## 5 Discussion  
As mentioned above, the Ka-band of the radar backscatter contains two components--Bragg and non-polarized components. Bragg scattering can be determined both by free capillary waves directly excited by wind and by bound waves--parasitic capillary ripples  
Figure 12: Internal wave profile from acoustic (ADCP) scattering data (**middle**), variability of Ka-band NRCS (**top**), wind velocity and direction (**bottom**): red curve—velocity, back curve—direction. The arrow indicates a rear slope of the IW. had wavenumbers which also fitted the Bragg resonance condition in the Ka-band range. The NP component was determined by the nonlinear structures such as bulges/toes, and possibly some initial part of PCR wave trains at a sufficiently high steepness of carrying cm-dm-scale GCW. For further interpretation of the experimental results, we discuss first how strongly Bragg scattering in our experiments contributed to the total radar return at VV polarization. To estimate this contribution, some auxiliary (without IW) investigations were performed to record the Ka-band radar backscatter at VV and HH polarizations; the HH-polarized measurements were carried out by simply turning the radar antenna around its axis by 90 degrees. Relationships between microwave scattering at two co-polarizations are usually characterized by the polarization ratio (PR), which is a ratio of VV- to HH-polarized radar returns. The polarization ratio as a function of surfactant concentration depicted in Figure 13 demonstrates that in our experiments, the PR values for a clean water surface are close to those predicted by Bragg theory for VV-polarized radar return. This means that Bragg scattering mostly determines Ka-band VV-polarized backscattering, whereas the role of the Bragg mechanism decreases when the surfactant concentration grows (see Figure 13, right panel, illustrating the ratios of Bragg and non-polarized components to total VV backscatter as functions of OLE concentration). One should note that the radar backscatter at high OLE concentrations tended to non-polarized scattering. This is consistent with the results of wave tank studies of the action of surface films on the profile of steep cm-dm-scale GCW [37; 38]. The effect revealed in [37; 38] is explained by the fact that PCR is more effectively suppressed by surface films than bulge/toe structures, and the latter determine mostly non-polarized scattering. Let us now consider VV polarized radar contrasts as functions of OLE surface concentration. The total VV-polarized contrasts calculated according to the model described above using Equations (11), (14)-(17) are depicted in Figure 14. For our calculations, we used the following expressions for the wind growth rate [28]:  
\[\beta(k,u*)=0.04k^{2}u*^{2}/\omega(k), \tag{22}\]  
and for the wave damping coefficient [27]:  
\[\gamma(k,E)=2\
u k^{2}(1-X+XY)/(1-2X+2X^{2}) \tag{23}\]  
Figure 13: Polarization ratio vs. surfactant concentration at a wind velocity of 2 m/s (**left**) and relative Bragg and non-polarized components normalized by the total VV radar backscatter (**right**). where \(\
u\) is the kinematic water viscosity, \(X=\frac{Ek_{\xi}^{2}}{\rho(2\
u)^{1/2}\omega^{3/2}},Y=\frac{Ek_{\xi}}{4\rho\
u \omega}\), and we assumed that \(m=3\) and \(n=1\). The relative intensities of Bragg and non-polarized components in Equation (17) were taken from the experiment according to Figure 13. Note that radar return suppression is determined solely by the suppression of cm-dm-scale GCW contributing both to Bragg and NP scattering, because Bragg scattering from free mm-scale Bragg waves is absent. According to the theory, wind waves in the cm-dm wavelength range of the spectrum are suppressed by about 1.5-2 times, which is approximately consistent with the experiment (see, Section 4).