Observed wavenumber‐frequency properties of microwave backscatter from the ocean surface at near‐grazing angles

This paper presents microwave observations of the ocean surface obtained with a high‐resolution X band imaging Doppler radar during March 1994. Besides providing time‐series imagery of the ocean surface, the radar has sufficient spatial resolution to produce wave vector‐frequency (three‐dimensional) spectra of observed power and Doppler frequency modulations due to the wind waves. The wave vector‐frequency spectra provide a complete representation of the mean spatio‐temporal properties of vertically polarized X band ocean backscatter at near‐grazing incidence. Evident in the spectra are the expected contributions from linear gravity waves exhibiting high coherences between power and Doppler (orbital) velocity and consistent with the composite‐surface‐model view of sea surface microwave backscatter. Occasionally, gravity wave harmonics possibly indicative of wave nonlinearities are observed. Also readily identified in the spectra are spatially broadband modulations due to localized scattering sources which are influenced by young gravity waves and the wind. Contributions from these apparently “non‐Bragg” scattering sources account for a significant part of the observed coherent modulations. The propagation velocity of this modulation is consistent with scattering from short gravity waves well beyond the spectral peak.