Session: Ocean II

The assessment of the accuracy of the skin sea-surface temperature (SST) retrievals from the SLSTR’s on the Sentinel-3’s is a vital prerequisite for application of the satellite data to any scientific problem. Underestimating errors and uncertainties will lead to ill-founded scientific conclusions, and over-estimates will cast doubt unnecessarily on what are sound conclusions and new knowledge. Correct assessment of errors and uncertainties requires independent measurements of skin SST of high accuracy. Such validation measurements can be provided by well-calibrated infrared radiometers mounted on ships. The second generation of Marine-Atmospheric Emitted Radiance Interferometers (M-AERI’s) have recently been developed and two are now deployed on cruise ships of Royal Caribbean Cruise Lines that operate in the Caribbean Sea, North Atlantic and Mediterranean Sea. M-AERI’s are Fourier Transform Infrared Interferometers that take measurements of the emission from the sea surface and atmosphere from which skin SSTs can be derived. In addition, two Infrared SST Autonomous Radiometers (ISAR’s) are mounted alternately on a car transporter ship of NYK Lines that crosses the Pacific Ocean between Japan and the USA. Both M-AERI’s and ISAR’s are self-calibrating radiometers having two internal blackbody cavities to provide at-sea calibration of the measured radiances, and the accuracy of the internal calibration is periodically determined by measurements of a NIST-traceable blackbody cavity in the laboratory. The deployments of the M-AERI’s and ISAR’s are funded by NASA for the SST retrieval algorithm development and validation for the MODIS’s on Terra and Aqua, and for VIIRS on S-NPP, but these shipboard measurements can make a significant contribution to the validation of the skin SST retrievals of the SLSTR’s. The presentation will provide an overview of the instrument capabilities, their past, present and future deployments, and examples of the validation results of MODIS and VIIRS.

Along-track altimetric measurements of Sea Surface Heights (SSH) are very well suited to quantify across-track currents. However, the spatial resolution of derived 2D velocities is restricted to scales above 100-150 km and the limited number of altimeters can lead to errors in the location of currents. On the contrary, infrared measurements of Sea Surface Temperature (SST) are well suited to locate flow patterns but it is difficult to extract quantitative estimations of ocean currents. To overcome the previous constrains we have developed a methodology to exploit the synergy between SST and SSH measurements to provide enhanced 2D surface currents. Our approach combines the usage of advanced signal processing techniques, such as wavelet analysis, with dynamical approaches like the Surface Quasi-Geostrophic (SQG) equations. Among all existing platforms, Sentinel-3 is the best placed to exploit this methodology since it provides simultaneous observations of both fields: SST and SSH. This approach has been tested using observations provided by Envisat (RA, AATSR), since it has sampling  characteristics similar to those of Sentinel-3 (SRAL,SLSTR). Besides, we have compared SAR mode CryoSat-2 observations with the currents derived from SST to asses the potential of the SRAL instrument. Finally, we have started to explore the potential of instruments like OLCI to provide additional information about ocean currents.

Although conventional radar altimetry products (Jason1, Jason2, LRM Cryosat-2, etc) have a spatial resolution between 5 and 10 km (due to the size of the radar footprint), the observation of ocean scales in the along track direction smaller than100 kmis limited by the existence of a “spectral bump”, i.e. a geographically coherent error between 10 and 20 km, observed on all conventional altimeters.

Dibarboure et al (2014) have largely discussed the bump artefact which appears to be a mixed effect between the Brown model used for retracking the data (which has been designed for a homogeneous scene and is not fully relevant during backscattering events) and the criteria for selecting the 20 Hz valid data. They showed that dedicated processing could better discard outliers and reduce the energy in the mean PSD of the sea level. More recently, dedicated editing procedure based on the mispointing angle on AltiKa (Poisson et OSTST 2014) has also enhanced the description of the corrupted data. In parallel, other groups are also working on retracking as Amarouche and Sandwell, also showing some significant improvement.

Another important issue for understanding the oceanic turbulence at short scales is the use of the new delay Doppler technique that should yield measurement less altered by the heterogeneities issue since the thin stripe-shaped synthetic footprint of SARM is reduced to 300 m in the along track direction. Preliminary results have shown that the SARM data derived from Cryosat-2 do not show the spectral bump.

As the delay Doppler processing have become more mature in the last couple of years, the content of the shortest spatial scales is further investigated here by analyzing several oceanic regime acquired in SAR mode with Cryosat-2 mission and comparing the results with other LRM processing (AltiKa, Jason-2 and Cryosat-2) either with standard ground processing retrackings or with new retracking approaches.

This work allows understanding the outcomes of the SARM processing that we could expect from Sentinel-3 mission.