Benefits of fully focused SAR altimetry to coastal wave height estimates

Benefits of fully focused SAR altimetry to coastal wave height estimates: A case study in the North Sea

Schlembach, Florian (Technische Universität München)
Ehlers, F. (TU Delft Physical and Space Geodesy)
Kleinherenbrink, M. (TU Delft Mathematical Geodesy and Positioning)
Passaro, Marcello (Technische Universität München)
Dettmering, Denise (Technische Universität München)
Seitz, Florian (Technische Universität München)
Slobbe, D.C. (TU Delft Physical and Space Geodesy)

2023

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 sandbanks, 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 observe that accurate and precise SWH records can be estimated in the nearshore zone within 1–3 km from the coast using satellite SAR altimetry. 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%.

Algorithm
Case study
Coastal retracking
Coastal SWH variation
Coastal zone
CORAL
FF-SAR
Fully focused SAR
North sea
Retracking
SAMOSA
SAR altimetry
Satellite altimetry
Sentinel-6 Michael Freilich
Significant wave height
SWH decay
UF-SAR
Unfocused SAR

http://resolver.tudelft.nl/uuid:aef10c4d-ba98-4419-b966-1f6dc1d7dead

https://doi.org/10.1016/j.rse.2023.113517

2023-09-06

0034-4257

Remote Sensing of Environment: an interdisciplinary journal, 289

Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.

Institutional Repository

journal article

© 2023 Florian Schlembach, F. Ehlers, M. Kleinherenbrink, Marcello Passaro, Denise Dettmering, Florian Seitz, D.C. Slobbe