On the spatial homogeneity of the wave spectra in deep water employing ERS-1/2 and ENVISAT SAR PRI products
Nelson Violante-Carvalho(1), Ian Robinson(2), Christine Gommenginger(2), Luiz Mariano Carvalho(1) and Brunno Goldstein(1)
(1) Rio de Janeiro State University UERJ, R. São Francisco Xavier, 524, 20550-900, Brazil
(2) National Oceanography Centre, Southampton, European Way, Southampton, SO14 3ZH, United Kingdom
Synthetic Aperture Radar (SAR) on-board satellites is the only source of directional wave spectra with continuous and global coverage. In SAR image mode the instrument acquires 100 by 100 km images but due to on-board storage limitations it can be operated only with a ground station in sight. The SAR wave mode (SWM) was introduced to overcome this coverage limitation since the much smaller 5 by 5 km imagettes are stored on-board and transmitted once per orbit to ground stations. Millions of SWM imagettes have been acquired since the launch in the early 1990's of the first European Remote Sensing Satellite ERS-1 and its successors ERS-2 and ENVISAT, which has opened up many possibilities specially for wave data assimilation purposes. With the better understanding of the imaging processes, retrieval algorithms have been proposed and the full directional spectra extracted from SAR data are now available.
Advances in our understanding of surface processes translating into better estimates through numerical simulations will necessarily require improvements in satellite remote sensing retrievals and more comprehensive schemes for the assimilation of this information into wave models. The retrieval of the directional wave spectrum from SAR images is not a trivial task. The main limitation lies in the fact that the SAR ocean wave imaging mechanism is strongly non-linear due to the vertical orbital movements induced by the waves which causes a Doppler offset in the image plane with smearing and loss of information beyond a high wavenumber cut-off.
SAR Level1 products for wave mode from ERS and ENVISAT have exactly the same characteristics as image mode of the same instruments, except for varying incidence angle. Most of the image mode data are near coastal regions and therefore the transfer functions need to be adapted to account for the non-linearities and modified dispersion relation induced by the limited depth. For ENVISAT HH polarized data, some modifications are needed to account for the different nature of dominant scattering mechanisms and consequently the non-linearities are much more pronounced due to the larger relative contribution of quasi-specular reflexion versus Bragg scattering.
The main goal of the present work is to extract the wave spectrum from image mode SAR (ERS-1/2 and ENVISAT SAR PRI products). From the 100 by 100 km image, several small images of 5 by 5 km are selected and the wave spectra are computed. The spatial homogeneity of the wave field in deep water is investigated against directional buoy measurements. The gradients of the wave parameters are compared and the assumption of spatial homogeneity of the waves is investigated.