Evaluation of Sentinel-3A Wave Height Observations Near the Coast of Southwest England

Abstract

Due to the smaller ground footprint and higher spatial resolution of the Synthetic Aperture Radar (SAR) mode, altimeter observations from the Sentinel-3 satellites are expected to be overall more accurate in coastal areas than conventional nadir altimetry. The performance of Sentinel-3A in the coastal region of southwest England was assessed by comparing SAR mode observations of significant wave height against those of Pseudo Low Resolution Mode (PLRM). Sentinel-3A observations were evaluated against in-situ observations from a network of 17 coastal wave buoys, which provided continuous time-series of hourly values of significant wave height, period and direction. As the buoys are evenly distributed along the coast of southwest England, they are representative of a broad range of morphological configurations and swell conditions against which to assess Sentinel-3 SAR observations. The analysis indicates that SAR observations outperform PLRM within 15 km from the coast. Within that region, regression slopes between SAR and buoy observations are close to the 1:1 relation, and the average root mean square error between the two is 0.46 ± 0.14 m. On the other hand, regression slopes for PLRM observations rapidly deviate from the 1:1 relation, while the average root mean square error increases to 0.84 ± 0.45 m. The analysis did not identify any dependence of the bias between SAR and in-situ observation on the swell period or direction. The validation is based on a synergistic approach which combines satellite and in-situ observations with innovative use of numerical wave model output to help inform the choice of comparison regions. Such an approach could be successfully applied in future studies to assess the performance of SAR observations over other combinations of coastal regions and altimeters.

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

“…If the error Err buoy exceeds 1 m for buoy i, this buoy is considered as unreliable and its records are discarded. Given that the majority of the buoys match well with the various altimeter estimates, a median error of this magnitude implies a poorly calibrated buoy or else one that is too far from the altimeter track as to not provide representative ground truth values (see [26]).…”

“…Nencioli and Quartly [26] developed a methodology to use model data to help inform the choice of suitable match-ups, showing that some locations 20 km apart could be poorly correlated, whereas other locations much further away but with good exposure would give equivalent measures for validation purposes. In particular, it was noted that a small number of the buoys selected from our validation exercise were in well sheltered locations unrepresentative of those conditions further offshore.…”

“…The involved teams have made improvements on the retrackers and have assessed the resulting performance in comparison against the baseline algorithms but not against several other retrackers. Yang and Zhang [25] have validated the baseline retrackers SAMOSA and MLE-4 of Sentinel-3A/3B (S3A/B) datasets against open-ocean buoy data of the U.S. National Data Buoy Center (NDBC) network, whilst Nencioli and Quartly [26] have performed a detailed analysis of the same using both models and buoys around the southwest UK. Fenoglio-Marc et al [27] have validated the performance of the standard SAMOSA retracker from the CS2 mission in the study area of the German Bight.…”

Sensitivity of Altimeter Wave Height Assessment to Data Selection

“…These values were determined by Monaldo [29] based on examination and auto-correlation analysis of altimeters and buoys, respectively. 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.…”

The Sea State CCI dataset v1: towards a Sea State Climate Data Record based on satellite observations

“…The uncertainty in estimates due to fading noise typically increases with SWH, but is much more pronounced in the near-shore region (Figure 15b potential for improved SWH accuracy near land (Nencioli and Quartly, 2019) and reduced sensitivity to fading noise through "multi-looking" (Raney, 1998).…”