Comparison of the two-dimensional directional wave spectra retrieved from spaceborne synthetic aperture radar images using the MPI scheme against directional buoy measurements*

Spaceborne Synthetic Aperture Radar (SAR) is to date the only source of two-dimensional directional wave spectra with continuous and global coverage when operated in the so-called SAR Wave Mode (SWM). Since the launch in 1991 of the first European Remote Sensing Satellite ERS-1, and more recently with ENVISAT, millions of SWM imagettes containing detailed spectral information are now available in quasi-real time. This huge amount of directional wave data has opened up many exciting possibilities for the improvement of our knowledge of the dynamics of ocean waves. However, the retrieval of wave spectra from SAR images is not a trivial exercise due to the nonlinearities involved in the mapping mechanism. In this paper we review the main features of the SAR ocean wave imaging mechanisms and give a detailed description of the Max-Planck Institut (MPI) retrieval algorithm which is running operationally at the European Centre for Medium-Range Weather Forecasts (ECMWF). Some examples of retrieved spectra are compared against directional buoy measurements obtained in deep water in the South Atlantic and against WAM spectra. The main characteristics of the MPI retrieval scheme are discussed and some of its deficiencies and strengths are identified.

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