Microwave backscattering from surf zone waves

Abstract

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 220 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.

Quantification of Two-Dimensional Wave Breaking Dissipation in the Surf Zone from Remote Sensing Data

“…It must be noted that, in general, these values σ t 0 may vary among different locations and grazing angles. For the present case, they have been derived by Catalán et al [17].…”

“…Optical (henceforth video) data include high intensity returns for both remnant foam and rollers. Microwave data (radar) at this frequency range can yield high backscatter intensities for both steepening and breaking waves [17]. However, foam and steepening waves rarely co-occur.…”

“…Thus, quantifying the evolution of rollers can be relevant for estimating dissipation. The signal of the roller is prominent in many remote sensing modalities, such as passive acoustic (e.g., [15]), optical (e.g., [16]), microwave (e.g., [17]), and infrared (e.g., [18]). Moreover, a wide range of platforms can be used to deploy the sensors, from nearshore towers and antennas (e.g., [4], for a review) to satellites (e.g., [19]).…”

On the Interpretation of Coherent Marine Radar Backscatter From Surf Zone Waves

“…Lin et al [42] and Catalán et al [43] studied surf zone Doppler data acquired by a coherent pulsed radar, which is very similar to the radar used in the present work. While the study of Lin et al [42] focuses solely on integrated Doppler velocity and backscatter intensity data, Catalan et al [43] analyzed also the range evolution of the Doppler spectra (integrated over two minutes) and the range-time evolution of the Doppler velocity modulated by shoaling and breaking waves. The results of Catalán et al [43] also confirm the simultaneous occurrence of high backscatter intensities and Doppler velocities close to the phase speed associated with actively breaking waves.…”

“…While the study of Lin et al [42] focuses solely on integrated Doppler velocity and backscatter intensity data, Catalan et al [43] analyzed also the range evolution of the Doppler spectra (integrated over two minutes) and the range-time evolution of the Doppler velocity modulated by shoaling and breaking waves. The results of Catalán et al [43] also confirm the simultaneous occurrence of high backscatter intensities and Doppler velocities close to the phase speed associated with actively breaking waves. Thus, within the 2 minute averaging period, a single breaker is enough to completely dominate the Doppler signal.…”