In-situ Airflow Measurements over Surface Waves using Particle Image Velocimetry

The input of wind energy by wave growth into the upper ocean is an important process for the global energy budget. However, observing and measuring the near surface physics that control these fluxes remains challenging, especially in field experiments.

We were able to capture small-scale motions in the airflow above surface waves. A high resolution 2D Particle Image Velocimetry (PIV) system was developed for velocity measurements within the first micrometers to centimeters above the air-water interface. The system, installed on a single pile platform in the Szczecin Lagoon (Baltic Sea coast, Germany) at a fetch of approximately 20 to 25 km, was remotely operated and could be rotated to measure the airflow velocities in a range of wind directions. In this study, we focus on a peak wave age (c_p/u_*) of 14.2, classified as "growing sea", with a slope (ak_p) of 0.08 and a 10-m wind speed of 5.7 m/s.

We observe modulations of the airflow by locally generated wind waves, including small sheltering events downwind of sharp wave crests. The pattern of the vertical wave-coherent velocity field shows a critical layer where the wind speed equals the wave speed. The phase of the observed vertical velocity eigenfunction is in agreement with the linear theory of Miles (1957, J. Fluid Mech., doi: 10.1017/S0022112057000567). In addition, we find a dimensionless wave growth rate using wave slope, wave age, and wave-coherent momentum flux, which is consistent with other studies.