Breaking wave contribution to low grazing angle radar backscatter from the ocean surface

Abstract

[1] The anomaly of radar sea spikes, defined here as the non-Bragg scattering events with backscattering cross-section of horizontal polarization exceeding that of the vertical polarization, has been associated with steep wave features possibly going through wavebreaking process, with or without whitecap manifestation. This property is exploited for using a dual polarized radar as a remote sensing breaking wave detector. Field data collected in the ocean covering wind speeds from 7 to 15 m/s, grazing angles from 1.4 to 5.5°, and with different levels of background swell influence are analyzed to quantify the radar cross-section and Doppler velocity from sea surfaces with and without wave breaking. Key results of the breaking effects are increasing significantly the Doppler velocity of both polarizations (about 50% faster), enhancing the horizontally polarized backscattering cross-section drastically (with 10-15 dB increase), and producing relatively small change in the vertically polarized cross-section (about 1-2 dB increase). The presence of swell (in the same direction of wind waves) reduces both the radar backscatter and the impact of breaking waves on radar return. By inference, the swell presence decreases the ocean surface roughness and breaking activity. These results are consistent with earlier in-situ surface wave measurements and our expectation of swell modification of breaking process due to interaction of short waves and the orbital velocity of long swell.

Investigation of Low-Grazing-Angle Microwave Backscattering From Threedimensional Breaking Sea Waves

“…It is also noted that V V polarized Bragg scattering is still dominant in large angles shown in Figures 6(b) and (d), which is properly predicted by tilting modulation of Bragg roughness. In addition, the numerical results are consistent with the PDF of the normalized cross-section at LGA in [41].…”

Sea Surface Microwave Scattering at Extreme Grazing Angle: Numerical Investigation of the Doppler Shift

“…The backscattering cross sections and mean Doppler velocities were analyzed separately on breaking and nonbreaking populations (a breaking event being characterized by a polarization ratio VV/HH greater than 1). Reference [9,Table 2] shows that both the average Doppler velocity and average cross section are stronger for the breaking population, this effect being more pronounced in HH polarization and at extreme grazing angles. Since the mean velocity is proportional to the mean Doppler shift, one can compare its evolution as a function of the grazing angle with our numerical simulations.…”

“…However, the model can be useful in unveiling the physical mechanisms at the origin of the Doppler shift, and its qualitative predictions can be analyzed in the light of experimental observations. A certain number of experiments with coherent radars illuminating the sea surface at LGA can be found in the literature, most of them operating in X-band (e.g., [6], [9], [12], [13], and [46]- [48]) and more rarely in L-band [8] and Ku-band [49]. A systematic conclusion of the aforementioned studies is the elevated level of NRCS return in HH polarization as compared to standard composite Bragg theory, the spiky nature of the backscattered power, and the occurrence of fast scatterers in the Doppler spectra.…”

“…In order to investigate the specific role of breaking at LGA, field data [9] were acquired in the ocean in wind conditions from 7 to 15 m/s. The backscattering cross sections and mean Doppler velocities were analyzed separately on breaking and nonbreaking populations (a breaking event being characterized by a polarization ratio VV/HH greater than 1).…”

“…It has been known for a long time that the backscattered signal from the sea at grazing incidence possesses some particular features which are not visible at moderate incidences, namely, the occurrence of pronounced spikes in the time series and the presence of "fast scatterers," which is faster than Bragg wave, in the Doppler spectrum (e.g., [6]- [8]), especially in horizontal polarization. Even though there have been a certain number of attempts to elucidate the apparition of fast scatterers (e.g., [7], [9], and [10]), there is still no wellacknowledged theory to explain the non-Bragg mechanisms at grazing incidences. The usually invoked responsible phenomena are breaking waves, facet specular reflection by long waves, multiple-path and multibounce scattering, wedge diffraction by bound waves, and shadowing effects.…”