The damping of ocean surface waves by a monomolecular film measured by wave staffs and microwave radars

Abstract

The damping of ocean surface waves by a monomolecular oleyl alcohol film of about 1.5–3 km2 in area is measured in the North Sea by wave staffs, a coherent X band microwave scatterometer mounted on a sea‐based platform, and an incoherent Ku band microwave scatterometer carried by an aircraft under moderate wind conditions (wind speed u10 = 3.5–7.7 m s−1). The observed wave attenuation by the monomolecular surface film measured by a wave staff in the frequency band between 3.2 and 16 Hz is in the range of about 40–60%, with only a slight increase with frequency. From this result it can be predicted that slicks affect microwave backscattering similarly in the L band (λ0 ≈ 20 cm) as in the Ku and X bands (λ0 ≈ 2 cm). It is shown by additional wave tank experiments that a direct influence of oleyl alcohol surface films on wave damping is confined to frequencies f ≥ 2 Hz, but a further indirect effect of oleyl alcohol films on the damping of ocean waves in the frequency range between 0.12 and 0.7 Hz by modifying the wind input and wave‐wave interaction mechanisms is indicated from our results. A possible directional dependence of the wave‐damping effect caused by surface films is discussed.