[1] The laboratory experiments on investigation of aerodynamic resistance of the waved water surface under severe wind conditions (up to U 10 ≈ 40 m s À1 ) were carried out, complemented by measurements of the wind-wave spectra. The tendency to saturation of the surface drag was observed for wind speeds exceeding 25 m s À1 , accompanied by the saturation of wind-wave slopes. The effect of surface drag saturation can be explained quantitatively within the quasi-linear model of the air boundary layer above the waved water surface, when the contribution of the short-wave part of the wind-wave spectrum to aerodynamic resistance of the water surface is taken into account.

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This paper models the impact of the presence of foam on the short-wave component of surface waves and momentum exchange in the atmospheric boundary layer at high winds. First, physical experiments were carried out in a wind-wave flume in which foam can be artificially produced at the water surface. Tests were conducted under high-wind-speed conditions where equivalent 10-m wind speed ranged from 12 to 38 m s−1, with measurements made of the airflow parameters, the frequency–wavenumber spectra of the surface waves, the foam coverage of the water surface, and the distribution of the foam bubbles. Analysis of the resulting data indicates that the surface drag coefficient correlates with the fraction of foam coverage and the mean square slope (MSS) of the water surface, and that, at a certain wind speed, the MSS decreases with an increase in the fraction of foam coverage. Based on these results, we suggest a simple model for eddy viscosity in the turbulent boundary layer over a fractionally foam-covered wave surface. The measurements in a laboratory environment are shown to be in good agreement with the predictions of a quasi-linear model of the atmospheric boundary layer over a waved water surface that adopts this eddy viscosity. Adaptation of the proposed model to field conditions is discussed, and the synergetic effect of foam at the water surface and spray in the marine atmospheric boundary layer on ocean surface resistance at high winds is estimated so as to be able to explain the observed peaking dependence of the surface drag coefficient on the 10-m wind speed.

Abstract. Drag and mass exchange coefficients are calculated within a self-consistent problem for the wave-induced air perturbations and mean velocity and density fields using a quasi-linear model based on the Reynolds equations with down-gradient turbulence closure. This second part of the report is devoted to specification of the model elements: turbulent transfer coefficients and wave numberfrequency spectra. It is shown that the theory agrees with laboratory and field experimental data well when turbulent mass and momentum transfer coefficients do not depend on the wave parameters. Among several model spectra better agreement of the theoretically calculated drag coefficients with TOGA (Tropical Ocean Global Atmosphere) COARE (Coupled Ocean-Atmosphere Response Experiment) data is achieved for the Hwang spectrum (Hwang, 2005) with the high frequency part completed by the Romeiser spectrum (Romeiser et al., 1997).