A coupling modulation model of capillary waves from gravity waves: Theoretical analysis and experimental validation

According to Bragg theory, capillary waves are the predominant scatterers of high‐frequency band (such as Ka‐band) microwave radiation from the surface of the ocean. Therefore, understanding the modulation mechanism of capillary waves is an important foundation for interpreting high‐frequency microwave remote sensing images of the surface of the sea. In our experiments, we discovered that modulations of capillary waves are significantly larger than the values predicted by the classical theory. Further, analysis shows that the difference in restoring force results in an inflection point while the phase velocity changes from gravity waves region to capillary waves region, and this results in the capillary waves being able to resonate with gravity waves when the phase velocity of the gravity waves is equal to the group velocity of the capillary waves. Consequently, we propose a coupling modulation model in which the current modulates the capillary wave indirectly by modulating the resonant gravity waves, and the modulation of the former is approximated by that of the latter. This model very effectively explains the results discovered in our experiments. Further, based on Bragg scattering theory and this coupling modulation model, we simulate the modulation of normalized radar cross section ( NRCS) of typical internal waves and show that the high‐frequency bands are superior to the low‐frequency bands because of their greater modulation of NRCS and better radiometric resolution. This result provides new support for choice of radar band for observation of wave‐current modulation oceanic phenomena such as internal waves, fronts, and shears. Key Points Free capillary waves are coupled and modulated by gravity waves Gravity wave modulation coefficient approximates that of free capillary wave The proposed coupling modulation model effectively explains the results of wind‐wave tank experiment