Estimation of sea surface short-wave spectra from co-polarized radar backscattering cross-section

Geophysical Model Functions (GMFs) describing Normalized Radar Backscattering Cross-Section (NRCS) of the ocean surface are analyzed to reconstruct short wind wave spectra responsible for the radar return. Ocean NRCS is represented as a sum of polarized Bragg and non-polarized backscattering components. Unlike VV or HH NRCS, the polarization difference, PD = VV − HH, does not contain NP return and thus is more suitable for Bragg wave spectrum retrieval. Omni-directional spectrum at Bragg wave number and its angular width are expressed via Fourier coefficients of GMF-based PD, Two-Scale Bragg model prediction, and mean-square slope of titling waves. Available L-, C-, Ku-, Ka-band dual co-polarized GMFs are used to retrieve the short wave spectrum. Comparison of the radar-derived spectra with theoretical models and independent optical stereo-photo measurements is also performed. A significant azimuthal isotropy of the L-band waves at light winds is detected and should be accounted for in theoretical models of short wind wave spectra.