Effect of sea state on the momentum exchange over the sea during neutral conditions

On the basis of an extensive data set from the air-sea interaction station Ostergarnsholm in the Baltic Sea, the dependence of drag on the ocean of wave state parameters has been studied for near-neutral conditions. For developing sea, the drag depends on wave age, u*/c(p) (u* friction velocity and c(p) the phase speed of the dominant waves), in agreement with recent findings over the World Ocean, strongly supporting that the Ostergarnsholm station can be relied upon to give results representative of open ocean conditions. For such conditions, it is demonstrated that the logarithmic wind law is indeed valid. For mixed sea/swell the logarithmic wind law is not valid and the drag coefficient, C-D depends on two parameters representing the wave state: u*/ c(p) and E-1/ E-2, where E-1 is the energy of the relatively long waves ( having a phase velocity larger than the wind speed at 10 m) and E-2, the short wave energy. Thus, plotting C-D as a function of u*/ c(p) gives a clear ordering of the data in parallel, sloping bands according to the value of E-1/ E-2. Thus, whereas very young and slow waves affect the atmospheric flow similar to rigid roughness elements, with the occurrence of longer waves, an entirely different mechanism gains successively more importance and dynamical coupling with the atmospheric turbulence occurs. It may be speculated that the often observed kink in the wind profile represents the upper bound of a wave-boundary-layer, which is thus an order of magnitude deeper than predicted and observed during growing sea conditions.