Small-Scale Airflow Dynamics Above Surface Waves

Small-scale physics at the wavy air-sea interface influence the fluxes of momentum, heat, mass, and energy across the ocean surface. We present 2D PIV measurements of the kinematics in the airflow as low as 100 µm above the water surface for different wind-wave conditions (wave ages C_p/u* between 1.4 and 66.7). Laboratory data were obtained at University of Delaware’s (42-m long) wind-wave-current facility, using a multi-camera multi-laser setup that combines PIV with laser induced fluorescence. Field PIV measurements are also reported.
We observe coherent turbulent structures within the buffer and logarithmic layers of the airflow above the air-water interface, whereby low velocity air is ejected away from the surface, and higher velocity fluid is swept downward. Wave-phase-resolved quadrant analysis suggests that these events are modulated by the waves. We directly observe airflow separation events over young wind waves (C_p/u*<3.7), whereby high spanwise vorticity layers detach from wave crests and surface viscous stress dramatically drops. This causes, on average, intense wave-phase-locked turbulence downwind of wave crests. Using a surface-following coordinate system, we estimate the impact of such events on the momentum budget within the constant stress layer, and we discuss the viscous, wave-coherent and turbulent contributions to the total wind stress.