Strong Modulation of Short Wind Waves and Ka-Band Radar Return Due to Internal Waves in the Presence of Surface Films. Theory and Experiment

Stanislav A Ermakov,Irina A Sergievskaya,Ivan A Kapustin

doi:10.3390/rs13132462

Abstract

Strong variability of Ka-band radar backscattering from short wind waves on the surface of water covered with surfactant films in the presence of internal waves (IW) was studied in wave tank experiments. It has been demonstrated that modulation of Ka-band radar return due to IW strongly depends on the relationship between the phase velocity of IW and the velocity of drifting surfactant films. An effect of the strong increase in surfactant concentration was revealed in convergent zones, associated with IW orbital velocities in the presence of a “resonance” surface steady current, the velocity of which was close to the IW phase velocity. A phenomenological model of suppression and modulations in the spectrum of small-scale wind waves due to films and IW was elaborated. It has been shown that backscatter modulation could not be explained by the modulation of free (linear) millimeter-scale Bragg waves, but was associated with the modulation of bound (parasitic) capillary ripples generated by longer, cm–dm-scale waves—a “cascade” modulation mechanism. Theoretical analysis based on the developed model was found to be consistent with experiments. Field observations which qualitatively illustrated the effect of strong modulation of Ka-band radar backscatter due to IW in the presence of resonance drift of surfactant films are presented.

Highlights

It is well known that internal waves (IWs) play an important role in ocean dynamics, in particular in energy fluxes, mixing processes, the exchange of nutrients and gases, etc., and the detection and characterization of IW remains a challenging problem
This paper aims to report on the results of laboratory/field studies of strong variability of capillary ripples due to IWs in the presence of surfactant films
The first experimental set was with the density difference between the lower and upper layers of 0.07 g/cm3, and the estimated IW phase velocity was 23 cm/s

Summary

Introduction

It is well known that internal waves (IWs) play an important role in ocean dynamics, in particular in energy fluxes, mixing processes, the exchange of nutrients and gases, etc., and the detection and characterization of IW remains a challenging problem. Orbital surface currents caused by IWs modulate the spectrum of small-scale wind waves, resulting in significant variations of microwave radar return. Different mechanisms by which IWs can be observed on the sea surface as bands with reduced and/or increased intensity of small-scale, gravity-capillary waves (GCWs) have been described in the literature. The most often discussed are hydrodynamic modulation (so-called straining or kinematic mechanism) of short wind waves due to non-uniform orbital velocities of the IW (see, e.g., [1,6,7,8,9,10]), and a “film modulation mechanism” associated with the redistribution of surfactants on the sea surface and with corresponding modulation of the surface wave damping rate The straining mechanism is shown to be effective for those GCWs for which group velocities are close to the IW phase velocity—a resonance between IWs and surface waves.

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