The energetics of breaking events in a resonantly forced internal wave field
A series of vertical density profiles was taken in a stratified tank in which a standing internal wave was forced to amplitudes at which it became unstable and, as a result of the instability, localized patches of mixing were generated within the fluid. By resorting the density profiles the available potential energy in the patches could be calculated and, by comparison with the average buoyancy flux in the tank (determined from density profiles taken before and after each mixing run), an average efficiency of utilization of available potential energy, ηAPE, was calculated. Along with previous measurements of the flux Richardson number, Rif, ηAPE was used to show that the mean value of the overturn Froude number, Frf, in the patches was 1, thus implying a balance between the rate of release of available potential energy and dissipation in the mixing patches. On the other hand, the patch-averaged overturn Reynolds number, Ret, was so low that, based on the results of previous laboratory experiments on stratified mixing in the wake of a biplanar grid, most of the patches cannot have been actively mixing at the time of sampling.
It is shown that the temperature and conductivity gradient spectra in different patches can be interpreted in a way consistent with the visualization of mixing events, that is, showing an evolution from the generation of an initially unstable density distribution, through the formation of coherent structures as the fluid restratifies and finally the degeneration of these structures into the finer scales of motion at which mixing occurs.(Received December 18 1990)
(Revised August 21 1991)