Impact of dominant waves on short wind waves and momentum flux at the sea surface: a laboratory experiment
Résumé
An experiment was carried in the large Marseille-Luminy wind wave tank to investigate the coupled evolution of small-scale wind waves and the air turbulent boundary layer without and with the presence of a long wave generated mechanically at the water surface by a submerged wavemaker. Wave observations were made by means of a two-dimensional laser scanning slope system and a linear array of 40 capacitance wave gauges aligned in the wind direction. The velocity field in air was measured by the classical hot wire and Pitot tube techniques. The effects of the long wave steepness and the wave scale separation ratio on wind and wave field parameters were examined in detail using various combinations of fetch, wind and paddle conditions. Thus, it is found that the development of capillary-gravity or short gravity waves generated directly by wind is deeply affected by the presence of a longer wave at the water surface. In the capillary range, significant changes in shape and energy level can be observed in the wavenumber slope spectra whereas the frequency wave height spectra are not modified, exhibiting the same wind-dependent energy level whatever the long wave conditions. Moreover, the slopes and scales of short waves are strongly modulated by the dominant waves, making the small-scale water surface roughness essentially inhomogeneous. Contrarily, the air friction velocity varies only slightly when a mechanical wave is generated at the water surface. The significant increase in the water surface roughness length observed however indicates that the mean velocity field is largely affected by the pressure field induced in the air flow by the long wave. At last, improvement in understanding of physics of the air-sea interface derived from these new findings is discussed.