ENERGY-DISSIPATION BY BREAKING WAVES

Recent field measurements by Agrawal et al. have provided evidence of a shallow surface mixed layer in which the rate of dissipation due to turbulence is one to two orders of magnitude greater than that in a co mparable turbulent boundary layer over a rigid wall. It is shown that predictions by Phillips of the energy lost by breaking surface waves i n an equilibrium regime and laboratory measurements by Rapp and Melvil le of the mixing and turbulence due to breaking together lead to estim ates of the enhanced dissipation rate and the thickness of the surface layer consistent with the field measurements. Wave-age-dependent scal ing of the dissipation layer is proposed. Laboratory measurements of d issipation rates in both unsteady and quasi-steady breaking waves are examined. It is shown that an appropriately defined dimensionless rate of dissipation in unsteady breaking waves is not constant, but increa ses with a measure of the wave slope. Differences between dissipation rate in quasi-steady and unsteady breakers are discussed. It is found that measurements of the dissipation rate in unsteady breakers are con sistent with independent estimates of the turbulent dissipation. The a pplication of these results to models of dissipation due to breaking a nd air-sea fluxes is discussed.