The Air–Sea Momentum Flux in Conditions of Wind Sea and Swell

TLDR

The study measured wind, heat, and vapor fluxes aboard a SWATH vessel in deep water, simultaneously recording directional wave spectra, and analyzed 126 17‑minute runs to assess how wind‑sea and swell orientations affect momentum transfer and the validity of inertial dissipation methods. Results show that counter‑ and cross‑swells can increase drag coefficients beyond those of pure wind sea, and that eddy‑correlation and inertial‑dissipation wind‑stress estimates diverge under complex conditions, suggesting the traditional inertial‑dissipation method is unsuitable in swell‑disturbed marine boundary layers.

Abstract

During the Surface Wave Dynamics Experiment, direct measurements of momentum, 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 water off the state of Virginia. Directional wave spectra were obtained simultaneously from a 6- or 3-wire wave-staff array mounted at the bow of the ship. One hundred and twenty-six 17-minute runs of flux and wave data obtained with the ship steaming slowly into the wind are examined for the effects of the relative direction of the wind sea and background swell on the momentum transfer. The adequacy of the inertial dissipation method, which depends on the high-frequency turbulent fluctuations for evaluating the wind stress, is also examined for any effects of swell. The results show that the presence of counter- and cross-swells can result in drag coefficients that are much larger than the value for a pure wind sea. The eddy correlation and inertial dissipation methods for measuring wind stress are found to diverge during the complex sea conditions. The authors interpret the latter observations as an indication that the traditional inertial dissipation method, in which the pressure and transport terms in the kinetic energy balance equation are assumed to be in balance, may be unsuitable for use in a marine boundary layer disturbed by swell.