Abstract
Nowadays, uncertainties related to the determination of ocean tides remain a major issue for the exploitation of altimetry data in coastal areas. Using Sea Surface Height (SSH) observations from a new GNSS-based system mounted on an Unmanned Surface Vehicle (USV), we develop a crossover methodology to assess tide models under altimetry tracks. To this purpose, we address the Pertuis Charentais area, a semi enclosed sea located in the centre of the Bay of Biscay (France), as a field and modelling case study. We have developed a barotropic model configuration, based on SCHISM platform, using tidal elevations of an up-to-date regional atlas as boundary conditions. To test the impact of boundary conditions, we propose a second configuration where we applied uniform empirical biases in phases and amplitudes on M3 and MN4 constituents. In addition, the survey was designed to highlight the contribution of third and fourth-diurnal waves that are strongly amplified on the shelf and is used to assess model performances under the pass 216 of Sentinel-3A. Our results show that the second configuration reduces the Root-Mean-Square Error (RMSE) of the survey crossover residuals by more than 60%, leading to half of the residuals below 2.5 cm. This improved solution also reduces by 20% the RMSE computed with data from tide gauges located in the inner part of the Pertuis Charentais. Therefore, our study reinforces the importance for coastal tide modelling of an accurate tidal forcing, especially for shallow water waves. We finally discuss the impact of the remaining M4 error on crossover residual heights. By introducing an empirical correction term based on M4 observations at tide gauges, we further reduce the RMSE of crossover residuals by 15–25%. With this innovative study, we demonstrate the interest of combining crossover validation methods and USV systems to spatially extend our understanding of coastal areas dynamics. This will be crucial in the scope of the future SWOT mission, for which the tide correction accuracy must be assessed over the large-extent areas covered by swaths observations.
Highlights
In order to assess the ability of the two model configurations to reproduce tide variability, amplitude and phase of modelled and observed sea-levels are computed for 13 constituents at 5 stations (LSDO, LROC, Aix island tide gauge (AIX), BOURC and COT, see Figure 5) using the python implementation of the harmonic analysis from UTide package software [34]
We propose an original application of the use of new sea-level mapping tools in combination with a crossover methodology to assess the performances of two hydrodynamic models developed for the Pertuis Charentais area
After a classic tide gauge validation, we assess the accuracy of the two model configurations under a Sentinel-3A track by performing a crossover analysis on sea-level data collected in July 2020, using PAMELi, an Unmanned Surface Vehicle (USV) developed at La Rochelle University and equipped with the Cyclopée system
Summary
Satellite altimetry recently reached an unprecedented level of global coverage with seven missions flying simultaneously. While altimeters have been originally designed for open ocean and have improved our understanding of the large-scale ocean dynamic, the exploitation of coastal altimetry data remains a challenge that mobilizes a large effort in the scientific community. The future Surface Water Ocean Topography (SWOT) mission [1,2]. Will help to solve this issue and certainly revolutionize our view of the coastal waters by mapping Sea Surface Height (SSH) with an unprecedented spatial resolution. Based on a radar altimeter sending pulses over a surface, classical nadir altimeters (e.g., Jason series) suffer from their large inherent footprint, which perturbates the reflected signal
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