Enhanced turbulence beneath short wind-induced water waves as derived from Lagrangian flow fields

Breaking waves and wind stress are a primary source for near-surface turbulence. Particle Tracking Velocimetry (PTV) was used developed to study turbulence beneath short wind-induced waterwaves at the circular wind-wave facility of the Institute for Environmental Physics. A thick light sheet (1-4 cm) was applied to illuminate small polystyrol seeding particles. The depth of the light sheet was chosen such that the particles stay long enough in the illuminated area to enable tracking over several wave periods. The flow field is observed by a digital CCD camera. Recording image sequences at up to 200 Hz allow an extensive study of the flow field. An automatic tracking algorithm was developed for the evaluation of the trajectories. Lagrangian flow field measurements offer an ideal approach to the study of drift profiles in the turbulent wave region, in addition yielding the thickness of that layer. Also bulk velocity and surface velocities can be derived. A measure for the turbulence was gained by the calculation of the friction velocity profile by correlating horizontal and vertical velocity components (eddy correlation technique). These profiles show a very interesting behavior. While the friction velocity profile is constant in the bulk, an abrupt enhancement of Reynolds stress from the bulk towards the water surface up to a factor of 10 is observed. The enhanced dissipation of kinetic energy beneath strong-breaking surface waves under fetch conditions was measured at Lake Ontario. Here, an enhancement dissipation factor of 5-60 was found. Our measurements now indicate that micro-scale wave breaking is sufficient to produce significant turbulence enhancement. This result contributes significantly to the understanding of gas exchange through the aqueous boundary layer and indicates that wave/turbulence interaction deserves further more detailed attention.