| Breaking internal waves are responsible for much of the vertical redistribution of mass, momentum, and pollutants in the stratified ocean and atmosphere. However, these breaking events must be accurately parameterized in order to correctly model local, regional, and global oceanic and atmospheric circulation. The parameterization of internal wave breaking turbulence can only be obtained from detailed, small-scale laboratory and numerical studies; for this purpose, we have designed and carried out a set of laboratory experiments examining the details of internal wave breaking.; The laboratory facility is capable of generating several types of breaking internal waves, and the detailed flow visualization of these events has pointed out marked differences in the turbulent processes even among similar types of breaking internal waves. The focus of the experiments is internal waves in a two-layer stratification; flow is visualized quantitatively with a high-resolution Planar Laser-Induced Fluorescence (PLIF) system, and complementing measurements of density and interfacial distortion are provided by other instruments. The experiments show that monochromatic long waves, forced to breaking through a laterally-contracting channel, break due to a spectacular Kelvin-Helmholtz instability originating in the high-shear wave crest and trough regions. This instability is strongly temporally- and spatially-modified by the oscillations of the driving wave shear. Unlike the frequently-studied steady stratified shear layer, which is often used to draw conclusions about breaking internal waves, the wave instability here is not governed by the canonical Ri = 1/4 stability limit. Instead, the wave time scale poses an additional constraint on instability, lowering the critical Richardson number below 1/4. Experiments were carried out to quantify this instability threshold; these experiments, have illuminated the wavenumber dependence of the critical wave steepness at which progressive monochromatic internal waves break. The results of separate experiments involving focused, breaking polychromatic internal wave trains are also described. These experiments illustrate a dramatic convective wave breaking, not unlike shoaling water waves, in which the wave tumbles forward (and sometimes backward) in a convective plunge, vigorously stirring the interfacial fluid. The impetus of this convective breaking is the background shear provided by the long waves in the packet. |
