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Sea State

Validation of S6-MF sea state measurements using triple collocation analysis

The Sentinel-6/Jason-3 Tandem provides a unique opportunity for in-depth investigations of the uncertainties and error characteristics of altimeter sea state data. Given the high temporal and spatial variability of sea state, conventional inter-comparison methods (e.g. cross-overs) are unable to isolated the contributions to observed uncertainties due to natural variability, random instrument errors and systematic instrument/processing biases. In the case of Sentinel-6MF, with its key role in ensuring long-term continuity of the altimeter reference data record,

understanding these uncertainties and the consistency of its sea state measurements in the context of other satellites, and in different ocean conditions, is particularly critical. The S6-MF/Jason-3 Tandem will give the opportunity to evaluate, for the first time, the performance of the new S6-MF SAR Interleaved mode (Gommenginger et al., 2013) directly against S6-MF LRM and Jason-3 (LRM), and to explore the relative merits of difference modes and satellites (biases, random errors, continuity) in different oceanic conditions (e.g. high waves, swell, low winds). This study uses triple collocation as the central methodology to assess data from Sentinel-6MF and Jason 3, different S6-MF operating modes, independent in situ fiducial data and global models. Triple collocation is a powerful statistical tool that makes it possible to quantify measurement uncertainties in three independent datasets, without assumptions about the quality of either data source. The method was applied successfully by NOC during the Sentinel-3A/B Tandem phase and served to establish the excellent consistency of sea state measurements from the two satellites (Clerc et al., 2020), and to quantify the instrument performance in LRM and SAR modes. In the case of Sentinel-3 however, the 6-months Tandem phase resulted in a relatively small number of collocations with buoys, and the study concluded that the triple collocation analyses would have benefitted (for the sake of robustness) from a longer tandem period. The longer, 12-months, Tandem between S6-MF and Jason-3 should provide a great opportunity to repeat and extend these analyses and achieve greater levels of confidence.

This activity comprises the following elements:

  • Triple collocation applied to S6-MF LRM, Jason 3 (LRM) and wave buoy data obtained by match-up during the Tandem phase (Significant Wave Height and Wind speed). This will evaluate the consistency of S6-MF LRM data against Jason-3 and the same fiducial observations, with regards to the long-term reference data records (Timmermans etal., 2020a ; 2020b).
  • Triple collocation applied to S6-MF SAR, Jason 3 (LRM) and wave buoys obtained by match-up during the Tandem phase (Significant Wave Height and Wind speed). This will examine the uncertainties of S6-MF interleaved SAR measurements against the common reference from Jason-3 and buoys.
  • Triple collocation applied to S6-MF SAR, S6-MF LRM and Jason 3 (LRM) over different ocean regions during the Tandem Phase. These analyses complement the other tasks by providing a broader view of performance across the globe, including regions where in situ measurements are rare or absent (e.g. Southern Ocean, Central Pacific).

REFERENCES

  • Clerc S, Donlon C, Borde F, Lamquin N, Hunt SE, Smith D, McMillan M, Mittaz J, Woolliams E, Hammond M, Banks C. Benefits and lessons learned from the Sentinel-3 tandem phase. Remote Sensing. 2020 Jan, 12(17):2668.
  • O’Carroll, A. G., Eyre, J. R. and Saunders, R. W., Three-Way Error Analysis between AATSR, AMSR-E, and In Situ Sea Surface Temperature Observations. Journal of Atmospheric and Oceanic Technology (25) 2008.
  • Gommenginger, C. Martin-Puig, L. Amarouche, and R. K. Raney, Review of State of Knowledge for SAR Altimetry over Ocean, Version 2.2, EUMETSAT, EUM/RSP/REP/14/74930421, November 2013.
  • Timmermans, B., C. Gommenginger, G. Dodet and J. R. Bidlot (2020). Global wave height trends and variability from new multi‐mission satellite altimeter products, reanalyses and wave buoys. Geophys. Res. Lett., 47 (9).
  • Timmermans, B., A. G. Shaw, and C. Gommenginger (2020), Reliability of extreme significant wave height estimation from satellite altimetry and in situ measurements in the coastal zone, Journal of Marine Science and Engineering, 8(12), 1039.

