An investigation of wave-induced momentum flux through phase averaging of open ocean wind and wave fields
Wetzel, Suzanne W.
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This thesis presents an investigation of the influence of the dominant, wind-driven, surface waves on the vertical flux of horizontal momentum in the marine surface layer over open ocean conditions. Through a procedure which involves phase averaging the wind fields at the period of the dominant waves, the wave-induced component of the atmospheric fluctuations is isolated and vertical profiles of mean wave-induced momentum flux are computed. Previous investigators have used phase averaging to remove the turbulence from an oscillatory signal, but the absence of a monochromatic wave field in open ocean conditions complicates this approach. This difficulty is overcome by choosing only the time periods characterized by the most monochromatic-like waves present and filtering those sections of wind data with a narrow band-pass filter centered around the dominant wave frequency before beginning the phase averaging process. The analysis is carried a step further by investigating the dependence of wave-induced momentum flux on sea state through bin averaging according to wave age. This results in a set of profiles which express the ratio of wave-induced momentum flux to the total flux as a function of the wave age parameter c/U10. These profiles all tend to zero with height, and remain negligible at all heights over fully developed seas. Over younger seas, this ratio becomes increasingly more positive (corresponding to a positive value for - uw) with decreasing wave age; while over older seas, this ratio becomes increasingly more negative with increasing wave age. This provides strong quantitative evidence that there is a significant flux of momentum to the atmosphere from decaying waves, and a transfer of atmospheric momentum to developing waves.
Submitted in partial fulfillment of the requirements for the degree of Master of Science at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution September 1996
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