Abstract. A series of airborne scatterometer experiments designed to collect C and Ku band ocean backscatter data in regions of high ocean surface winds has recently been completed. More than 100 hours of data were collected using the University of
High-resolution dual-polarization X-band images of the ocean surface were obtained at a grazing angle of about 3 . Area extensive imaging allowed us to study backscatter properties of sea spikes and to compare radar measurements with visual surface features evident from video recordings. The vertically polarized radar images consist of distributed scatter whose amplitude and Doppler velocity are modulated by larger scale gravity waves consistent with Bragg scattering and composite surface theory (CST). The horizontally polarized radar images are dominated by spatially discrete scattering centers (or sea spikes) moving at velocities comparable to the phase velocities of gravity waves beyond the spectral peak. These sea spikes also exist in the corresponding V-pol radar images, but are less prominent due to the dominant Bragg backscatter. Sea spikes are characterized by polarization ratios H/V that often exceed unity, typically by about 5 dB. Comparison of the larger spikes with simultaneous co-registered video recording of the surface indicates that approximately 30% of observed sea spikes are associated with actively breaking waves (whitecaps) while the remainder are identified with "steep" wave features. By classifying the larger sea spikes according to their corresponding surface features, we find Doppler velocities for sea spikes due to whitecaps noticeably faster (about 50%) than other sea spikes, though the distributions for both overlap significantly. We also find little measurable difference in the polarization ratios of the two classes of sea spikes as observed on the open ocean.Index Terms-Sea surface electromagnetic scattering.
Abstract. Low-grazing angle (LGA) radar sea spikes were observed with a highresolution, dual-polarization, X band imaging radar deployed on the floating instrument platform (FLIP) during the Marine Boundary Layer Experiment (MBLEX) held off the California coast during April-May 1995. Spatiotemporal statistics of observed sea spike events are presented, including duration, velocity, and directional distributions. The dependence of these parameters on wind and wave conditions is compared to theoretical predictions and to similar measurements of breaking wave signatures obtained with passive acoustic techniques by Ding and Farmer [1994]. The density of sea spikes (events per unit time per unit area) and the fractional surface coverage are estimated as a function of friction velocity and compared to theoretical predictions. Though we find consistency between the dynamical aspects of sea spike events and of acoustic wavebreaking signatures, we do not observe a predicted cubic u, dependence of sea spike density. Differences may be a consequence of the specific nature of low-grazing angle scattering at X band (and higher) frequencies. We observe an approximately quadratic dependence on u, of sea spike fractional surface coverage which is insensitive to the choice of backscattered power threshold over a 9 dB range. IntroductionBreaking waves on the ocean surface are believed to be an important part of air-sea interactions. They limit the height of ocean waves, mix surface waters, transfer energy from the wave field to currents, and enhance the fluxes of heat, mass, and momentum through the generation of turbulence and entrainment of air [Melville, 1996]. Because wave breaking is a nonlinear and intermittent process, direct measurement of wave breaking in the field is extremely difficult. For this reason the development of optical, acoustical, and microwave remote sensing methods is desirable.Breaking events have long been attributed as a source for microwave backscatter, especially for horizontally polarized radiation at near-grazing angles [Wetzel, 1990]. "Sea spikes," the colloquial term for high-intensity bursts of radar backscatter typically observed at low grazing angles, are attributable both to actively breaking waves and to scattering features (i.e., wedges, bores, plumes) bound near the crests of steep waves. A number of radar studies of wave breaking in the laboratory have attempted to identify the specific mechanisms for scattering and to measure the dependence of the microwave signature on breaking strength. Kwoh and Lake [1984] attributed discrete bursts of backscatter from short gravity waves at moderate to high incidence angles to "gentle" breaking. Banner and Fooks [1990, 1991a, b] used a CW scatterometer and boresighted video to study both the detection of breaking waves and the dependence on environmental parameters. They found an approximately cubic relation between u, and the number of sea spikes observed at moderate incidence attributable to breaking, in agreement with theoretical predictions by ...
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