In situ measurements of capillary‐gravity wave spectra using a scanning laser slope gauge and microwave radars

Abstract

Capillary-gravity wave spectra are measured using a scanning laser slope gauge (SLSG), and simultaneously by X and K band Doppler radars off the Chemotaxis Dock at the Quissett campus of the Woods Hole Oceanographic Institution at Woods Hole, Massachusetts. Wave spectral densities estimated from the radar measurements using the Bragg theory agree with those measured using the SLSG at the Bragg wavenumber to within a few decibels, suggesting that Bragg scattering theory is valid for the conditions of this experiment. The observed degree of saturation of capillary-gravity waves is in reasonable agreement with measurements by J•ihne and Riemer (1990) obtained from measurements in a large wind-wave flume at intermediate wind speeds, but our data indicate a higher degree of saturation at very low wind speeds. The rate at which the slope-frequency spectrum falls off, however, is much lower in the field than in laboratories, even at moderate winds, suggesting long waves are responsible for a large Doppler shift of capillary-gravity waves. Close examination of combined wavenumber-frequency slope spectra also reveals significant smearing of the spectra in the frequency domain due to long waves. These observations confirm that spatial measurements (wavenumber spectra measurements) are essential for characterizing short capillary-gravity waves, since this strong Doppler shift will dramatically change apparent frequency spectra. in idealized conditions [Hara and Mei, 1994]. The parameterization of the surface roughness in practical conditions still relies heavily on empirical observations [e.g., Charnock, 1955]. Many theoretical, as well as experimental, studies are available on the generation, the nonlinear wave-wave interaction, and the dissipation of the capillary-gravity waves Paper number 94JC00531. 0148-0227/94/94JC-00531 $05.00 separately. In particular, recent numerical [Ruvinsky et al., 1991], theoretical [Longuet-Higgins, 1992], and experimental [Perlin et al., 1993] studies have advanced our understanding of the interaction between steep gravity waves and parasitic capillary waves near their crests. How-ever, studies of the combined effects of the three forcing mechanisms, namely, wind input, wave interaction, and wave dissipation, are still scarce either theoretically [Janssen, 1986[Janssen, , 1987; Hara and Mei, 1994] or experimentally. As a result, theoretical modeling of capillary-gravity waves in a field environment has been limited to the simple approach based on the conservation of wave action with the above three mechanisms added independently. Further progress of theoretical modeling is hindered partly because reliable direct ob'servations of capillary-gravity waves in a field environment are almost nonexistent except for the many radar backscatter measurements that come with their own uncertainties owing to the lack of knowledge regarding the mechanism of microwave reflection at the air-sea interface.Conventional single-point wave-height or wave-slope measurements are not appropriate to characterize capi...

An Improved Spectrum Model for Sea Surface Radar Backscattering at L-Band

“…In the experiment of Banner et al [37], their two-dimensional spectrum for short gravity waves shows a greater power spectral density at crosswind than alongwind. Hara et al [38] found that gravity waves (k = 49 rad/m) may exist and propagate against the wind from their observed spectra, and this can lead to a similar conclusion as Banner et al [37]. Further, the fitting procedure of the ∆ ratio showed negative values in the short gravity wavenumber range, which means that the energy of the spectrum in the crosswind direction can exceed the energy in the alongwind direction [28].…”

“…Further, the fitting procedure of the ∆ ratio showed negative values in the short gravity wavenumber range, which means that the energy of the spectrum in the crosswind direction can exceed the energy in the alongwind direction [28]. There are no convincing physical explanations for this phenomenon [37,38]; however, some scholars have illustrated a number of kinetic characteristics of small-scale waves that can be instructive. Elfouhaily et al [21] demonstrated that a few of the shorter waves ridden on longer waves may travel perpendicular to the wind direction due to the interaction between wave.…”

A simple theory of capillary–gravity wave turbulence

“…Instead, it experiences an accelerated roll-off at a rate exceeding the rates of both gravity and capillary spectra roll-offs. The accelerated roll-offcommences at wavenumbers which show no noticeable dependence on the wind energy input (Jfihne & Riemer 1990), although the influence of long waves appears to be important (Hara et al 1994). This behaviour finds a simple explanation in the framework of the present theory.…”

“…(Jfihne & Riemer 1990) and field (Hwang et al 1993;Hara, Bock & Lyzenga 1994) observations showed that, contrary to intuitive expectations, the wavenumber spectrum in the capillary-gravity range does not exhibit a monotonic transition from the gravity wave to the capillary wave regime. Instead, it experiences an accelerated roll-off at a rate exceeding the rates of both gravity and capillary spectra roll-offs.…”

Characterization of Sun Glitter Statistics in Ocean Video

“…A number of controlled experiments have been performed in wind-wave tanks under various set-ups, including those described in [66][67][68][69][70][71][72][73][74][75][76][77].…”