An Empirical Imaging Model for Sar Ocean Wave Measurements

Continuous ocean wave measurements on a global basis are still only possible using spaceborne synthetic aperture radar. For more than a decade the European satellites ERS-1 and ERS-2 have acquired about 1500 globally distributed SAR images dayly. ENVISAT is capable of recording some 3000 imagettes. However strong uncertainties in the SAR ocean wave imaging models used so far are well known. These models are either purely theoretical or semi-empirical. A new empirical model for the SAR ocean wave imaging process is proposed within this study. It is derived based on a global data set of ERS-2 wave mode spectra and colocated two-dimensional ocean wave spectra from the numerical ocean wave model WAM. A least-square minimisation approach is used to fit a quasi-linear model function calculated as an optimal estimate of the ocean to SAR transfer function. The empirical transfer function is compared to the theoretical expression used in the literature. Comparing simulated and observed SAR spectra the quality of the empirical model is tested. Furthermore the model is used to evaluate significant wave heights based on a quasi-linear inversion approach. Both statistics and 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, especially using ENVISAT data.