## 6 Conclusions  
* A phenomenological model of the modulation of short wind waves and Ka-band radar return due to internal waves in the presence of surfactant films has been elaborated. Wave tank studies on the IW modulation of surfactant concentration and of radar  
Figure 15: RMTF magnitude and phase for non-resonance (**left**) and resonance (**right**) calculated according to model (curves) and experimental data (symbols).  
backscatter in the field of IW orbital currents were carried out for two different cases. In the first case, the IW phase velocity was large compared to the velocity of surfactant films due to wind drift; in the second case, these velocities were close to each other--non-resonance and resonance cases, respectively. It has been shown theoretically and in experiments that surfactants at resonance were strongly comparable to non-resonance cases concentrated over the IW troughs or the rear slopes of IW;
* It has been observed that Ka-band radar backscatter was reduced when a film was applied on water and the radar contrast increased with surfactant concentration and, in practice, did not depend on whether resonance on non-resonance conditions were realized;
* Modulation of radar return (the radar modulation transfer function) practically did not depend on surfactant concentration for non-resonance case, but strongly increased with concentration at resonance. The results were explained by different modulation mechanisms for these two cases, i.e., the straining modulation of wind waves due to IWs was a dominant mechanism for non-resonance cases, whereas the modulation of radar return at resonance was determined mostly by the strong modulation of surfactants. In both cases, the radar backscatter was modulated due to variations of parasitic capillary ripples generated by steep cm-dm-scale waves (cascade modulation), rather than of free Bragg capillary mm-scale waves. Some field observations have been presented, which qualitatively confirmed the effect of strong modulations of Ka-band radar backscatter due to IWs in the presence of resonance drift of surfactant films;
* It has been shown that the developed modulation model was consistent with experimental results;
* The mechanism of strong (cascade) modulation of microwave Ka-band radar backscatter studied in this paper is very important for correct interpretations of IW signatures in radar imagery of the sea surface. The quasi-linear models of hydrodynamic straining modulation combined with Bragg theory underestimate the IW radar signature contrasts, whereas the proposed cascade model, taking into account the strong nonlinearity of short wind waves, gives more realistic values of the contrasts and the RMTF. We plan to extend the model in the future to a wider range of wind wave spectra in order to explain IW signatures in satellite imagery in X/S radar bands.  
Conceptualization, S.A.E.; methodology, S.A.E. and I.A.S.; investigation, I.A.S. and I.A.K.; data curation, I.A.S. and I.A.K.; writing, S.A.E. All authors have read and agreed to the published version of the manuscript. This work was performed within the State Assignment (project no. 0729-2020-0037).  
The authors declare no conflict of interest.