Combining laboratory experiments and ocean observations of breaking waves to estimate whitecap coverage with a spectral wave model

Wind-driven air-entraining breaking wave whitecaps play a crucial role in air-sea interaction and upper ocean physics: they dissipate wave energy, enhance ocean-atmosphere gas exchange and generate sea spray aerosols. Providing global, sea-state dependent estimates of breaking wave whitecap coverage is necessary to improve whitecap-based parameterisations of bubble-mediated air-sea gas fluxes and aerosol particles.

We will present an energy balance model that connects individual whitecap foam properties to breaking wave energy dissipation and use the model to provide estimates of breaking wave whitecap coverage derived from a spectral wave model. The energy balance model has been validated with breaking wave laboratory experiments and compared to previous datasets from the literature. The combined experimental data, which span almost 3 orders of magnitude in energy dissipation, show that the volume of the sub-surface two-phase flow integrated in time during active wave breaking is effectively linearly proportional to the total energy dissipated by breaking. This result is then used to estimate breaking wave whitecap coverage using the European Centre for Medium Range Forecasts (ECMWF) spectral wave model, ECWAM, and these model results are compared to field measurements. The model results and the field measurements largely agree to within a factor of 2 when the effect of swell waves and wave development on wave breaking are explicitly accounted for. Insights on the importance of constraining the bubble plume penetration depth beneath whitecaps and the relative contribution of whitecaps in balancing wind energy input to the upper ocean will be discussed.