Measurements of the Airflow Structure Above Waves.

The coupling between wind and surface waves strongly influences the fluxes of momentum, heat and mass across the Ocean surface. We present detailed direct measurements of airflow kinematics within the first 10 cm above waves, and investigate the instantaneous and mean along-wave structure of the wind stress. Laboratory measurements were achieved for 17 different wind-wave conditions, with wave ages C_p/u_* between 1.4 and 66.7, in the large (42-m long) wind-wave facility at University of Delaware's Air-Sea Interaction laboratory. A novel imaging system was especially developed for this study, combining Laser-Induced Fluorescence (LIF) and Particle Image Velocimetry (PIV) techniques. We were able to obtain high resolution two-dimensional velocity measurements in the airflow within and above the viscous sublayer, along with wave-profile and wave-phase data. We observe intermittent upward ejections of low velocity air downwind of wave crests, as well as airflow separation events. These events are usually accompanied by a dramatic drop in the measured surface viscous stress, a flux of vorticity away from the surface, and an intensification of the wave-coherent turbulent shear stress. On average, below the critical height (defined as U(z_c)=C_p), we observe an intense coupling of the phase-averaged airflow velocities (measured) with underwater wave orbital motions (computed). Above z_c, the airflow generally displays a strong sheltering effect downwind of wave crests. Finally, we present preliminary field measurements.