Observations of the sea surface by coherent L band radar at low grazing angles in a nearshore environment

Abstract

Coherent microwave L band signals have been collected in a Mediterranean coastal zone in horizontal transmit/horizontal receive (HH) and vertical transmit/vertical receive (VV) polarization modes at low grazing angles and under light to strong wind conditions. Doppler spectra have distinct morphologies depending on polarization mode and wind conditions. VV spectra and, only under light wind conditions, HH spectra present features that are consistent with an interaction process of Bragg type between electromagnetic waves and the sea surface, including the Doppler effects imparted by surface currents. At high wind, fast scatterers associated with greater velocities than Bragg velocity can be detected on HH spectra, depending on the direction of propagation of long waves and not on wind direction, these directions being not necessarily the same in the coastal zone considered. For a given radar range the backscattered power in VV was found to be almost constant, which suggests saturation of the wave spectrum in the short‐wave range and isotropy of the short‐wave spectral energy in the downwind half plane. For vertical polarization the observed variations of the relative backscattering coefficient with grazing angle are supported by the slightly rough surface scattering theory, but the HH/VV polarization ratio is much greater than predicted by this theory. The data suggest that at low grazing angles, non‐Bragg scattering effects play a major role at horizontal polarization, especially in high‐wind–high‐wave conditions.