Fig. 2: Airflow separation events over slow ocean surface waves, measured by Particle Image Velocimetry (PIV).

The velocity fields (horizontal component u, a–d) are in a frame of reference traveling at wave crest speed (c = 1.4 m/s). The dashed line indicates where u = c for the first PIV snapshot (a). The vorticity fields (spanwise vorticity ω, clockwise positive e–h) demonstrate the detachment of the surface viscous shear layer past the crest of the wave. The velocity resolution is 1 velocity vector every 1.2 mm². The snapshots are taken every 1/15 of a second (66 ms). The age of this wave (with a wavelength  ~1 m) is c/u_* = 6.4 or c/U₁₀ = 0.22, which classifies it as a ''slow” wave with respect to the wind speed blowing over it⁸. It is riding on the crest of a longer wave (with a wavelength  ~ 70 m). U₁₀ is the mean wind speed at a height of 10 m above the water surface, and u_* is the friction velocity, a measure of the total air–sea momentum flux.