Strong variations of radar return from the sea surface due to breaking surface waves

Abstract

Recent studies of microwave radar return at moderate and large incidence angles have shown the backscattering is determined by resonance (Bragg) surface waves of cm-scale wavelength range, and by non polarized (non Bragg) component which is associated with wave breaking and quasi specular reflection. This paper is focused on results of field studies of non-Bragg backscattering from the clean water surface and from the water surface covered with surfactant films. The study was carried out using dual polarized X-band radars in the coastal zone of the Black Sea in 2017-2019 at an incidence angle of about 60 degrees. It was found that the radar return contains a Non Bragg component not related to the breaking crests and specular tilt areas at wind speeds from a threshold of the wind wave generation up to wind velocities of the order of 10 m/s. The part of the non-Bragg component not related to the wave breaking crests decreases strongly in the areas of film slicks. At high wind velocities the non-Bragg component out of the spikes is strongly modulated (several times larger than the Bragg component) in the long-wave field, in film slicks the modulation of the non-Bragg component increases. Analysis of the Doppler shifts showed that the velocities of the non-Bragg scatterers correspond to the dm-scale free surface waves and vary slightly in the areas of film slicks. Thus, we concluded that nonlinear features associated with the dm-scale wind waves cause the non-Bragg scattering.