The use of ERS-1 synthetic aperture radar for measurement and modeling of the directional wave spectrum

The study presented in this work examines the use of Synthetic Aperture Radar (SAR) onboard the first European Remote Sensing Satellite (ERS-1) for the measurement of wind waves and discusses its applications such as wave data assimilation and inverse modeling. Since the launch of ERS-1 in 1991 the directional wave spectrum became available with high spatial and temporal global coverage. With the subsequent launch of its successors ERS-2 and ENVISAT millions of SAR wave mode (SWM) imagettes have been and are still being acquired providing a unique data set for the improvement of the understanding of the dynamics of surface gravity waves. Over four years of directional measurements acquired by a heave-pitch-roll buoy moored in tropical deep waters around 150 km offshore are employed to investigate the mechanisms of SAR wave imaging and the retrievals of wave spectra from SAR images. new approach for the partitioning of the frequency spectrum is implemented and applied to isolate the wind sea from the swell contaminated spectra. Specifically the influence of swell on wind sea growth is analyzed in terms of their direction of propagation and separation in frequency space, with direct connection on how long waves modulate short ones. The relatively poorly understood Hydrodynamic and Tilt Modulation Transfer Functions (MTF) are discussed in the light of the findings. For the first time the most widely implemented Max-Planck Institut (MPI) SWM retrieval scheme was assessed through detailed statistical intercomparisons and selected qualitative validations against directional buoy data and against the third generation wave model WAM data - which was used as first guess to the inversion. The MPI scheme deteriorates the first guess increasing the bias and the errors of the retrievals of significant wave height, mean wave length and mean direction of propagation of short waves.