Estimating the effective energy transfer velocity at air‐sea interface

Abstract

An analytical expression of the effective energy transfer velocity relating surface wind stress and energy dissipation is derived from the parameterization functions of wave energy dissipation and ocean surface friction coefficient. The equation is applied to analyses from two field experiments at very different stages of wave development. Discussions on relevant issues such as the effect of wind field steadiness, scaling, and spectral resolution are presented. A key result from this investigation is that the effective energy transfer velocity is confined in a narrow range, with the majority of data points within 1.5 and 3 m/s, over a wide range of wind speed (5–16 m/s) and dimensionless frequency (0.8–4.2) of the wavefield. A sharp increase in the energy transfer coefficient and effective energy transfer velocity occurs at a wind speed near 6 or 7 m/s. Data points that deviated significantly from the parameterization prediction appear to be associated with wind field unsteadiness, which is not accounted for in the formulation of parameterization function. The breaking process is local in space or time as well as in wave number or frequency. Global (mean) quantities such as dominant wave speed and wind friction velocity are not the proper scaling factor for the breaking velocity or energy transfer coefficient. The scaling factor should reflect the local characteristics (steepness, acceleration, or orbital velocity) of the wavefield that lead to waveform instability.