Doppler radar measurements of wave groups and breaking waves

Abstract

A 3-GHz Doppler radar has been used to study wave dynamics and backscatter from the sea surface at low grazing angles. Vertical polarization results are dominated by Bragg scatter even at low (•8 ø) grazing angles. Horizontal polarization results, however, show a strong upwind-downwind asymmetry with additional, highvelocity intermittent scatter in the upwind direction associated with steep or breaking waves. These characteristics have been exploited to distinguish spilling breaking events from the background Bragg scatter. While these "spikes" at a single range may appear random in time, the combined range and time information reveals a well-determined propagation pattern. It is shown that for a developing sea in deep water, group behavior modulates the occurrence of wave breaking. The frequency-wavenumber spectrum shows a clear separation between the linear dispersion curve and nonlinear effects related to breaking. The most important nonlinear feature is a line near the dominant wave group velocity which is identified with the spectrum of breaking intermittency. The slope of this line suggests that the wave components which are most likely to break lie at frequencies significantly above the dominant wave frequency. backscatter mechanism at microwave frequencies [Wright, 1968; Plant and Keller, 1990; Poulter et al., 1994]. At low grazing angles, Bragg scatter from horizontal polarization for a slightly roughened sea is predicted to be -30 dB below that of vertical polarization. Additional mechanisms at these angles such as wedge diffraction [Lyzenga et al., 1983] and backscatter from breaking plumes [Wetzel, 1990] have been invoked to explain the strong observed scatter. Despite data dating back more than 20 years a satisfactory theoretical explanation of backscatter at low grazing angles remains elusive.In this paper we investigate the properties of sea backscatter at low grazing angles (-8 ø) using an S band radar. At these angles, Bragg backscatter using horizontal polarization will theoretically be small. In section 3, properties of the backscatter mechanisms are sought in terms of the polarization ratio, azimuthal dependence, velocity of scatter, and temporal behavior. A key feature of the radar is the simultaneous acquisition of full Doppler spectra over a significant range extent. In section 4 the range coverage of the radar is exploited to show the propagating nature of the upwind HH backscatter. This is used to provide information on the group modulation of wave breaking of deep water waves where the breaking is predominantly spilling breaking. Finally in section 5, linear wave and group modulation information is derived from the frequency-wavenumber spectrum. Experimental DescriptionThe radar used in these studies operates at 3 GHz ()• = 10 cm) with an FMCW modulation [Poulter et al., 1995a]. An adaptation using interrupted modulation (FMICW) has been developed to allow a single antenna to be switched between the transmitter and the receiver [McGregor et al., 1994]. This greatly enhances the portabili...

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

“…R increases from À22.2 dB (a = 3°) to À13.8 dB (a = 0.9°) in cross-wave conditions and from À13.9 dB to À1.1 dB in up wave conditions. Such high values of R, which can approach one, have been already observed [Lee et al, 1995[Lee et al, , 1996Smith et al, 1996]. By curve fitting the data points and considering the variation with a obtained for s VVrel 0 , the relative backscattering coefficient in HH varies as a 1.06 and a 0.16 for cross-wave and up wave conditions, respectively.…”

Passive Acoustic Determination of Wave-Breaking Events and Their Severity across the Spectrum

“…These utilize the acoustic, optical, reflective, and other properties of breakers, distinguishing them from the more homogeneous background wave field. Without giving a comprehensive review of these methods, it is important to cite radar observations of the microwave backscatter from breakers (Jessup et al 1990;Loewen and Melville 1991;Smith et al 1996;Phillips et al 2001), sonar observations of bubble clouds produced by breaking wind waves (Thorpe 1992), infrared remote sensing of breaking waves (Jessup et al 1997), conductivity measurements of void fraction produced by breakers (Gemmrich and Farmer 1999;Lammarre and Melville 1992), and aerial imaging and its analysis to obtain wave-breaking statistics (Melville and Matusov 2002).…”

Using wavelet spectrum analysis to resolve breaking events in the wind wave time series

“…Gemmrich and Farmer (1999) used void fraction conductivity measurements at sea to describe the scale and occurrence of breaking waves. Smith et al (1996) and Phillips et al (2001) studied the speed distribution spectral properties of breaking events using high range resolution radar.…”