Laboratory generation of internal waves from rough topography

In the first of a multistage process to understand the generation of internal waves from rough topography, we have performed laboratory experiments to study wave generation over and in the lee of model topography. We have chosen to study periodic, finite-amplitude topographies (with topographic height to separation distances, H/L = 0.1 and 0.2), representative of the earth's major mountain ranges as well as the repetitious topographic features of the ocean floor. The topographic shapes are selected to encompass varying degrees of roughness, from smoothly varying sinusoidal hills to steeper triangular and rectangular hills.

The model hills are towed at a range of speeds along the surface of a uniformly salt-stratified fluid. The experiments show that internal waves are generated not only by flow over the hills but also by flow over "fluid hills" in the lee, or boundary-trapped lee waves, and by vigorous turbulence that is created far in the lee of steeper topographies. Waves are visualized and their characteristics measured using a non-obtrusive optical technique called "synthetic schlieren".

For the sinusoidal hills, experimental results are compared with linear theory predictions, which stictly apply to waves generated by flow over smooth hills with small values of H/L. For low tow speeds, U, the internal wave frequencies are consistent with those predicted by linear theory. However, when U reaches a sufficiently high threshold, in which linear theory predicts no propagating waves, internal waves are still excited in the lee of topography with frequency an approximately constant fraction of the buoyancy frequency, N. In these cases, boundary-trapped lee waves are observed and couple with the vertically propagating internal waves.

The effect of turbulence on wave generation is further explored using the steeper triangular and rectangular topographies. In these cases, turbulent lee waves emerge even at low towing speeds and vigorous turbulence develops far in the lee as the tow speed increases. Smaller-scale internal waves are observed below this turbulent region, the properties of which are compared to those generated by topographic forcing and lee waves.