John A. McGregor

Doppler radar measurements of wave groups and breaking waves

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°) grazing angles. Horizontal polarization results, however, show a strong upwind-downwind asymmetry with additional, high-velocity 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.

S band Doppler radar measurements of bathymetry, wave energy fluxes, and dissipation across an offshore bar

The use of a shore-based microwave Doppler radar for the remote sensing of ocean wave propagation over an offshore sand bar was investigated. The radar yielded high spatial resolution measurements of several quantities of importance in the study of bar dynamics. The spatial variation in wave phase with distance along the radar beam direction was used to calculate bathymetry with sufficient accuracy and resolution to clearly reveal the presence of the bar and the tidal cycle variation of water depth. This real-time bathymetry enabled the calculation of ocean wave energy fluxes from the radar velocity data. These energy fluxes showed good agreement with predictions from a numerical wave propagation model, except in regions of significant wave breaking, where the discrepancy between measured and modeled fluxes was found to be closely related to the modeled wave dissipation rate.

Microwave backscatter from the sea surface: Bragg scattering by short gravity waves

Resonant Bragg scattering forms the basis for the composite or dual-scale model for microwave backscatter from the sea surface. The scatterers are short surface waves that are spatially resonant with the incident electromagnetic waves. Bragg scattering is most easily identified when the intrinsic velocity of the scattering agents can be deduced and compared with theoretical surface wave phase velocities. In this paper we present S band (wavelength 10 cm) microwave backscatter data taken at low grazing angles for three situations where the scatterer velocity could be separated from the surface currents. These include a situation where pack ice acts as an additional tracer on the water surface, a wind direction reversal, and an azimuthal scan during low wind speed conditions where two Bragg peaks are visible in the Doppler spectra. These data show that under these conditions the scattering is dominated by propagating 5-cm resonant waves. Doppler spectra recorded at low grazing angles are interpreted in terms of the angular distribution of the scattering waves, the contributions of wave orbital velocities, and the effects of geometric shadowing.

Nonlinear features in wave-resolving microwave radar observations of ocean waves

Transformation of sea-surface Doppler microwave backscatter observations from the space-time domain to the wavenumber-frequency domain separates linear wave energy from nonlinear effects. Here observations and modeling are used to investigate the sources of these nonlinearities. Wave breaking and electromagnetic shadowing are examined with emphasis on their relative effects both inside and outside the region of the wavenumber-frequency spectrum associated with the linear dispersion equation. Shadowing significantly reduces the variance levels within the linear spectral region. In addition, shadowing is less directly related to changes in variance outside this region, i.e., that region associated with nonlinearity in the wave field. Wave breaking has less of an effect on the variance within the linear region than shadowing. However, the modeled wave breaking does have a greater tendency to increase variance levels at frequencies less than that of the linear wave field, for any given wavenumber. Aliasing and emphasis of crest backscatter are also explored to explain features seen in some wavenumber-frequency intensity images. Two-dimensional data allow the linear wave spectrum to be separated from nonlinear effects. This results in improved wave height spectrum estimation.

Wind‐wave development across a large shallow intertidal estuary: A case study of Manukau Harbour, New Zealand

Abstract Locally generated wind‐waves in estuaries play an important role in the sediment dynamics and the transport of biota. Wave growth in estuaries is complicated by tidally varying depth, fetch, and currents. Wave development was studied at six sites along a transect across Manukau Harbour, New Zealand, which is a large intertidal estuary with a tidal range of up to 4 m. Three meteorological masts were also deployed across the measurement transect to measure wave forcing by the wind. A spatial variation in wind speed by up to a factor of 2 was observed which has a significant effect on wave development at short fetches. The wind variation can be explained by the extreme change in surface roughness at the upwind land‐water boundary. The tidally varying depth results in non‐stationary wave development. At the long fetch sites wave development is dictated by the tidally varying depth with peak frequencies continuing to decrease after high water, whereas wave height is attenuated by bottom friction. The non‐dimensional energy and peak frequency parameters commonly used to describe wave growth, clearly exhibit depth limiting effects, but with wider scatter than in previous studies in simpler environments. The peak frequency predictions of Young & Verhagen (1996a) fit our data well. However, the wide variability of energy limits the usefulness of standard growth prediction curves in such situations, and highlights the requirement for a validated, shallow‐water numerical model.

Ocean surface currents obtained from microwave sea‐echo Doppler spectra

Ocean currents may be determined from the positions of Bragg peaks in Doppler spectra of radar echoes from the sea surface. In this paper the applicability of this technique at microwave frequencies is investigated. Advantages of high range resolution and sensitivity to currents within centimeters of the ocean surface ensue from the short Bragg wavelength at microwave frequencies. A disadvantage is that the technique is restricted to low wind speed, fetch-limited sea conditions for which individual Bragg peaks are resolvable in the Doppler spectra. Experiments at low grazing angles with an S band (3 GHz) Frequency-modulated interrupted continuous wave radar show that in these conditions surface drift currents can be measured to better than ±2 cm s−1 accuracy over a large range of azimuth angles with respect to the wind direction. In conditions under which the Bragg peaks are not resolvable, currents may be measured by using local wind direction measurements to correct the mean Doppler velocity for the relative contributions of the advancing and receding Bragg peaks. Experiments revealed that the accuracy of current measurements obtained in this manner was better than ±5 cm s−1. The technique was applied to the mapping of tidal currents in an inlet, demonstrating the significant potential of the radar measurements for nearshore and coastal dynamics studies.