SAR ocean wave measurements based on an empirical imaging model

Spaceborne Synthetic Aperture radar is still the only instrument providing continuous wave measurements on a global basis. For more than a decade, the European satellites ERS-1 and ERS-2 have acquired about 1500 globally distributed SAR images daily. It is well known however that strong uncertainties exist in the SAR ocean wave imaging models used so far, which are either purely theoretical or semi-empirical. In this study, a new empirical model for the SAR ocean wave imaging process is proposed. The empirical model is derived based on a global data set of ERS-2 wave mode spectra and collocated two-dimensional ocean wave spectra from the numerical ocean wave model WAM. A quasi-linear model function is fitted using a least-square minimization approach to calculate an optimal estimate of the ocean to SAR transfer function. The empirical transfer function is compared to the theoretical expression used in the literature. The quality of the empirical model is tested by comparison of simulated and observed SAR spectra. Furthermore, the model is used to estimate significant wave heights based on a quasi-linear inversion approach. Statistics as well as global maps of wave parameters are presented. The study is a contribution to the optimization of the operational use of global SAR data for the assimilation of numerical wave forecast models