SAR Mode Altimetry Observations of Internal Solitary Waves in the Tropical Ocean: algorithm development of a method of detection
Adriana M. dos Santos-Ferreira, José C.B. da Silva and Meric Srokosz
Internal waves (IWs) of tidal frequency (i.e., internal tides) are successfully detected in sea surface height (SSH) by satellite altimetry. Shorter period internal solitary waves (ISWs), whose periods (and spatial scales) are an order of magnitude smaller than tidal internal waves, have been generally assumed too small to be detected with conventional altimeters. This is because conventional (pulse-limited) radar altimeter footprints are somewhat larger than or of similar size, at best, than the typical wavelengths of the ISWs.
Here it is demonstrated that the synthetic aperture radar altimeter (SRAL) on board the Sentinel-3A can detect short-period 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 (σ0) 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 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 kilometres along-track. Furthermore, it was possible to calculate the mean square slope (mss) for the dual-band (Ku and C bands) altimeter of Sentinel-3, which made the ISW signatures unambiguously identified because of the large mss variations in exact synergy with OLCI (Ocean Land Colour Imager) images. Hence, the detection method is validated in cloud-free sunglint OLCI images. It is shown that both σ0 and SLA yield realistic estimates for routine observation of ISWs with the SRAL. The detection method that is used relies on the parameter mss which is calculated from σ0. This is a significant improvement from previous observations recently reported for conventional pulse-limited altimeters (Jason-2).
Several case studies of ISW signatures are interpreted in various regions of the ocean, and a Matlab code is developed to be used in any deep ocean region. Wavelets were used for a first analysis of the mss variations because ISWs can be readily identified in high frequencies signals. Other geophysical parameters such as SLA are used to exclude phenomena that are unlikely to be ISWs.