Round Robin Assessment of Radar Altimeter Low Resolution Mode and Delay-Doppler Retracking Algorithms for Significant Wave Height

Schlembach, Florian; Passaro, Marcello; Quartly, Graham D.; Kurekin, Andrey A.; Nencioli, Francesco; Dodet, Guillaume; Piollé, Jean-François; Ardhuin, Fabrice; Bidlot, Jean‐Raymond; Schwatke, C; Seitz, Florian; Cipollini, Paolo; Donlon, Craig

doi:10.3390/rs12081254

Cited by 34 publications

(39 citation statements)

References 53 publications

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“…We make use of a compilation of buoy and altimeter data gathered for the Sea State CCI Round Robin exercise [5]. The in situ data come from the operational archive at the European Centre for Medium-range Weather Forecasting (ECMWF) augmented by a few sources (see Section 2.2 of [5]). Because only buoys near the selected altimeter tracks were used, this means that only a small fraction of the ECMWF dataset was employed.…”

Section: Buoy Datamentioning

confidence: 99%

“…The various candidate algorithms are described in some detail in the Round Robin evaluation [5]; to avoid repetition, only the briefest details are recapped here.…”

Section: Altimeter Datamentioning

confidence: 99%

“…More detailed information on all of these algorithms is given in the earlier paper [5] and in the Algorithm Theoretical Basis Document [18]. They are summarised in Table 1.…”

Section: Altimeter Datamentioning

confidence: 99%

“…Nencioli and Quartly [30] showed that aspects of the coastline morphology and orientation to the prevailing waves can be important in the coastal zone, and this is an issue that we address later. In this paper, we re-use the match-up database provided for the Round Robin paper [5], so the information is only recapped here. Jason-3 and Sentinel-3A both provide waveforms and, thus, opportunity for SWH estimates at a rate of ∼20 Hz, corresponding to an along-track displacement of ∼294 m (J3) or ∼334 m (S3A).…”

Section: Data Match-upmentioning

confidence: 99%

“…In an endeavour to improve the accuracy and consistency of these multi-satellite compendia, the European Space Agency (ESA) has inaugurated a "Sea State" component to their Climate Change Initiative (CCI) programme [4]. One focus of this work has been to instigate a Round Robin process to evaluate a number of candidate algorithms, and assess their reliability in providing data, with low noise levels and minimal outliers, which compare well with model forecasts and the limited coastal buoy network [5].…”

Section: Introductionmentioning

confidence: 99%

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Sensitivity of Altimeter Wave Height Assessment to Data Selection

Quartly¹,

Kurekin²

2020

Remote Sensing

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This paper addresses the issue of how the selection of buoys and the calculation of altimeter averages affect the metrics characterising the errors of altimetric wave height estimates. The use of a 51-point median reduces the sensitivity to occasional outliers, but the quality of this measure can be improved by demanding that there is a minimum number of valid measurements. This had a marked impact in both the open ocean and the coastal zone. It also affected the relative ordering of algorithms’ performances, as some fared poorly when a representative value was gleaned from a single waveform inversion, but had a much better ranking when a minimum of 20 values were used. Validation procedures could also be improved by choosing altimeter-buoy pairings that showed a good consistency. This paper demonstrated an innovative procedure using the median of the different retrackers analysed, which can be easily extended to other data validation exercises. This led to improved comparison statistics for all algorithms in the open ocean, with many showing errors less than 0.2 m, but there was only one strong change in the relative performance of the 11 Jason-3 retrackers. For Sentinel-3A, removing the inconsistent coastal buoys showed that all of the new algorithms had similar errors of just over 0.2 m. Thus, although improvements were found in the procedure used for the Sea State Round Robin exercise, the relative rankings for the buoy calibrations are mostly unaffected.

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Section: Buoy Datamentioning

confidence: 99%

“…The various candidate algorithms are described in some detail in the Round Robin evaluation [5]; to avoid repetition, only the briefest details are recapped here.…”

Section: Altimeter Datamentioning

confidence: 99%

“…More detailed information on all of these algorithms is given in the earlier paper [5] and in the Algorithm Theoretical Basis Document [18]. They are summarised in Table 1.…”

Section: Altimeter Datamentioning

confidence: 99%

Section: Data Match-upmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Sensitivity of Altimeter Wave Height Assessment to Data Selection

Quartly¹,

Kurekin²

2020

Remote Sensing

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Benefits of Fully Focused SAR Altimetry to Coastal Wave Height Estimates: A Case Study in the North Sea

