Comparison of optical and radar measurements of surf and swash zone velocity fields

Abstract

[1] Surf zone bore celerities and swash zone surface currents were measured on a shallow sloping, low-energy beach using two remote sensing methods that differ fundamentally in their imaging mechanisms. Microwave Doppler radar measures electromagnetic backscatter from small-scale ocean surface roughness while video-based particle image velocimetry (PIV) relies on image texture resulting from variations in light reflectivity from the ocean surface. Imagery from the two methods showed high correlation, in which regions of high radar backscatter corresponded to visibly identifiable waves and bores propagating across the surf zone. Correlation coefficients between radial velocity time series sampled using the two methods at multiple locations across the surf zone were typically greater than 0.5 for frequencies less than a noise cutoff of 0.25 Hz. Similarly, spectra were found to be coherent at the 95% level with a nearly zero phase shift between the two signals near the broad spectral peak between 0.02 and 0.25 Hz. However, some significant differences were evident. PIV was capable of estimating both cross-shore and alongshore surface velocities while the use of one microwave Doppler radar system restricted surface velocity estimates from that technique to line of sight (radial) only. PIV was found to be more capable of estimating swash zone (uprush and backwash) surface velocities as a smoother water surface in the swash zone adversely affected radar reflectivity. In contrast, microwave Doppler radar was found to be more capable of estimating the surface velocity between bores when insufficient image texture was recorded in the video imagery. Both techniques were capable of measuring surf zone bore celerities through comparison to a shallow water model and to independent celerity estimates extracted from the slope of individual bore trajectories. Typical normalized errors were roughly 25% for radar and 15% for PIV.

Optical and Microwave Detection of Wave Breaking in the Surf Zone

“…This value proved to be independent of the changes in the ambient environmental conditions during the experiment. Being consistent with the findings of previous research [16], [25], [57], it was found that the strongest backscatter comes from the steep and/or breaking waves. However, unlike most results from a deeper water, it is apparent from the present results that, in the surf zone, the depth-limited breaking waves are the more dominant mechanism.…”

Microwave radar cross sections and Doppler velocities measured in the surf zone

“…Although the means of the distributions (Figure 3) associated with the secondary peaks agree well with the predicted phase velocities (Figure 4), pixels with the largest NRCS have higher Doppler velocity values (Figure 3) than predictions. Thus mean Doppler velocities of surf zone bores estimated from the largest individual values of NRCS (Figure 4, squares) (see also Puleo et al [2003, Figure 9c], who use the same data used here) rather than those estimated from the peak of the distribution (i.e., the highest‐valued contours in Figure 3) as used here are significantly larger than predicted velocities (Figure 4). The physical mechanism associated with these largest velocities remains uncertain.…”

“…Radial velocity versus cross‐shore coordinate (and depth). Asterisks are theoretically predicted shallow water phase velocities (), diamonds are observed radial velocities corresponding to the means of the joint histograms for radar beams from −2.49° to +2.49°, triangles are the observed radial velocities corresponding to the peaks of the histograms, and squares are the observed radial velocities corresponding to pixels with the largest NRCS values [from Puleo et al , 2003]. The RMS difference between the observations (diamonds) and the predictions (asterisks) is 0.17 m/s.…”

“…Vertically polarized backscattered power and Doppler velocities were measured in the nearshore region at Scripps Beach, La Jolla, California [ Puleo et al , 2003] from 1700 to 1800 UT on 10 October 2000 using a second‐generation Focused Phased Array Imaging Radar (FOPAIR) [ McIntosh et al , 1995]. FOPAIR is an X band microwave Doppler radar designed to image the sea surface with meter‐scale resolution.…”

Remote sensing of breaking wave phase speeds with application to non-linear depth inversions

“…Some field observations have suggested that nearshore wave speeds are bounded by this limit (Inman et al, 1971;Thornton and Guza, 1982); although, in a few cases the limit has also been exceeded (Suhayda and Pettigrew, 1977;Lippmann and Holman, 1991;Puleo et al, 2003). In general, there is significant variability between the observed phase speeds and the theoretical predictions.…”