Exploiting differences and processing techniques to study ocean swell waves and high sea states and mitigate their impact on S6-MF SSH measurements

The S6-MF mission continues the innovative record of altimetric Delay/Doppler technique [Raney, 1998] started with the Cryosat-2 mission. While this latter operates only over selected oceanic regions, the S3-A/B missions perform this acquisition of such data worldwide. With the recent launch of the S6-MF satellite, this technological advancement with respect to traditional altimeters goes one step further by bringing new capabilities such as: (1) novel processing technics that enhance the SAR altimeter capability for providing topographic, wave and backscattering features of the surface at smaller scales and with even more lower measurements noise than what has already been achieved by S3-A/B from the interleaved operating mode; (2) simultaneous generation of both conventional low-resolution mode (LRM) and SAR mode data [Phalippou et al., 2012], but also (3) the provision of LR-RMC data [Moreau et al., 2021].

While various studies pointed out significant benefits of SAR over LRM in terms of improved measurement errors and finer along-track spatial resolution [Boy et al., 2017], some downsides specifically to SAR altimetry have also been highlighted. Indeed, the retrieved topography, wave and backscattering features are sensitive to long ocean waves. The impact depends strongly on the period of the waves and their energy, but also on the orientation of the satellite track with respect to them [Aouf et Phalippou, 2015; Abdalla and Dinardo, 2016; Moreau et al., 2018; Reale et al., 2018; Rieu et al., 2020]. Another important effect of swell is the increase of the high-frequency noise on the estimated parameters, but also of the SSH variance at longer wavelengths because of the aliasing [Reale et al., 2020; Rieu et al., 2020]. In addition, to this swell effect, orbital velocities [Boisot et al., 2016; Bushhaupt, 2019; Egido et al., 2020, Amarouche et al., 2019; Tran et al., 2020] induced by all sea states and not limited to swell only, can also alter the SAR mode signal leading to observed biases in SWH data which can in turn induce a bias in SSH through the SSB correction. Other phenomena can furthermore affect the delay/Doppler measurement leading possibly to additional biases in SSH estimation. They may be related to nonlinear effects of waves leading to upwave/downwaves SSH and SWH biases and variability [Tran et al. 2020]. Sentinel-6 MF should be impacted by the same kind of limitations related to sea state than S3-A/B data. The preliminary analysis performed within the commissioning activities on the newly acquired data seems to confirm that so far. However, one can expect some differences of behavior due to differences in some instrument characteristics: pulse repetition frequency, altitude, integration time length …

Three issues and corresponding analysis axes have been identified to answer to ESA questions:

  • Sea-state impact assessment: What is the potential impact of ocean wave conditions on the long-term sea state and sea level time-series?
  • Swell detection: Is it possible to detect swell by combining SAR altimeter data processed in different ways and then to define new products including additional swell information?
  • SSH correction: Is it possible to propose approaches to mitigate negative SSH impacts due to SAR processing and reduce regional biases that would enter into the sea level record?

Assessment of the feasibility of each of them in terms of data availability, technical difficulties and workload will performed. Selection of the aspects to be further analyzed during the second phase and proposal of the corresponding work plan to be agreed by ESA.

