Dependence of wind stress across an air–sea interface on wave states

Based on a combination of laboratory and field data, the influence of wind waves on wind stress with respect to sea-surface roughness z0 and drag coefficient CD was comprehensively analyzed. Wave steepness is an internal parameter denoting wave stability and is directly related to z0. Wave age is an external parameter representing the ability of wind to input energy into waves. In low and moderate winds, wave steepness increases monotonically with decreasing wave age, which is equivalent to the growth relationships of wind waves. In high winds, wave steepness reaches a maximum due to intense wave breaking and remains as a constant, whereas wave age continues to decrease with increasing wind speed. The dimensionless roughness z0/Hs as a function of wave steepness is more suitable than wave age, because wave height is related to wave age through the growth relationships of wind waves. In low and moderate winds, wave steepness can be replaced by wave age; CD increases with the wind speed, with a weak dependence on wave state. In high winds, intense wave breaking results in the collapse of the relationship between wave steepness and wave age, and CD decreases significantly with decreasing wave age and levels off with wind speed. Further, the reduction of CD in high winds is a matter of fact if both their data are available.