Third Generation Wind Wave Model Frequency Dependence of the White-Capping Dissipation Source Function

This paper proposes new frequency dependence for the empirical formulas presently used to determine wave energy dissipation in ocean wave models. Using an energy focusing technique, several unidirectional transient wave trains were generated. Each of the transient wave train contained an isolated plunging or spilling breaker. By comparing the energy spectra of free-wave components before and after breaker it was found that: 1)the energy loss as function of frequency is almost exclusively from wave components at frequencies higher than the spectral peak frequency; 2)although the energy density of the wave components near the peak frequency are the largest, they do not significantly gain or lose energy after breaking; and 3)wave components of frequencies significantly below or near the peak frequency gain a small portion (about 12%) of energy lost by the high-frequency waves. The empirical formulas presently used to determine white-capping dissipation (Komen et al. 1994; Tolman and Chalikov 1996; Booij 1999) do not agree with the above spectral distribution of energy dissipation. Analysis of the dissipation distribution obtained by Meza et al. (2000), suggest that the dependence of the dissipation rate on the frequency should be described by, (ωωp)(1−(ωωp)E(ω), where ω is the wave frequency, ωp is the spectral peak frequency and E(ω) is the energy density spectrum. An energy dissipation source function with such a frequency dependence is being implemented and tested in a third generation wind wave model.