Wave Product
Get as PDFSwell field example
The SAR wave product is derived from the visible modulation of the radar echo by long ocean waves. This modulation of radar echo is coming partly from a modulation of the sea surface roughness by long waves, parly from the changing local incidence angle caused by tilting of the sea surface by long waves, and partly because of effects of orbital velocities of long waves. As a result, only long waves (wavelength larger than twice the actual resolution) are observed usually called swell. For Single Look Complex (SLC) SAR products, the limit is therefore about 60m in wavelength and for ASAR medium resolution products, the limit is around 175m. Most of the time, wind sea have significant wave scales bellow this limit and therefore cannot be observed, whereas swell have most of the time wavelength larger that this limit. As a consequence SAR wave product is also aften called swell product. In case of high winds, the resolution in the azimuth direction (satellite flight direction) is even degraded by some blurring caused by random orbital velocities of unresolved wave scales. The limit of detected ocean wave scales can then go up to 200m and sometimes even more in case of extreme winds. This additional wind dependant wavelength limit holds for both SLC and other type of SAR products.
In conditions of favorable winds (over the 2m/s small waves generation threshold) the SAR wave product can provide a quantitative estimation of the 2D wave spectrum with an accuracy on integrated parameters, estimated for each wave partition, of about 10 deg in direction, 30m in wavelength, and 0.3m in significant wave height.
The SAR wave product available on SOPRANO (numerical netcdf file) is a collection of 2D wave spectra estimated on contiguous blocs of SAR data within the whole SAR product. SAR wave product resolution or the grid spacing between two consecutive estimate of wave spectrum has been set to about 5km for image mode products (100km swath) and 20km for wide swath products (400km wide). Twice this resolution could easily be achieved but would result in much larger files. In addition, we provide the integrated parameters (significant wave height, dominant wavelength, dominant direction) of the two largest wave partition found in the 2D spectra, in terms of eneregy. The first partition beeing the one with the highest energy. Partionning concept is described in details in this ENVIWAVE project document about wave spectrum partitioning
On the graphical SOPRANO representation, only the dominant wave partition is represented, in terms of significant wave height (color), dominant direction and wavelength (arrow). the arrow length is proportionnal to the dominant wavelength for the most recent processed products. Similar information is available on a global scale from the wave mode of the same instrument. In this case, wave spectra are estimated every 100km along satellite track and dominant direction is used to propagate the observations at a group velocity estimated from the observed wavelength along a great cycle ray. the propagation allow to combine several observations at a given time into a Level3 product that can provide swell tracking from their source to their destination. This L3 product is displayed as an animated gif over the last 7 days of observations with a 3 hours time step.
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