Remote observations of the initial generation of surface waves

We present laboratory experiments on the initial generation of surface waves and related Langmuir circulations. Using quantitative remote optical and infrared flow visualization techniques, we acquired collocated surface wave slope and temperature. We show that the classical wave generation problem is accompanied by other phenomena, including Langmuir circulations, that occur over comparable space and time scales. Of particular interest is the influence of the subsurface velocities associated with the Langmuir circulations on the structure of the growing wave field. Direct measurements of wave variables show a clear cross-wind modulation which is qualitatively consistent with geometrical optics and wave action conservation. The temporal evolution of the directional slope spectra show that the wind waves are initially generated near the minimum phase speed. Energy is then transferred to both lower and higher wavenumbers; as the wave field evolves to a fetch-limited state with the appearance of parasitic capillary waves. Once the wave field has grown sufficiently, microscale breaking is observed in both the slope and infrared images. We show that the generation of the surface waves by the wind triggers a transition to turbulence of the near-surface flow