Abstract's details

Coastal sea state representation, variability and uncertainty analysis from Jason-3 and Sentinel-6 Michael Freilich tandem phase experiment.

Ben Timmermans (National Oceanography Centre, United Kingdom)


Christine Gommenginer (National Oceanography Centre (Southampton), United Kingdom); Chris Banks (National Oceanography Centre (Liverpool), United Kingdom)

Event: 2023 Ocean Surface Topography Science Team Meeting

Session: Coastal Altimetry

Presentation type: Type Forum only

Contribution: PDF file


Accurate knowledge and understanding of the sea state and its variability is crucial to numerous oceanic and coastal engineering applications. Owing to a general sparsity of high quality in situ records in many regions, remote observations of sea state from satellite altimeter are becoming more widely used. However, questions remain over measurement uncertainty and long term sea state variability, particularly close to the coast, where collocation and intercomparison of observations is challenging. For example, accurate collocation requires consistency of spatiotemporal sea state properties of in situ observations and altimeter data but this poses challenges in the presence of local sea state gradients. These are driven by local small scale effects, such as sheltering, currents and water depth, which themselves may vary substantially with geographic location. Furthermore, since analysis of sea state uncertainty benefits from a large number of collocated samples, standardized altimeter sampling methods, that are often employed for efficiency and expediency, such as those based upon fixed geographic area or extent, introduce site specific biases, and in general obfuscate sources of uncertainty in subsequent analysis.

In this work we propose a methodology based upon in situ measurements from moored NDBC buoys, and 1 Hz along-track significant wave height altimeter data, to examine local sea state properties, establish collocation criteria and analyse long term sea state variability. We first exploit the Jason-3 record to demonstrate the limited sensitivity of altimeter sampling to geographic extent, at deep water sites offshore. This is linked to the homogeneity of sea state variability in the open ocean over large spatial scales (> 100 km). In contrast, we reveal the strong spatial heterogeneity of sea state properties closer to the coast (< 50 km). Having established the altimeter sampling methodology, we show its impact on sea state uncertainty estimation at a large number of coastal sites in the U.S. Pacific. We then extend the analysis to data from the Sentinel-6 Michael Freilich mission that flew concurrently with Jason-3 during the Sentinel-6 MF – Jason-3 tandem phase experiment. The tandem phase data provides over one year of closely collocated observations of significant wave height, acquired from both low resolution and SAR altimetry modes aboard Sentinel-6 MF. We make use of this unique experimental configuration and dataset to evaluate differences between the various measurement platforms at coastal scales.
Ben Timmermans
National Oceanography Centre
United Kingdom