REFERENCES

  • Abdalla, S., S. Dinardo, ‘‘Does swell impact SWH from SAR altimetry?”, 2016 SAR Altimetry Workshop, La Rochelle, France, Oct. 31, 2016.
  • Amarouche, L., Tran, N., Herrera, D., Guerin, C.-A., Dubois, P., Aublanc, J., Boy, F., 2019. Impact of the ocean waves motion on the Delay/Doppler altimeters measurements. OSTST Meeting 2019, Chicago, Illinois, United States, Oct. 21–25.
  • Aouf, L. and Phalippou, L., ‘‘On the signature of swell for the Cryosat-2 SAR-mode wave data”, OSTST Meeting 2015, Reston, Virginia, USA, Oct. 20 – Oct. 23, 2015. Available online at https://meetings.aviso.altimetry.fr/fileadmin/user_upload/tx_ausyclsseminar/files/OSTST2015/IPM-02-ostst_Aouf_sarmode_2015_1.pdf.
  • Boisot, O., L. Amarouche, J-C. Lalaurie and C-A. Guérin (2016), “Dynamical Properties of Sea Surface Microwave Backscatter at Low-Incidence: Correlation Time and Doppler Shift,” IEEE Trans. Geosci. Remote Sens., 54, 7385-7395, doi : 10.1109/TGRS.2016.2601242.
  • Boy, F., Desjonquères, J.-D., Picot, N., Moreau, T., Raynal, M., 2017. CRYOSAT-2 SAR Mode Over Oceans: Processing Methods, Global Assessment and Benefits. IEEE Trans. Geosci. Remote Sens. 55, 148–158. https://doi.org/10.1109/TGRS.2016.2601958.
  • Buchhaupt, C., 2019, “Model Improvement for SAR Altimetry,” Darmstadt, Technische Universität, ISBN 978-3-935631-44-0, doi: https://tuprints.ulb.tu-darmstadt.de/9015.
  • Dinardo, S., B. Lucas and J. Benveniste, ‘‘SAR altimetry at 80 Hz”, OSTST Meeting 2014, Lake Constance, Germany, Oct. 28 – Oct. 31, 2014. Available online at https://meetings.aviso.altimetry.fr/fileadmin/user_upload/tx_ausyclsseminar/files/SAR_Altimetry_at_80_Hz_OSTST_2014.pdf.
  • Egido, A., S. Dinardo, and C. Ray, 2020. The case for increasing the posting rate in delay/Doppler altimeters, Adv. Space Res. https://doi.org/10.1016/j.asr.2020.03.014.
  • Moreau, T., Tran, N., Aublanc, J., Tison, C., Le Gac, S., Boy, F., 2018. Impact of long ocean waves on wave height retrieval from SAR altimetry data. Adv. Space Res. 62 (6), 1434–1444. https://doi.org/10.1016/j.asr.2018.06.004.
  • Moreau, T., Cadier, E., Boy, F., Aublanc, J., Rieu, P., Raynal, M., Labroue, S., Thibaut, P., Dibarboure, G., Picot, N., Phalippou, L., Demeestere, F., Borde, F., Mavrocordatos, C., 2021. High-performance altimeter Doppler processing for measuring sea level height under varying sea state conditions, Adv. Space Res., https://doi.org/10.1016/j.asr.2020.12.038.
  • Phalippou, L., Caubet, E., Demeestere, F., Richard, J., Rys, L., Deschaux-Beaume, M., Francis, R., and Cullen, R., 2012. Reaching sub-centimeter range noise on Jason-CS with the Poseidon-4 continuous SAR interleaved mode, Ocean Surface Topography Science Team 2012, Venice, Italy, 27–28 September 2012, available at: https://www.aviso.altimetry.fr/fileadmin/documents/OSTST/2012/oral/02_friday_28/05_instr_processing_IIb/05_IP2B_Phalippou.pdf.
  • Raney, R.K., 1998. The delay/doppler radar altimeter. IEEE Trans. Geosci. Remote Sens. 36 (5), 1578–1588. https://doi.org/10.1109/36.718861.
  • Reale, F., Dentale, F., Carratelli, E.P., Fenoglio-Marc, L., 2018. Influence of Sea State on Sea Surface Height Oscillation from Doppler Altimeter Measurements in the North Sea. Remote Sens. 10 (7), 1100. https://doi.org/10.3390/rs10071100.
  • Reale, F., Pugliese Carratelli, E., Di Leo, A., Dentale, F., 2020. Wave orbital velocity effects on radar Doppler altimeter for sea monitoring. J. Marine Sci. Eng. 8 (6), 447. https://doi.org/10.3390/jmse8060447.
  • Rieu, P., Moreau, T., Cadier, E., Raynal, M., Clerc, S., Donlon, C., Borde, F., Boy, F., Maraldi, C., 2020. Exploiting the Sentinel-3 tandem phase dataset and azimuth oversampling to better characterize the sensitivity of SAR altimeter sea surface height to long ocean waves. Adv. Space Res. https://doi.org/10.1016/j.asr.2020.09.037, ISSN 0273-1177.
  • Tran, N., Amarouche, L., Boy, F., 2020. Impact of the ocean waves on the Delay/Doppler altimeters: Analysis using real Sentinel-3 data. OSTST Meeting 2020, Virtual, Oct. 19–23.