Laboratory measurements of turbulence below wind-generated waves, with and without surfactants

The fluxes of momentum and mechanical energy between the atmosphere and the ocean are coupled with the complex small-scale interactions between wind and wind-generated waves. Small wind waves may carry a significant portion of the air-sea momentum flux, and their growth and dissipation are therefore critical components for the momentum budget at the ocean surface.

We present novel laboratory measurements of turbulence and viscous stress under wind-generated waves using PIV (Particle Image Velocimetry), under slick-free and slick-covered water surfaces. Three surface-active substances were used, with different visco-elastic properties, for wind speeds ranging from 4 m/s to 8 m/s.  Additionally, LIF (Laser-induced fluorescence) imagery was acquired to characterize the surface, including the distribution of bound and freely propagating capillary waves. The bulk of the measurements was performed at a fetch of 15.5 m.

The surfactants strongly modify the onset of capillary waves characteristic of microscale breaking wind-waves, which in turn modifies the growth and evolution of the wind-waves, as well as the turbulent dynamics below microscale breaking wave crests. We will discuss the dynamical role of capillary waves and turbulence near the crest of wind-waves, for the wind-wave energy budget.