Space‐time properties of radar sea spikes and their relation to wind and wave conditions

Abstract

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 ...

Optical and Microwave Detection of Wave Breaking in the Surf Zone

“…These anomalies occur most often for active microwave sensors with low grazing angle (LGA) viewing geometries and include high intensity bursts of backscatter (sea spikes) [24], [25], [40], polarization ratios (HH/VV, where HH corresponds to horizontal transmit-horizontal receive and VV corresponds to vertical transmit-vertical receive) exceeding unity [22], [24], [41], [42], and a broadening of the Doppler spectrum and large Doppler offsets [43], [44]. The breaking waves have been often cited as the source for these anomalies, and the characteristics of the anomalies have been used for breaking wave detection.…”

Breaking waves and near-surface sea spray aerosol dependence on changing winds: Wave breaking efficiency and bubble-related air-sea interaction processes

“…Similarly, based on feature tracking [5] or Doppler analysis [7,8] of microwave radar sea spikes, which are closely associated with breaking waves, the average breaking wave velocity is smaller with swell presence compared to that in pure wind seas (figure 5).…”

“…In summary, the breaking efficiency consideration provides a consistent explanation of the behavior of aerosols, Figure 5. The breaking velocity deduced from (a) feature tracking [5] and (b) Doppler processing [7][8] of radar sea spikes. The breaking velocity is smaller in mixed seas (blue) in comparison to the value in wind seas (red).…”

“…In previous investigations of the length and velocity scales of breaking waves [4][5][6][7][8], it is found that the breaking wavelength (velocity) scale is shorter (slower) in mixed seas compared to that in wind seas. From the consideration of breaking wave geometric similarity [9], the size of the breaking patches and the air cavities entrained into water is proportional to the breaking wavelength.…”

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

“…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.…”