Near-Surface Velocity and Coherent Structures Under Low-Wind Conditions in a Wind-Wave Tank

Despite evidence that wave phase-dependent effects at the air-sea interface such as airflow separation and sheltering can affect surface stresses, coupled- and wave models still neglect such effects. To improve our understanding of phase-dependent effects on air-sea momentum flux and surface stress, we conducted a comprehensive experiment at the University of Miami SUSTAIN wind-wave laboratory using a range of wind-wave forcing conditions. As part of the experiment, we collected velocity fields on the water-side of the air-sea interface with Particle Image Velocimetry (PIV). Under low wind speed conditions, the velocity field in the top few centimeters of the water column exhibits small-scale features consistent with boundary-layer coherent structures. The findings from PIV measurements are in agreement with results from numerical simulations using a LES model representation of the SUSTAIN tank. The results suggest the flow exhibits dynamics consistent with ramp-like structures which may lead to advection of momentum from the boundary into the interior of the flow. Surface waves can alter the dynamics of these near-surface coherent structures, change the momentum profile in the boundary layer and thus impact near-surface turbulence and viscous stress.