Wind stress and surface roughness at air-sea interface

Based on the compiled data of thirty independent observations, the report presents the wind-stress coefficient, the surface roughness and the boundary layer flow regime at the air-sea interface under various wind conditions. Both the wind-stress coefficient and the surface roughness are found to increase with the wind velocity when U10 is less than 15 m/sec and reach a saturated value for U10 greater than 15 m/sec. Based on the oceanic wave observations, it states that this discontinuity at U10 = 15 m/sec is due to an increase in the wind velocity (measured at the significant wave amplitude above the mean water level) beyond the average wave phase velocity. This finding provides a well-defined separation for the often quoted terms 'light' and 'strong' winds, and also explains the existence of the so-called 'critical wind velocity.' It also shows that the surface roughness is governed by the amplitude of the short gravity waves rather than the mean square surface slope. Charnock's relationship is shown to be applicable to most of the data and Charnock's proportionality constant is determined. Finally, two approximate formulae for the wind-stress coefficient, one for light wind and the other for strong wind are suggested.