Abstract
It is demonstrated that the synthetic aperture radar altimeter (SRAL) on board of the Sentinel-3A can detect short-period internal solitary waves (ISWs) with scales of the order of a kilometer. A variety of signatures owing to the surface manifestations of the ISWs are apparent in the SRAL Level-2 products over the ocean. These signatures are identified in several geophysical parameters, such as radar backscatter ( σ 0 ) and sea level anomaly (SLA). Radar backscatter is the primary parameter in which ISWs can be identified owing to the measurable sea surface roughness perturbations in the along-track direction resulting from the sharpened SRAL footprint. The SRAL footprint is sufficiently small (300 m in the along-track direction) to capture radar power fluctuations over successive wave crests and troughs, which produce rough and slick surface patterns arrayed in parallel bands with scales of a few kilometers along-track. Furthermore, it was possible to calculate the mean square slope ( s 2 ¯ ) for the dual-band (Ku and C bands) altimeter of Sentinel-3, which made the ISW signatures unambiguously identified because of the large s 2 ¯ variations in exact synergy with ocean and land color instrument (OLCI) images. Hence, the detection method is validated in cloud-free sun glint OLCI images. It is shown that both σ 0 and SLA yield realistic estimates for routine observation of ISWs with the SRAL. The detection method that is used relies on the parameter s 2 ¯ which is calculated from σ 0 . This is a significant improvement from previous observations recently reported for conventional pulse-limited altimeters (Jason-2). An algorithm is developed to be used in any ocean region. Wavelets were applied for a first analysis of the s 2 ¯ variations because ISWs can be readily identified in high-frequency signals. Other geophysical parameters such as SLA were used to exclude phenomena that are unlikely to be ISWs.
Highlights
Altimeter ocean backscatter is most described as the specular backscatter from all surface roughness elements with length scales greater than about three times the incident wavelength of the microwaves
Most satellite altimeters operate in dual-frequency, the Ku-band (13.6 GHz) and C-band (5.3 GHz). This dual-frequency allows for correction of the reduction in phase velocity caused by free electrons within the ionosphere, denominated “Ionospheric Correction”, which is frequency dependent, so one correction is developed from the difference in time delay at the altimeter’s Ku- and C-band
Several theoretical studies suggest that there is a more direct altimeter inference to be made in terms of surface s2, which should parallel to the optical measurements of ocean s2 versus wind speed provided in Reference [1]
Summary
Altimeter ocean backscatter is most described as the specular backscatter from all surface roughness elements with length scales greater than about three times the incident wavelength of the microwaves. Off the Amazon shelf break deep waters of the tropical ocean, some of the most intense ISWs in the ocean are found to propagate for hundreds of kilometers offshore, whose propagation direction is not far from the diurnal (descending) Sentinel-3A satellite tracks This region allows us to study and test any detection method based on sea surface manifestations of ISWs. The waves are believed to originate from the steep slopes of the shelf break as internal (tidal) waves, which subsequently evolve nonlinearly and disintegrate into solitary internal waves (or solitons) due to an imbalance between nonlinear and dispersive effects on the linear internal tide.
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