OBSERVATIONS OF NEARSHORE AND SURF ZONE WIND STRESS

Field data of the wind stress over surf zone waves is presented from an open ocean beach on the East Australian Coast. Two ultrasonic anemometers were deployed at nominal heights of 5 and 10 m above the water surface in the intertidal and inner surf zones, with concurrent measurements of water levels and offshore wave param-eters. Considering near-neutral conditions only, the wind drag coefficients were found to systematically change with the wind angle of approach relative to the shoreline, and are much smaller for longshore wind than during onshore wind. The concept of an apparent wave steepness changing with wind direction is suggested to explain this behaviour. The drag coefficients over the surf zone during onshore wind and near-neutral conditions were determined to be almost twice the values expected at the same wind speed and open ocean conditions. The ob-served Charnock coefficient was similarly an order of magnitude larger than open ocean values. A wave celerity of the order of that expected in the inner surf zone is required to explain the observed large roughness and drag coefficients using existing wave-age dependent parameterisations. This suggests that the slower wave celerity in the surf zone is an important contributor to the increased wind stress, in addition to the sawtooth wave shape