Air-sea momentum flux in the presence of frequent wave breaking events: theory and laboratory observations

Under high-wind conditions, breaking surface waves likely play an important role in the air–sea momentum flux. Previously, a coupled wind–wave model was developed for very young seas (strongly forced wind waves). The model is based on the momentum and energy conservation in the air and the wave energy balance. The model predicts that the wind stress is dominated by the form drag of breaking waves, and that the breaking-wave distribution in the dominant wave scale strongly depends on wind forcing. The objective of this study is to validate different components of this coupled wind–wave model against laboratory observations of wind stress, wind speed, wave spectra, and the breaking-wave distribution. First, the air-side momentum/energy balance is examined by comparing the observed wind speed/stress profiles and the total (integrated) momentum/energy input to breaking and non-breaking waves. This allows us to estimate the form drag due to airflow separation at breaking wave crests. Second, the wave energy balance is examined by estimating the energy input from wind and the energy dissipation due to wave breaking. This allows us to estimate the energy dissipation rate per unit length of breaking wave crests. Our preliminary results suggest that the form drag of breaking waves plays an important role in air-sea momentum flux in very young seas, although it is not as large as previous estimates. The estimated energy dissipation rate by breaking waves is near the upper range of previous estimates.