Meteo-Marine Parameters and Their Variability Observed by High Resolution Satellite Radar Images
Lehner, Susanne1; Pleskachevsky, Andrey1; Rosenthal, Wolfgang2
1German Aerospace Center (DLR), GERMANY; 2GAUSS, GERMANY
Space borne SAR (Synthetic Aperture Radar) is a unique sensor providing two dimensional information of the ocean surface. Due to its high resolution, daylight, weather independency and global coverage SAR is particularly suitable for many ocean and coastal observations. The new TerraSAR-X (TS-X) satellite radar acquires images of the sea surface with a high resolution up to 1m, individual ocean waves with wavelengths under 30m are detectable. The spatial variability of the wave parameters measured in TS-X scenes, wave refraction, wave groupiness and wave breaking are investigated.
Two-dimensional information of the ocean surface retrieved using TS-X data has been validated and fitted with collocated hindcast model results and buoy data for different oceanographic applications: derivation of the high-resolution wind field (XMOD algorithm) and integrated sea state parameters (empirical XWAVE algorithm). Both algorithms are capable to take into account fine-scale effects and this two-dimensional information can be successfully applied to validate numerical models. The refraction of long swell, rendered by high resolution SAR images has been also studied using wave ray tracking techniques. The changing of wavelength and direction in the rays allows detecting underwater structures (sandbars, reefs) and obtaining bathymetry. Wave ray technique also allows obtaining wave energy flux propagation towards the coast and its dissipation. The height of individual breaking waves can be obtained from SAR-image signatures. Retrieved amplitudes of the breaking waves are somewhat lower than predicted by wave models in the shoaling zone, likely due to a stronger dissipation than given by the model.
The large spatial coverage of images in order of 100km resolves the impact of the wind gustiness on surface waves under storm conditions. In particular, the appearance of wave groups with extreme high amplitude and wavelength are of interest. Mesoscale wind gust, moving as an organized system across the sea, continuously transferring energy and momentum into the growing waves are often associated with specific cloud formations in form of cells visible in optical satellite images. E.g. analysis of information from different optical sensors (Thermal Infrared images from AVHRR Advanced Very High Resolution Radiometer onboard NOAA satellites, MSG-1 Meteosat Second Generation true color composite images, MERIS-ENVISAT) shows that during many storms characterized by extremely high individual waves ring pattern cloud formations with a diameter of 30km-90km are observed. These are associated with open cells due to convection in the atmosphere during cold air outbreaks. According to SAR measurements, the footprints of these mesoscale patterns produce a local increase in the wind field at the sea surface with a sharp gradient along their leading edge. Superimposing the observed gusts into conventional coarse input wind field for numerical spectral wave model results in the local increase of significant wave height. A group of cells for real storm condition produces a local increase of significant wave height of more than 6m during a short time window of 10min-20min (passing of the cell) within the cell area and especially in a narrow area of about 2.5km at the center of a cell. Wave groups including extreme individual waves with wavelength of more than 370m appear under the cell's footprint. This corresponds well with measurement of a rogue wave group with length of about 400m registered during storm "Britta" and damaged the deck of research platform FiNO-1 in 18m above Mean Sea Level in 2006.