Wind wave tank measurements of wave damping and radar cross sections in the presence of monomolecular surface films

Abstract

Measurements of the damping of small gravity and gravity‐capillary water surface waves covered with monomolecular organic films of different viscoelastic properties were performed in the wind wave tank facility of the University of Hamburg. The wind speed dependence of the radar cross sections for X and Ka band was measured with upwind looking microwave antennas. It is shown that Marangoni damping theory, which describes the damping of water surface waves by viscoelastic surface films, is not the only damping mechanism in wind wave tank experiments where the wind sea is not fully developed. The other source terms of the action balance equation, i.e., the energy input into the water waves from the wind, the nonlinear wave‐wave interaction, and the dissipation by wave breaking, are affected differently by the various substances. It is hypothesized that this difference is caused by the different viscoelastic properties of the substances, i.e., by the different intermolecular interactions of the film molecules. A slight dip in the wind dependence of the radar cross section at Ka band at wind speeds of 8–9 m/s was measured, which corresponds to comparable reductions of the mean squared wave height and wave slope. Polarization ratios (i.e., the ratios of the radar backscatter at vertical and horizontal polarization) higher than those predicted by simple Bragg scattering theory for X band at low wind speeds and different incidence angles are explained within a (three‐scale) composite‐surface model. At higher wind speeds, where the polarization ratio decreases rapidly, breaking by wedges and spilling breakers is hypothesized to become more dominant. The dependence of the polarization ratio on the coverage of the water surface with a slick is explained qualitatively by means of the composite‐surface model. Finally, it is stated that wind wave tank measurements in the presence of monomolecular surface films are useful for the verification of theories concerning radar backscattering, wave damping, and wind‐wave and wave‐wave interactions.