hills. This new mechanism for internal wave generation may be an important factor in mixing over rough topography in the ocean.

Stratified flow over asymmetric and double bell-shaped obstacles

“…h/w d , where w d is the downstream half-width of the obstacle. Sutherland (2002) illustrated how the first term can be eliminated with h/w d ≈ 0, but it seems plausible that the transition cannot be "infinitely" sharp: one should detect gradual (but certainly nonlinear) changes when h/w d approaches zero. This simple argumentation might be related with the observed anomaly in Fig.…”

“…He identified stationary lee-waves trapped in the wake of the obstacle and vertically-propagating internal waves of slightly different frequencies but of the same amplitudes. Sutherland (2002) concluded that the dynamics are governed by nonhydrostatic nonlinear effects. Note that his parameter range [U, N, H, h] was essentially the same as in our experiments.…”

“…Sutherland (2002) performed experiments with "infinitely" asymmetric obstacles having a long plateau of constant height and a smooth sinusoidal leeward slope. This step-like geometry enhances the role of wave generation by pressure gradient forces.…”

“…Here, we concentrate on two particular aspects of orographic waves: the effects of asymmetric obstacle shapes and the wave generation by two neighboring "mountains". Asymmetric obstacles were experimentally investigated by Baines and Hoinka (1985), and recently by Sutherland (2002). Much more experiments were performed by symmetric objects with a wide range of geometric aspect ratios (see Baines, 1995).…”

Stratified flow over topography: wave generation and boundary layer separation

“…Both studies ignored the influence of internal waves upon separation and, conversely, ignored the impact of boundary layer separation upon internal wave generation. In experiments studying stratified flow over a step [3], vertically propagating waves and boundary-trapped lee waves were found to couple resonantly. The period of both was an approximately constant fraction of the buoyancy period.…”