THE AIR-SEA MOMENTUM FLUX IN CONDITIONS OF WIND SEA AND SWELL

During the Surface Wave Dynamics Experiment, direct measurements of mo mentum, heat, and water vapor fluxes were obtained from a mast on the foredeck of a SWATH (small water-plane area, twin hull) ship in deep w ater off the state of Virginia. Directional wave spectra were obtained simultaneously from a 6- or 3-wire wave-staff array mounted at the bo w of the ship. One hundred and twenty-six 17-minute runs of flux and w ave data obtained with the ship steaming slowly into the wind are exam ined for the effects of the relative direction of the wind sea and bac kground swell on the momentum transfer. The adequacy of the inertial d issipation method, which depends on the high-frequency turbulent fluct uations for evaluating the wind stress, is also examined for any effec ts of swell. The results show that the presence of counter-and cross-s wells can result in drag coefficients that are much larger than the va lue for a pure wind sea. The eddy correlation and inertial dissipation methods for measuring wind stress are found to diverge during the com plex sea conditions. The authors interpret the latter observations as an indication that the traditional inertial dissipation method, in whi ch the pressure and transport terms in the kinetic energy balance equa tion are assumed to be in balance, may be unsuitable for use in a mari ne boundary layer disturbed by swell.