Energy dissipation of wind‐generated waves and whitecap coverage

The energy dissipation per unit area of the ocean surface attributed to fetch‐ or duration‐limited wind‐generated waves can be expressed in terms of wind speed, significant wave height and peak wave frequency. Such a parameterization equation can be exploited for obtaining a first order estimation of the rate of energy input through the air‐sea interface in the world's oceans using satellite output of wind speed, wave height and wave period. For general wind wave events in the ocean with event duration longer than one hour, the energy dissipation (in W/m2) is equal to the product of the density of air, wind speed cubed and a proportionality coefficient between 0.00037 and 0.00057. Using the equation to calculate the wave energy dissipation, the whitecap coverage is proportional linearly to the energy dissipation. The threshold energy dissipation for whitecap inception is between 0.013 and 0.038 W/m2, which corresponds to a threshold wind speed of between 2.5 and 3.6 m/s. The proportionality coefficient is relatively constant for a wide range of wave growth conditions in comparison to the data scatter in the whitecap measurements. This may explain why it is so difficult to establish an unequivocal dependence on the explicit surface wave parameters in the whitecap data. The weak explicit wave signal can be detected after the cubic wind speed dependence is removed.