Evaluation of SAR-based Sea State Parameters and Roughness Length Derivation over the Coastal Seas of the United States

Abdalmenem Owda*, Andrey Pleskachevsky, Xiaoli Guo Larsen, Merete Badger, Dalibor Cavar, Charlotte Hasager

*Corresponding author for this work

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Abstract

This paper presents comprehensive validation of specific sea state parameters (SSPs) and SAR-derived wind speeds (uSAR ). The paper introduces a novel approach to retrieving roughness length (z0) based on wave steepness, following the retrieval of the short wavelengths necessary to estimate z0. The Synthetic aperture radar (SAR) onboard the Sentinel-1 (S1) satellite was used specifically the Interferometric Wide Swath Mode (IW) data. The data was processed using the CWAVE_EX algorithm for SSPs and CMOD-5 for uSAR CWAVE_EX was developed especially for coastal waters; the processing chain includes steps for SAR image denoising and eliminating image artifacts. SAR S-1 data inherently exhibits a substantial azimuthal cutoff length due to the data's high satellite altitude and SAR IW resolution. That complicates the retrieving of short wavelengths prevalent in coastal zones and needed to retrieve z0 The research focuses on the coastal seas of the United States, benefiting from the presence of an extensive network of ocean buoys for validation purposes. The complete SAR S1 A/B archive from 2014 to 2022 was first processed to retrieve SSPs and uSAR The validation for significant wave height (Zs), second moment wave period(Tm2) and uSAR was performed using in-situ measurements with about 6,000 collocations.Hs and Tm2 were compared against the corresponding parameters from hindcast spectral numerical model data with about 380,000 collocations. The comparisons between the retrieved Hs and Tm2 against the in-situ observations and hindcast wave model data yielded a root mean square error (RMSE) of 0.46-0.50 m and 0.9-1.1 sec. The RMSE of uSAR against in-situ observation was about 2 m/s with a bias of 0.78 m/s. The estimated z0 values from satellite-driven wave parameters were highly correlated with the z0 estimated from the in-situ observations, with a RMSE of 0.04×10-3 m and a bias of -0.01×10-3 m. The study highlights the possibility of using SAR remote sensing data for global mapping of z0, including coastal effects of local variability in sea state and wind field gustiness.
Original languageEnglish
JournalIEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing
Volume17
Pages (from-to)9415-9428
ISSN1939-1404
DOIs
Publication statusPublished - 2024

Keywords

  • SAR
  • Integrated sea state parameter
  • Air-sea interaction
  • Roughness length
  • CWAVE_EX
  • JONSWAP
  • Neutral stratified wind
  • Geophysical model function

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