During the MARSEN 79 experiment, attenuation of capillary and gravity water waves by two oleyl alcohol and one methyl oleate surface films (‘slicks’) was investigated. A slight influence of an oleyl alcohol slick occurs at frequencies between 0.5 Hz and 0.7 Hz and above 0.7 Hz (wavelength L = 3.2 m) wave attenuation becomes significant. A methyl oleate slick causes only a slight wave damping in the frequency range ƒ<5 Hz (L>6.7 cm). In the capillary wave range ƒ≥14 Hz the wave‐damping characteristics for the two surface film substances are comparable. In the high‐frequency capillary wave range ƒ>20 Hz, however, methyl oleate surface films act more strongly on the wave field, as was demonstrated by additional wind‐wave tunnel experiments. The different wave attenuation characteristics of these two surface active compounds are attributed to different interaction between their hydrophilic part and the adjacent water layer.

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The response of microwave cross sections of the sea at four different microwave frequencies to fluctuations in the wind was measured during the Synthetic Aperture Radar and X Band Ocean Nonlinearities-Forschungsplatform Nordsee experiment. Four separate CW microwave systems operating at L, S, X, and Ka bands and incidence angles near 45 ø measured the response of short surface waves with Bragg wavelengths of 14.1, 9.2, 2.1, and 0.6 cm, respectively, to near-surface wind fluctuations. Cross sections from the four microwave systems were correlated with winds from a sonic anemometer which was positioned near the sea surface areas illuminated by the microwave systems. We find that in the wind speed regime from 6.4 to 9.4 m/s, coherence functions relating the cross sections to fluctuations of the wind are appreciable only below 0.01 Hz and then only for the highest two microwave frequencies whose Bragg wavelengths correspond to gravity-capillary waves. For all microwave frequencies, coherence functions are very low from 0.01 up to 0.05 Hz, the highest frequency to which our measurements are meaningful. We show that modulation transfer functions (MTFs) relating the cross sections to wind fluctuations should be nearly equal to the wind speed exponent of the cross section at low frequencies. These MTFs were impossible to determine accurately at L and S bands due to the low coherence, but they were between 1 and 2.5 at X and Ka bands, in line with measurements of the wind speed exponent. We infer that the low coherence at L and S bands is due to their low response to the mean wind. Since we expect microwave cross sections at these incidence angles to be nearly linearly dependent on Bragg wave spectral density, we infer that gravity-capillary waves on the ocean respond directly to changing winds only on timescales longer than about 100 s and that gravity waves longer than about 9 cm respond to other factors more strongly than to the wind.

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