What are the mechanisms for non‐Bragg scattering from water wave surfaces?

Abstract

Abstract. Experimental data obtained in the past several years have provided conclusive evidence that non-Bragg scattering plays a major role in X-band microwave backscatter from water wave surfaces, and non-Bragg scattering events are especially noticeable at small grazing angles, for wind-roughened surfaces, in the presence of breaking waves. We have conducted scattering experiments under a variety of wind wave conditions in an attempt to determine the different mechanisms which contribute to non-Bragg scattering. At small grazing angles we find that non-Bragg scattering is due to fast scatterers generated by the wave breaking, and with increasing wave steepness and surface roughness, mechanisms of multiple scattering and multipath interference become increasingly important. -1 m) and the temporal radar backscatter was correlated with video images. They observed single-bounce backscatter from specular facets and gave quantitative results of the measured distribution of facet size. Closer scrutiny of their data reveals that the specular reflection they observed was not due to fast scatterers but rather to the slowest scatterers and was due to backscatter from the smoother patches in the troughs of larger waves. However, the notion of "specular reflection," which produces signals with HH = VV, was quickly adopted by theoreticians and modelers, and it was assumed that single-bounce reflection from specular facets was 123

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

“…However, it is still not clear whether all of these contributions are simultaneous or should be used alternatively, if at all. It is classically acknowledged that shadowing of the troughs diminishes the contribution of negative long wave orbital velocities, therefore enhancing the mean Doppler shift [7]. This is consistent with a recent numerical study [11] which identifies geometrical shadowing as one important factor in the increase of the Doppler shift.…”

“…Geometrical shadowing was proposed in earlier studies (e.g., [7]) as the responsible mechanism for the blow-up of the mean Doppler shift at LGA, because it reduces the contribution of negative orbital velocities in large troughs. However, some recent observations [12] tend to show that geometrical shadowing does not actually play a role in microwave coherent backscatter from the sea surface at grazing angles.…”

“…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.…”

“…Another controversy pertains to the actual role of breaking which is known to be related to the production of fast scatterer but not its unique origin (e.g., see discussions in [7] and [13]). The present numerical study confirms that nonbreaking hydrodynamical models can also account for very large Doppler shifts at LGA and shows that geometrical shadowing is not a relevant mechanism in this context.…”

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

“…The separation of HH and VV spectral peaks was interpreted as the manifestation of bound waves and/or non-Bragg scattering mechanisms occurring, for instance, in wave breaking conditions. The paper by Lee et al [1999] provides a stimulating discussion of these effects.…”