Laboratory Study of the Wind Structure above Waves

Airflow dynamics above waves strongly influence exchanges of heat, momentum and mass between the Ocean and the Atmosphere. We present experimental results on the detailed structure of the airflow above waves. The experiments took place at University of Delaware's large (42m long, 1m wide, and 1.25m high) wind-wave facility, which features a recirculating wind-tunnel and a mechanical wave generator. Winds ranging from 3 to 15 m.s^-1 (U₁₀) were combined with different mechanical wave conditions, including monochromatic waves, groups, and breakers, thus yielding a variety of wave ages and steepnesses. Airflow properties within and above the viscous sublayer were obtained at different fetches using PIV, Pitot tube and hot-wire probe measurements, while wave profiles and spectra were measured by dual beam laser-induced fluorescence. We observe intermittent separation of the viscous sublayer past the wave crest in some wind-wave conditions. Despite the intermittent aspect of the separation events, our results suggest that there is a substantial along-wave variability of the surface viscous tangential stress, even when averaged over all wave phases. These events, in turn, may affect wave growth and the air-water momentum balance.