Chapter 2 Development and application of satellite retrievals of ocean wave spectra

Abstract

Abstract The launch of SeaSat in 1978 demonstrated the feasibility of measuring ocean wave heights and imaging the corresponding two-dimensional wave field from space. With the launch of the first European Remote-sensing Satellite (ERS-1) in 1991, wave researchers and operational forecasters obtained global, continuous, quasi-real-time wave data for the first time. This led to the developments of sophisticated, so-called “third-generation” wave models, such as the Wave Model (WAM), and spectral retrieval algorithms for synthetic aperture radar (SAR) data. To achieve these goals, however, significant hurdles had to be overcome. Wave modelers had to develop numerically viable parameterizations of the nonlinear wave-wave interactions. The remote-sensing challenge was to understand and resolve the strong nonlinearities besetting SAR imaging of the moving ocean wave surface. This paper reviews the progress achieved over the last twenty years and summarizes wave data assimilation methods and other current applications of ERS quasi-real-time global SAR wave spectral data or SAR wave-mode product. Two applications are presented. A comparison of wave spectra predicted by WAM with spectra retrieved from ERS-1 on a global scale revealed that WAM overpredicted local wind-generated sea surface heights and underpredicted swell. The former can be largely attributed to wind-forcing errors, while the latter is most likely due to an overly strong swell dissipation in WAM. Assimilation of ERS-1 altimeter sea surface height data into the WAM spectra was found to not alter the qualitative conclusions of the comparison. A second application addresses the trans-ocean propagation of swell. Swell propagating from a storm in the South Pacific is traced over a period of ten days with ERS-1 SAR and compared with model predictions. Wind fields used for wave predictions are also compared with ERS-1 wind scatterometer data.