Evaluation of a linear expression for retrieval of directional wave spectra from complex sar look cross spectra

Abstract

The feasibility of a Linear Inversion Method applied to SAR cross spectra to retrieve wave directional spectra is investigated. The proposed method is compu tationally efficient and does not require any a priori information from wave or at mospheric models. Several test cases of complex SAR image spectra were simulated considering a broad range of values of steepness for different satellite configurations. The results of the simulated cases were employed to correct biases while retrieving directional wave spectra from Level 1 cross spectra. The nonlinearities, not accounted for when applying a linear inversion, are quantifiable as they depend on the satellite configuration and the relative position of the peak wave spectrum in relation to the azimuth cut-off. The wave spectra retrieved with the Linear Inversion Method from an Envisat ground track were evaluated against the ECMWF reanalysis ERA5 and compared to the results of ESA Level 2 WVW wave mode spectra. Both meth ods presented similar results in terms of RMSE and SI for significant wave height, peak wavelength and peak propagation direction. The shape of the directional wave spectra retrieved with the Linear Inversion Method presented better similarity with the ERA5 spectra, in comparison with the WVW spectra. Moreover, in around one third of the cases, the ESA method did not resolve the directional ambiguity. The preliminary results employing the Linear Inversion Method are encouraging, specially considering its simplicity and performance against the ESA distributed wave spectra