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Ocean Surface Topography from Space
SCIENCE
Investigating radar altimeter signatures of Internal Solitary Waves in the ocean


Author:

José C.B. Da Silva - (Faculdade de Ciências da Universidade do Porto – Departamento de Geociências, Ambiente e Ordenamento do Território)

Co-Investigator(s):
  Jean Tournadrec
Bertrand Chapronc
(Laboratoire d’Oceanographie Spatiale, IFREMER)
(Laboratoire d’Oceanographie Spatiale, IFREMER)


Abstract:
Investigating radar altimeter signatures of Internal Solitary Waves in the ocean
Advanced Synthetic Aperture Radar (ASAR) image acquired by ENVISAT on 24 August 2008 at 02:07 UTC showing strong surface manifestations of Internal Solitary Waves in the South China Sea. The white line is the ground track of Jason-2 satellite (cycle 005 pass 088) overpassing the region at nearly the same time. The colored strip on the ground, each side of the ground track, represents radar backscatter obtained from an inversion scheme proposed in Tournadre et al. (2011). SAR image and altimeter backscatter patterns are consistent, after correction of 37 min between the two data takes.
Internal Waves of tidal frequency (i.e. Internal Tides) are successfully detected in sea‐surface height (SSH) by satellite altimetry (Ray and Mitchum, 1996). Shorter period nonlinear internal waves or Internal Solitary Waves (ISWs), whose periods are an order of magnitude smaller than tidal internal waves, are however generally assumed too small to be detected with standard altimeters (at low sampling rates, i.e. 1 Hz). This is because the Radar Altimeter (RA) footprint is somewhat larger, or of similar size at best, than the ISWs typical wavelengths. We propose to investigate how new generation high sampling rate satellite altimeters (i.e. Jason-3 operating at 20 Hz and Sentinel-3 RA) are able to detect short-period ISW trains of waves in the ocean. It will be shown in this proposal that Jason-2 (20 Hz) along-track data hold a variety of short-period signatures that are consistent with surface manifestations of ISWs. Our observational method is based on satellite synergy with imaging sensors such as Synthetic Aperture Radar (SAR) and other high-resolution optical sensors (e.g. 250m resolution MODIS/VIIRS images and Sentinel-3 OLCI) with which ISWs are unambiguously recognized. EUMETSAT is responsible for the routine operations of Sentinel-3, whose data will provide exact synergy between OLCI (Ocean Land Colour Instrument), SLSTR (Sea and Land Surface Temperature Radiometer) in the thermal infrared wavebands and RA in SRAL mode. Straining of decimeter-scale surface waves due to ISW orbital currents is known to cause roughness variations along internal wave propagation fields. This effect was demonstrated by measurements of wind wave slope variances associated with short-period ISWs in the pioneer work of Hughes and Grant (1978). Mean square slope can be estimated from nadir looking RAs using a geometric optics (specular) scattering model (Brown, 1990; Jackson et al., 1992; Frew et al., 2007), and directly obtained from backscatter (sigma0) along-track records. Here, we propose to use differential scattering from the dual-band (Ku- and C-bands) microwave pulses of the Jason-2/3 high-rate RA and the dual-frequency Synthetic Aperture Radar Altimeter (SRAL) onboard Sentinel-3 to isolate the contribution of small-scale surface waves to mean square slope. The differenced altimeter mean square slope estimate, derived for the nominal wave number range 40–100 rad/m, will then be used to detect roughness variations in records of along-track high sampling rate RAs. Subsequently these high-frequency signatures will be compared with simultaneous or quasi-simultaneous satellite imagery with imprints of ISWs. We expect that the RA signatures consistent with ISWs will be also apparent in radargrams of the Sensor Geophysical Data Records (SGDR), in high resolution (20-Hz) data. The waveforms shape, in particular their trailing edge, is expected to be modified by the presence of ISWs with respect to waveforms unperturbed by short-period internal waves. Furthermore, altimeters of the Jason series can be viewed as high resolution imaging instruments whose geometry is annular and not rectangular. Hence, a method based on the computation of the imaging matrix and its pseudoinverse may be used to infer the surface backscatter at high resolution (300 m) from the measured waveforms (Tournadre et al., 2011). This method will be applied to the case of ISW observations. In this proposal we show preliminary evidence of the existence of radar backscatter variations consistent with ISW patterns on the ocean surface, and seek for a project that further investigates those signatures in the framework of small scale processes at the sea surface interface.



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