The response of wind drag to underlying swell slope
Résumé
Lack of knowledge of the development of momentum flux response to wave field steepness is hampering the development of more quantitative understanding of wind-induced energy fluxes which are critical to prediction of maritime safety under severe wave conditions. Laboratory experiments were undertaken to determine total drag response to mean swell steepness. It was established that : the introduction of monochromatic low frequency waves beneath microscale waves disrupts the microscale in such a way that the total stress borne by the surface can fall by up to 20%. Behaviour is systematic in terms of mean low frequency wave steepness. Once the low frequency waves achieve a sufficient mean steepness (~0.15), their presence augments the total stress with a very strong response observed above a mean steepness of 0.20. Consistent behaviour was observed across a range of wave scales and wind forcing. Consistent development of mean spectral energy in the frequency range 8 to 30 Hz was also observed which may have implications for remote sensing of the ocean. Limited form drag measurements obtained during this study show consistent development of form drag with mean wave steepness that has not previously been observed. A tentative qualitative description of the development of drag over the ocean is developed. Future studies which would clarify existing uncertainty regarding wind-induced momentum and energy fluxes to the surface of the open ocean are described.
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