Group Line Energy in Phase-Resolved Ocean Surface Wave Orbital Velocity Reconstructions from X-band Doppler Radar Measurements of the Sea Surface

Abstract

The wavenumber-frequency spectra of many radar measurements of the sea surface contain a linear feature at frequencies lower than the first order dispersion relationship commonly referred to as the “group line”. Plant and Farquharson, showed numerically that the group line is at least partially caused by wave interference-induced breaking of steep short gravity waves. This paper uses two wave retrieval techniques, proper orthogonal decomposition (POD) and FFT-based dispersion curve filtering, to examine two X-band radar datasets, and compare wave orbital velocity reconstructions to ground truth wave buoy measurements within the field of view of the radar. POD allows group line energy to be retained in the reconstruction, while dispersion curve filtering removes all energy not associated with the first order dispersion relationship. Results show that when group line energy is higher or comparable to dispersion curve energy, the inclusion of this group line energy in phase-resolved orbital velocity reconstructions increases the accuracy of the reconstruction. This increased accuracy is demonstrated by higher correlations between POD reconstructed time series with buoy ground truth measurements than dispersion curve filtered reconstructions. When energy lying on the dispersion relationship is much higher than the group line energy, the FFT and POD reconstruction methods perform comparably.

Sea Surface Roughness Observed by High Resolution Radar

“…Kammerer and Hackett [8] show that phase-resolved ocean wave fields are reconstructed from X-band Doppler radar measurements of the ocean surface by proper orthogonal decomposition (POD) more accurately than the conventional FFT-based dispersion curve filtering. The results indicate that the group line (a linear feature at frequencies lower than the first order dispersion relationship in wavenumber-frequency spectra of the ocean surface) influences the phase-resolved wave field.…”

Inversion of Wave Parameters With Shore-Based Coherent $S$-Band Radar Using Quasi-Binary Variational Mode Decomposition

“…Therefore, many studies focus on removing the nonlinear components from the wavenumberfrequency spectrum to improve the accuracy of ocean wave parameter inversion. Kammerer et al [19]- [21] demonstrated that retaining part of "Group-Line" energy in significant wave height inversion can improve inversion accuracy when "Group-Line" energy is greater or comparable to dispersion curve energy. Hwang et al [22] used a high-pass filter with a cutoff frequency of 0.04 Hz to filter out part of the "Group-Line" energy, but the wave period was overestimated.…”