SAR Mode Altimetry Observations of Internal Solitary Waves in the Tropical Ocean Part 1: Case Studies

Abstract

It is well known that 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, as the typical wavelengths of the ISWs. Here we demonstrate that the synthetic aperture radar altimeter (SRAL) on board the Sentinel-3A can detect short-period ISWs. 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 (sigma0), sea level anomaly (SLA), and significant wave height (SWH). Radar backscatter is the primary parameter in which ISWs can be identified owing to the measurable sea surface roughness perturbations in the along-track 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 kilometers. The ISW signatures are unambiguously identified in the SRAL because of the exact synergy with OLCI (Ocean Land Colour Imager) images, which in cloud-free conditions allow clear identification of the ISWs in the sunglint OLCI images. We show that both sigma0 and SLA yield realistic estimates for routine observation of ISWs with the SRAL, which 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 light of our knowledge of radar backscatter in the internal wave field. An analysis is presented for the tropical Atlantic Ocean off the Amazon shelf to infer the frequency of the phenomena, being consistent with previous satellite observations in the study region.

SAR-Mode Altimetry Observations of Internal Solitary Waves in the Tropical Ocean Part 2: A Method of Detection

“…Although Level-1b Ocean products are also available for the Sentinel-3 SRAL, here, we choose to use the Level-2 topography product denominated "enhanced measurement." It is understood that the Level-1b SAR measurements possess a broad set of solutions that all deserve to be thoroughly tested for a particular application [7], but for simplicity, in this initial effort we opted for Level-2 data which seems to be sufficient to detect at least the largest ISW signatures (see Reference [8]).…”

“…Here we use exact synergy between the OLCI image data and the SRAL high-rate along-track records, which are acquired simultaneously at the same position on the ocean surface by Sentinel-3A and have nearly the same spatial resolutions (300 m). This approach allows identification of ISWs in the along-track altimeter data records unambiguously [8]. Since we can directly compare the SRAL signal with radiance contrasts resulting from the ISWs in the OLCI images (provided that cloud-free conditions exist), a validation method for an independent detection method of ISWs in the SRAL can be readily established.…”

“…Since rain attenuation at the Ku-band is on an order of magnitude larger than that at the C-band, rain-contaminated observations from the Jason-2/3 dual-frequency altimeter are usually identified as an abrupt decrease in the σ 0 ratio between the Ku-band and C-band. However, in this study, we could not rely on this criterion to discard rain-affected measurements as those differenced dual-frequency σ 0 (high-frequency) fluctuations could also be due to IW surface manifestations (see also Reference [8]). At present, our method to deal with rain-affected measurements consists of a threshold in the integrated columnar liquid water content L z , here chosen as 0.1 kg/m 2 , and water vapor content (wv), whose limit was chosen as 60 kg/m 2 [26] (as measured by the MWR on board Sentinel-3A).…”

“…It was demonstrated in Reference [5] that the Jason-2 altimetry data with a high sampling rate (i.e., 20 Hz) hold a variety of short-period signatures that are consistent with surface manifestations of ISWs in the ocean. More recently Reference [8] showed that high-frequency signatures contained in the along-track SRAL of Sentinel-3A may result from ISW events. These authors base their conclusions on the exact synergy between the SRAL and OLCI (ocean and land color instrument).…”

Validation of Sentinel-3 SAR Level-2 and Level-3 Products in the Baltic Sea and Estonian lakes

“…CryoSat-2 and Sentinel-3 operating in SAR mode have a waveform footprint of 5 km 2 , much smaller than that covered by pulse-limited altimeters in 20-30 km 2 , such as SARAL/AltiKa and the Jason series. The along-track resolution of SAR altimeters has been increased to approximately 300 m along the flight path [8].…”