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CalVal Ocean

Validation of the S6-MF measurements over open ocean and characterization of potential differences/discrepancies with respect to Jason-3

Thanks to the long duration of Sentinel-6A/Jason-3 tandem phase (15months+), a large amount of spatially and temporally collocated data are being collected. This unique opportunity allows a precise evaluation of Sentinel-6A performances with respect to Jason-3. The goal is to allow a seamless transition between Jason-3 and Sentinel-6A as the reference mission.

A complete CalVal assessment is performed in order to identify any discrepancies/differences between Jason-3 and Sentinel-6A LR over ocean. Residuals between Sentinel-6A LR and Jason-3 datasets are analyzed globally over ocean but also over specific geographical areas, specific atmospheric and sea state conditions to highlight any source of dependency. Thanks to the large dataset, the level of uncertainties is low. When identified, discrepancies are investigated to understand its origin and to propose correction if necessary.

Improvements brought by HR modes are also assessed in comparison to LR S6 and Jason-3’s conventional mode and to other SAR data (from Sentinel-3A). Direct comparisons between HR RAW and RMC data have been performed over LX2 segments and over complete cycles thanks to the RAW2RMC on-ground converter.

This activity is strongly based on results obtained during CNES/EUMETSAT commissioning activities and during GPP project.

This activity aims to:

  • Compile and summarize the main outcomes obtained in the frame of the Sentinel-6 MF commissioning activities and identify the remaining open questions,
  • Perform investigations to fully assessed Seintinel-6A performances and discuss potential processing alternatives (L1B and L2 and post processing) that could allow to mitigate sensitivities and ultimately discrepancies between all acquisition modes of S6 and J3.

Evaluation of the performance of S6-MF measurements in coastal areas

The accuracy of altimetry measurements in the coastal areas is affected by the local departure of the radar signal from the known ocean response (due to inhomogeneities of the illuminated area) and the inaccuracy of the corrections, as well as of the tidal models, needed to isolate anomalies in the sea level variability (Cipollini et al., 2017).

Sea Surface Height (SSH) from Delay-Doppler (DD) instruments is generally more precise and reliable in the coastal zone if compared to previous standard low-resolution mode (LRM) altimetry missions, even without any specific coastal retracker. Despite the improvements, the quantity and the quality of sea state and sea level retrievals in the coastal zone is still significantly different than from the open ocean. For example, concerning Significant Wave Height (SWH), it has been observed that the amount of missing data and outliers in Sentinel-3 data for a distance to coast of less than 20 km amounts to almost 40% (Schlembach et al., 2020).

In the latest years, the reprocessing of Low Resolution Mode (LRM) missions with the ALES retracker (Passaro et al., 2014) has shown that meaningful information can be retrieved in general up to 3 km from the coast and in some cases until few hundreds of meters (Benveniste et al., 2020). ALES has been designed to improve the detection of sea level in the coastal zone by overcoming the difficulties in retrieving the information from contaminated radar waveforms. Despite the wide use, this retracker is not yet part of the ground segment of the Geophysical Data Records of the LRM missions and reprocessings, with the consequence that DD coastal performances are compared with LRM data that are not optimized for the coastal zone. Given that the coastal zone is explicitly a focus of S6-MF, there is a need to understand how reliable the data provided to the users are and what are the improvements compared to the coastal-optimized LRM data. Moreover, in view of future reprocessings, the best strategy concerning possible additional dedicated retrackers and the different modes of operations have to be found. Considering the latter, in particular, it is important to understand from the early stage of the mission whether the on-board RMC processing (Kuschnerus et al., 2018), suggested in the open ocean since the current default ground station network cannot support operations in SAR RAW mode everywhere in the ocean, alters the performances of the retrieval algorithms in the coastal zone.

This activity aims at providing an internal comparison of the coastal performances of S6-MF in its different modes of operation (LRM, SAR-RAW and SAR-RMC) and J3. The performances will be assessed in terms of range retrieval and significant wave height retrieval. In addition, considering the intercomparison of the products from the two missions, the consistency of the relevant geophysical correction (i.e. radiometer correction, dual-frequency ionosphere, sea state bias) will be checked. All the statistics will be referred to the 20-km limit from the global coastline, i.e. the area in which typically the general performance of satellite altimetry data is considered degraded.

The work is structured in three different tasks:

  • Retrack the J3 and S6-MF LRM waveforms with specific retrackers: ALES and heritage from other ongoing projects (for example, WHALES from Sea State CCI),
  • Performance assessment analysis in terms of intrinsic noise and outlier analysis in the coastal zone, for S6 LRM, SAR-RAW and SAR-RMC & retracked J3,
  • Intercomparison of S6 and J3 based on L2 products in the coastal zone will be performed focusing on bias, drift and their geographical patterns.

REFERENCES

  • Benveniste J., Birol F., Calafat F., Cazenave A., Dieng H., Gouzenes Y., Legeais J.F., Léger F., Niño F., Passaro M., Schwatke C., Shaw A., 2020. (The Climate Change Initiative Coastal Sea Level Team): Coastal sea level anomalies and associated trends from Jason satellite altimetry over 2002–2018. Nature Scientific Data, 7, 357. https://doi.org/10.1038/s41597-020-00694-w.
  • Bosch W., Dettmering D., Schwatke C., 2014. Multi-mission cross-calibration of satellite altimeters: constructing a long-term data record for global and regional sea level change studies. Remote Sensing 6(3): 2255-2281. https://doi.org/10.3390/rs6032255.
  • Cipollini P., Benveniste J., Birol F., Joana Fernandes M., Obligis E., Passaro M., Strub T., Valladeau G., Vignudelli S., Wilkin J., 2017. Satellite Altimetry in Coastal Regions. In: Stammer D. and Cazenave A. (Eds.), Satellite Altimetry Over Oceans and Land Surfaces.
  • Kuschnerus, M., Cullen, R., Fornari, M., Giulicchi, L., Moreau, T., Rieu, P., Boy, F., Makhoul, E., Roca, M, 2018. Sentinel-6 Poseidon-4 RMC mode processing and expected performance, OSTST 2018.
  • Passaro M., Dinardo S., Quartly G.D., Snaith H.M., Benveniste J., Cipollini P., Lucas B., 2016. Cross-calibrating ALES Envisat and CryoSat-2 Delay–Doppler: a coastal altimetry study in the Indonesian Seas. Advances in Space Research, 58(3), 289-303, https://doi.org/10.1016/j.asr.2016.04.011.
  • Passaro M., Cipollini P., Vignudelli S., Quartly G., Snaith H., 2014. ALES: A multi-mission subwaveform retracker for coastal and open ocean altimetry. Remote Sensing of Environment 145, 173-189, https://doi.org/10.1016/j.rse.2014.02.008.
  • Schlembach F., Passaro M., Quartly G.D., Kurekin A., Nencioli F., Dodet G., Piollé J.-F., Ardhuin F., Bidlot J., Schwatke C., Seitz F., Cipollini P., Donlon C., 2020. Round Robin Assessment of Radar Altimeter Low Resolution Mode and Delay-Doppler Retracking Algorithms for Significant Wave Height. Remote Sensing, 12(8), 1254. https://doi.org/10.3390/rs12081254.