Velocity structure immediately below wind-forced ocean waves

Surface gravity waves play a critical intermediate role in transferring momentum into the ocean. Laboratory measurements and limited field observations of dissipation under breaking suggest that high dissipation events associated with wave breaking have short lifetimes (OT, the wave period) and shallow (O1-2 A, the wave amplitude) depth distribution, complicating efforts to model momentum transfer from wind into the ocean mixed layer. However the kinematics of breaking oceanic wind waves remain poorly understood, as it is very difficult to measure the velocity field very close to the wave surface, in the shallow depth range directly forced by the wind-wave breaking.

A high resolution bistatic geometry, coherently sampled acoustic Doppler profiler has been developed to measure the velocity field in small scale oceanic boundary layers. This instrument has been successfully used to study the mean current and oscillatory wave bottom boundary layers in the surf zone and inner shelf in the last 5 years. A slightly modified, upward-looking version of this instrument was used during CBLAST to measure the velocity field immediately below ocean wind waves and shoaling waves. The system noninvasively measures three component velocity profiles with 1.5cm vertical bins over a 1.5m range, starting 0.5m above the instrument frame at a 40 Hz sample rate. These velocity profiles provide profiles of Reynolds’s stresses and shear measurements allowing shear production associated with intermittent micro-breaking and breaking events to be estimated in a surface-following coordinate system.

Examples of bubble density, velocity shear and Reynolds stress profiles will be shown as the local wind speed increased from 3 m/s to 9 m/s over a 2 hour period. These profile timeseries reveal near-surface shear and coherent rotational velocity structures below the waves.