Experimental study of the wind stress above waves

Airflow dynamics above waves strongly influence exchanges of heat, momentum and mass between the Ocean and the Atmosphere. In fact, estimating the wind stress at the ocean surface and relating it to sea state are important yet challenging problems for most coupled air-sea models. We present experimental results on the structure of the airflow above waves for several wind speeds, wave ages and slopes. The experiments were performed in the large (42-m long) wind-wave facility at University of Delaware's Air-Sea Interaction laboratory. The wave field and the airflow above were investigated using simultaneously laser-induced fluorescence and PIV. We were able to obtain high resolution velocity measurements in the airflow inside the wave boundary layer, within and above the viscous sublayer. We observe intermittent airflow separation events past the crest of the waves. These events are usually accompanied by a dramatic drop in the measured surface viscous stress, and a flux of vorticity away from the surface. Despite the intermittent aspect of this phenomenon, these events may affect the average distribution of the wind stress between viscous, wave and turbulent contributions, which impacts wave growth and the air-water momentum balance. Our results hold for wind speeds that would normally be considered low to moderate. Drag coefficient estimates as a function of wind speed will be discussed, as well as implications for models of air-sea momentum flux.