WIND-WAVE TANK MEASUREMENTS OF WAVE DAMPIMG AND RADAR CROSS-SECTIONS IN THE PRESENCE OF MONOMOLECULAR SURFACE-FILMS

Measurements of the damping of small gravity and gravity-capillary wat er 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 micr owave antennas. It is shown that Marangoni damping theory, which descr ibes 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 ac tion balance equation, i.e., the energy input into the water waves fro m the wind, the nonlinear wave-wave interaction, and the dissipation b y wave breaking, are affected differently by the various substances. I t is hypothesized that this difference is caused by the different visc oelastic properties of the substances, i.e., by the different intermol ecular interactions of the film molecules. A slight dip in the wind de pendence 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 mea n squared wave height and wave slope. Polarization ratios (i.e., the r atios 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 w ithin 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 depend ence of the polarization ratio on the coverage of the water surface wi th a slick is explained qualitatively by means of the composite-surfac e model. Finally, it is stated that wind wave tank measurements in the presence of monomolecular surface films are useful for the verificati on of theories concerning radar backscattering, wave damping, and wind -wave and wave-wave interactions.