Refined source terms in wind wave models with explicit wave breaking prediction. Part I: Model framework and validation against field data

Wave breaking in the open ocean is a widespread air–sea interfacial process with very significant geophysical and maritime importance, yet present spectral wave forecast models do not provide explicit forecasts of breaking wave properties. Recent advances in understanding the wave breaking process have made it possible to redress this deficiency.This paper describes a novel methodology that adds accurate forecasts of the spectral density of breaking crest length per unit area and associated breaking strength for the dominant wind waves to standard directional wave height spectrum forecasts.A threshold-based formulation for the breaking component of the dissipation rate source term is proposed within a broad bandwidth spectral wind wave model. An ‘exact’ form of the nonlinear source function was used to avoid spurious effects arising from faster approximate versions for this source term. A spectral wind input formulation compatible with these two source terms was chosen from the suite of existing forms. Our model was required to reproduce measured dimensionless energy evolution, mean squared slope, directional spreading, wind stress and total water-side dissipation rates. In addition, we sought to match modelled and observed breaking properties. This large set of constraints required a critical reassessment of the strengths of the wind input and dissipation rate source terms relative to the nonlinear spectral transfer term.Detailed comparisons are made between the model predictions and results from the unique FAIRS open ocean data set where breaking wave observations were gathered along with wind stress, wave height and water-side dissipation rate measurements. The model results closely reproduced the observed breaking wave properties in addition to the characteristics listed above.