Wide-field-of-view PIV measurements above and below microscale breaking waves

Abstract Small-scale physical processes within the first centimeters above and below ocean surface waves are important for atmosphere-ocean momentum and energy budgets. The complex feedback mechanisms between airflow separation, turbulence under microscale breaking wave crests, and associated parasitic capillary waves remain to be fully understood. A new, 51-cm-wide-field-of-view laser imaging system was developed, which involves high-resolution (one velocity vector every 133 µm) air–water PIV (particle image velocimetry, 33 µm/pixel) and LIF (laser-induced fluorescence, 52 µm/pixel). The system was installed at a fetch of 15.5 m in the 24-m-long, 1-m-wide, 1.5-m-high wind-wave tank of the University of Hamburg (Germany). Wind-generated waves were examined at a reference wind speed of 4.5 m/s. The wide-field-of-view air-side PIV measurements display intense airflow separation events past waves, with sheltering effects that affect several waves downwind. Chronological sequences of air and water-side PIV measurements (11 wide-field-of-view PIV frames per second) allow us to follow the evolution of microscale breaking waves and associated air/water dynamics, and they reveal the intimate coupling between highly intermittent airflow sheltering events, capillary waves, and turbulence below wave crests. Graphical abstract