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.
Patricio A. Catalán, email@example.com, Departamento de Obras Civiles, Universidad Técnica Federico Santa María, Valparaíso, Chile. Centro Nacional de Investigación para la Gestión Integrada de Desastres Naturales, CONICYT/FONDAP/1511007, Santiago, Chile. CCTVal-Centro Científico Tecnológico de Valparaíso, Basal Project FB021, Valparaíso, Chile
Merrick C. Haller, Coastal & Ocean Engineering Program, School of Civil and Construction Engineering, Oregon State University, Corvallis, Oregon, USA
William J. Plant, Applied Physics Lab, University of Washington, Seattle, Washington, USA