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Microwave backscattering from surf zone waves
The microwave backscatter properties of surf zone waves are analyzed using field observations.
By utilizing a preexisting, independent, water surface discrimination technique, the microwave returns were
extracted along individual waveforms and the data from shoaling (steepening) waves, surf zone breaking
waves, and remnant foam were separated and quantified. In addition, a wave tracking analysis technique
allows the returns to be examined on a wave-by-wave basis as individual waves progress through the shoaling
zone and break on a nearshore sand bar. Normalized radar cross sections (NRCS), polarization ratios,
Doppler spectra, and scatterer velocities were collected using a dual-polarized, X-band radar operating at
lower grazing angles than previously reported (1°–3.5°). The results indicate that the maximum NRCS levels
are from the active breaking portions of the wave and were consistently about -20 dB, regardless of radar
polarization, azimuth angle, wave height, or wind speed. In addition, breaking waves induce broadening of
the Doppler spectra and mean scatterer velocities that correlate well with the carrier wave celerity. Analysis
of the polarization ratios suggest that the active breaking portions of the wave are depolarized but that
higher polarization ratios (>0 dB) are found on the leading edges shoreward of the active breaking portions
of the waves, which indicates a clear distinction between two scattering regimes. These results are consistent
with scattering from a very rough surface that is being mechanically generated by the breaking process,
showing a good agreement with the expected grazing angle dependency of a Lambertian scatterer.Keywords: microwave backscattering, wave breaking, nearshore, remote sensin