## 5 Discussion
One should emphasize that the obtained Ka-band radar contrast is significantly larger than the hydrodynamic suppression of cm-dm-scale GCW, and this occurs because of the high sensitivity of nonlinear structures on GCW profiles to small reductions in the spectrum intensity of these GCW (a \"cascade suppression mechanism\"). One can conclude from Figure 14 that the phenomenological model described above is consistent with the experiment. The results of applying the model of radar backscatter modulation due to IW are presented in Figure 15. Here, the RMTF magnitude and phase as functions of surfactant concentration were plotted for two cases considered above; firstly, when IW phase velocities and wind drift were different and kinematic modulation dominated over the film effect, and secondly, when IW and drifting films were in resonance, resulting in strong modulations of surfactant concentration and in enhanced \"film modulation\" mechanisms. Figure 15 illustrates that the RMTF calculated according to the model was consistent with the measurements. Similar to radar contrasts, the modulations in our experiments were determined by strong (\"cascade\") modulation of the nonlinear structures generated on the profile of steep cm-dm-scale GCW. Now, we briefly discuss the results of field observations of radar backscatter variations over IWs presented in Section 3. The IW phase velocities estimated from radar panoramas of slicks and ADCP measurements were about 27-28 cm/s. These values are approximately consistent with the theoretical values of about 29 cm/s obtained from the dispersion relationships for IWs in a two-layer fluid (see [39]):  
\[C(k)=\sqrt{\delta\rho/\rho_{0}g\cdot\kappa^{-1}\cdot[cth(xh)+cth(\kappa H-h)]^ {-1}}, \tag{24}\]  
where \(\delta\rho/\rho_{0}\) is the relative density difference between the upper and low water layers with thicknesses \(h\) and \(H-h\), respectively. The wind was directed towards the shore, and we estimated the wind drift velocity to be 3% of the wind speed, which was about  
Figure 14: Total radar backscatter contrast. Black and red curves are model contrasts for wind velocities of 2.5 m/s, and 2 m/s, respectively: symbols—experiment. 12 cm/s. When summing the wind drift and the mean current of 17 cm/s measured in the upper layer at a depth of 1 m, one comes to the value of the total mean surface velocity of 29 cm/s. The measured and theoretical IW velocities were consistent with each other; therefore, one can suppose that the velocities in the bulk water are rather small and do not alter the IW phase velocity. The projection of the surface velocity on the IW propagation direction is close to the IW phase velocity. Hence, according to the theory (see, Section 2) surfactants should be concentrated over the troughs/rear slopes of IWs; this was supported by our observations. Strong reductions in Ka-band radar intensity in slicks, of about 10-15 dB (see, Figure 12), indicate that surfactants are effectively accumulated in the slick bands and strongly suppress short wind waves. We suppose that radar backscattering is determined by capillary ripples with Bragg wavelengths of about 0.5 cm, and these ripples are, in general, both free waves and bound PCR. The latter significantly contribute to the spectrum of capillary waves, very likely dominating over free wind ripples because the latter are ineffectively generated at low wind speeds and particularly in film-covered areas. Then, the proposed cascade mechanism of strong modulations of PCR associated with accumulation of surface films by IW can be effective, and this conclusion is consistent, at least qualitatively, with the observed strong suppression of radar return in the slick bands. More detailed descriptions and analyses of the field experiments will be given elsewhere.