Schlembach

Ehlers

Kleinherenbrink

et al. 2022

Preprint

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Estimating the three geophysical variables significant wave height (SWH), sea surface height, and wind speed from satellite altimetry continues to be challenging in the coastal zone because the received radar echoes exhibit significant interference from strongly reflective targets such as mud banks, sheltered bays, ships etc. Fully focused SAR (FF-SAR) processing exhibits a theoretical along-track resolution of up to less than half a metre. This suggests that the application of FF-SAR altimetry might give potential gains over unfocused SAR (UF-SAR) altimetry to resolve and mitigate small-scale interferers in the along-track direction to improve the accuracy and precision of the geophysical estimates. The objective of this study is to assess the applicability of FF-SAR-processed Sentinel-6 Michael Freilich (S6-MF) coastal altimetry data to obtain SWH estimates as close as possible to the coast. We have developed a multi-mission FF-SAR processor and applied the coastal retracking algorithm CORALv2 to estimate SWH. We assess different FF-SAR and UF-SAR processing configurations, as well as the baseline Level-2 product from EUMETSAT, by comparison with the coastal, high-resolution SWAN-Kuststrook wave model from the Deltares RWsOS North Sea operational forecasting system. This includes the evaluation of the correlation, the median offset, and the percentage of cycles with high correlation as a function of distance to the nearest coastline. Moreover, we analyse the number of valid records and the L2 noise of the records. The case study comprises five coastal crossings of S6-MF that are located along the Dutch coast and the German coast along the East Frisian Islands in the North Sea. We find that the FF-SAR-processed dataset with a Level-1b posting rate of 140 Hz shows the greatest similarity with the wave model. We achieve a correlation of ˜0.8 at 80% of valid records and a gain in precision of up to 29% of FF-SAR vs UF-SAR for 1-3 km from the coast. FF-SAR shows, for all cycles, a high correlation of greater than or equal to 0.8 for 1-3 km from the coast. We estimate the decay of SWH from offshore at 30 km to up to 1 km from the coast to amount to 26.4%+-3.1%.

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Benefits of Fully Focused SAR Altimetry to Coastal Wave Height Estimates: A Case Study in the North Sea

Schlembach

Ehlers²,

Kleinherenbrink³

et al. 2023

Preprint

View full text Add to dashboard Cite

Estimating the three geophysical variables significant wave height (SWH), sea surface height, and wind speed from satellite altimetry continues to be challenging in the coastal zone because the received radar echoes exhibit significant interference from strongly reflective targets such as mud banks, sheltered bays, ships etc. Fully focused SAR (FF-SAR) processing exhibits a theoretical along-track resolution of up to less than half a metre. This suggests that the application of FF-SAR altimetry might give potential gains over unfocused SAR (UF-SAR) altimetry to resolve and mitigate small-scale interferers in the along-track direction to improve the accuracy and precision of the geophysical estimates. The objective of this study is to assess the applicability of FF-SAR-processed Sentinel-6 Michael Freilich (S6-MF) coastal altimetry data to obtain SWH estimates as close as possible to the coast. We have developed a multi-mission FF-SAR processor and applied the coastal retracking algorithm CORALv2 to estimate SWH. We assess different FF-SAR and UF-SAR processing configurations, as well as the baseline Level-2 product from EUMETSAT, by comparison with the coastal, high-resolution SWAN-Kuststrook wave model from the Deltares RWsOS North Sea operational forecasting system. This includes the evaluation of the correlation, the median offset, and the percentage of cycles with high correlation as a function of distance to the nearest coastline. Moreover, we analyse the number of valid records and the L2 noise of the records. The case study comprises five coastal crossings of S6-MF that are located along the Dutch coast and the German coast along the East Frisian Islands in the North Sea. We find that the FF-SAR-processed dataset with a Level-1b posting rate of 140 Hz shows the greatest similarity with the wave model. We achieve a correlation of ~0.8 at 80% of valid records and a gain in precision of up to 29% of FF-SAR vs UF-SAR for 1-3 km from the coast. FF-SAR shows, for all cycles, a high correlation of greater than or equal to 0.8 for 1-3 km from the coast. We estimate the decay of SWH from offshore at 30 km to up to 1 km from the coast to amount to 26.4% +- 3.1%.

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Round Robin Assessment of Radar Altimeter Low Resolution Mode and Delay-Doppler Retracking Algorithms for Significant Wave Height

Cited by 34 publications

References 53 publications

Sensitivity of Altimeter Wave Height Assessment to Data Selection

Sensitivity of Altimeter Wave Height Assessment to Data Selection

Benefits of Fully Focused SAR Altimetry to Coastal Wave Height Estimates: A Case Study in the North Sea

Benefits of Fully Focused SAR Altimetry to Coastal Wave Height Estimates: A Case Study in the North Sea

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