Wind-Wave Interaction for Strong Winds

In this paper we revisit the problem of wind-wave interaction with emphasis on strong winds. For these events it is assumed that nonlinearity is so large that the slope of the wind waves has reached a limiting steepness. Recent observations suggest that the drag decreases with wind in the strong wind speed regime. In this paper we try to explain this. In the first step we introduce a model for surface gravity waves and calculate explicitely the background roughness length from the original approach of Janssen (1989). It is found that for young, steep windsea the background roughness length al-most vanishes giving a reduced drag. In addition, it is shown that for steep waves the slowing down of the wind by waves is a nonlinear process, hence the growth rate of the waves by wind depends in a nonlinear fashion on the wave spectrum. For strong winds it is found that, as waves are typically steep, this nonlinear effect gives a further reduction of the wind input. As a consequence, in these extreme circumstances the drag coefficient decreases with wind.