A comparison of in situ and airborne radar observations of ocean wave directionality

Abstract

The directional spectrum of a fully arisen ∼3 m sea as measured by an experimental airborne radar, the NASA Ku‐band radar ocean wave spectrometer (ROWS), is compared to reference pitch‐roll buoy data and to the classical SWOP (stereo wave observation project) spectrum for fully developed conditions. The ROWS spectrum, inferred indirectly from backscattered power measurements at 5‐km altitude, is shown to be in excellent agreement with the buoy spectrum. Specifically, excellent agreement is found between the two nondirectional height spectra, and mean wave directions and directional spreads as functions of frequency. This agreement is found despite certain discrepancies between the radar and buoy angular harmonics which are believed to be due to buoy instrumental effects. A comparison of the ROWS and SWOP spectra shows the two spectra to be very similar, in detailed shape as well as in terms of the gross spreading characteristics. Both spectra are seen to exhibit bimodal structures which accord with the Phillips' resonance mechanism. This observation is thus seen to support Phillips' contention that the SWOP modes were indeed resonance modes, not statistical artifacts.

Measuring currents, ice drift, and waves from space: the Sea Surface KInematics Multiscale monitoring (SKIM) concept

“…Hence, these basic measurement are highly resolved in range but averaged over the footprint diameter in the perpendicular (azimuthal) direction. This averaging is the basic principle of the wave spectrometer laid out by Jackson et al (1985). Namely, only the waves aligned with the line of sight produce a modulation of the signal in the range direction.…”

“…In general U S,R can be estimated from the directional wave spectrum. The details of this estimation with a rotating wave Doppler spectrometer, combining the ideas of Jackson et al (1985) and Caudal et al (2014), is presented in Appendix A. An overall accuracy of 10% for U S,R is expected from our preliminary algorithm.…”

Directional Wavenumber Spectra of Ocean Surface Waves

“…Bimodal directional distribution has been observed from spatial measurements using aerial stereo photographic technique (Phillips 1958;Cote et al 1960, Holthuijsen 1983, airborne radar system (Jackson et al 1985), land-based imaging radar (Wyatt 1995), and airborne scanning lidar system (Hwang et al 2000a,b). In contrast to the analysis of temporal measurements from wave gauge arrays or directional buoys, standard 2D Fast Fourier Transformation (FFT) procedure is sufficient to bring out the multi-modal feature in the directional distribution from 3D spatial topographic images.…”

“…J " 1U Numerical experiments (e.g., Komen et al 1984;Banner and Youngl994) £Z!T fl irectional dis,ribu,ion func,ion is sensi,ive <° S formulafons of the sources terms. Accurate determination of the directional distribute ,s needed to enhance our understanding of the ocean wave dynamS Td to validate the performance of nonlinear ocean wave models.…”

Comment on “Imaging radar observations of directional properties of ocean waves” by William McLeish and Duncan B. Ross

“…While it is certainly quite possible that the buoy data are biased high because of contamination by a background sea or by instrumental effects [cf. Jackson et al, 1985); hereinafter referred to as JWP] one may as well suspect the SLAR data. Several considerations in fact indicate that the buoy data (without background correction) are the more representative of the actual spreading.…”