Influence of wind wave breakings on a millimeter-wave radar backscattering by the sea surface

Yu.Yu. Yurovsky1, ✉, I.A. Sergievskaya2, S.A. Ermakov2, 3, 4, B. Chapron5, I.A. Kapustin2, 3, 4, O.V. Shomina2

1 Marine Hydrophysical Institute, Russian Academy of Sciences, Sevastopol, Russian Federation

2 Institute of Applied Physics, Russian Academy of Sciences, Nizhni Novgorod, Russian Federation

3 N.I. Lobachevsky State University of Nizhni Novgorod, Russian Federation

4 Volga State University of Water Transport, Nizhni Novgorod, Russian Federation

5 Russian State Hydrometeorological University, Saint-Petersburg, Russian Federation

✉ e-mail: yyyurovsky@gmail.com

Abstract

Results of field study of radar backscattering from the sea surface on 8 mm radio wave length are presented. The instantaneous Doppler shifts are analysed to examine kinematic properties of the scatterers. The constant Doppler shift due to the sea surface current is estimated from accompanying video observation data by tracing bright features (stand-alone bubbles, pieces of foam and small scale debris). Wave breaking is detected by binarization of the video sequence and further filtration of the events by lifetime and size. The threshold for the binarization is tuned to meet the empirical law for breaking wave coverage. It is shown that the contribution of the breaking waves to the NRCS increases with wind speed. The contribution is almost twice higher at HH polarization in comparison with VV polarization, but never exceeds 20-30%. The time averaged instantaneous Doppler velocities tend to phase speed of Bragg waves with increasing of incidence angle, but still are 1.5-2 times higher at HH polarization. At incidence angles less than 30° the mean instantaneous Doppler shift rapidly increases indicating switching to specular reflection mechanism. The analysis shows that discrepancy from Bragg backscattering model cannot be attributed to the observed breaking waves only. A possible explanation for this effect is discussed and assumed to be due to the spatial modulation by intermediate waves within radar footprint, the influence of parasitic capillary ripples, or micro-scale breaking events that are not detected by the presented video method.

Keywords

radar backscattering, millimeter waves, Doppler shift, surface current, wind wave breaking, parasitic ripples

For citation

Yurovsky, Yu.Yu., Sergievskaya, I.A., Ermakov, S.A., Chapron, B., Kapustin, I.A. and Shomina, O.V., 2015. Influence of wind wave breakings on a millimeter-wave radar backscattering by the sea surface. Physical Oceanography, (4), pp. 34-45. doi:10.22449/1573-160X-2015-4-34-45

DOI

10.22449/1573-160X-2015-4-34-45

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