Validation of Copernicus Sea Level Altimetry Products in the Baltic Sea and Estonian Lakes

Aive Liibusk,Kuo-Hsin Tseng,Tarmo Kall,Rivo Uiboupin,Sander Rikka,Ülo Suursaar

doi:10.3390/rs12244062

Abstract

Multi-mission satellite altimetry (e.g., ERS, Envisat, TOPEX/Poseidon, Jason) data have enabled a synoptic-scale view of ocean variations in past decades. Since 2016, the Sentinel-3 mission has provided better spatial and temporal sampling compared to its predecessors. The Sentinel-3 Ku/C Radar Altimeter (SRAL) is one of the synthetic aperture radar altimeters (SAR Altimeter) which is more precise for coastal and lake observations. The article studies the performance of the Sentinel-3 Level-2 sea level altimetry products in the coastal areas of the Baltic Sea and on two lakes of Estonia. The Sentinel-3 data were compared with (i) collocated Global Navigation Satellite System (GNSS) ship measurements, (ii) the Estonian geoid model (EST-GEOID2017) together with sea-level anomaly corrections from the tide gauges, and (iii) collocated buoy measurements. The comparisons were carried out along seven Sentinel-3A/B tracks across the Baltic Sea and Estonian lakes in 2019. In addition, the Copernicus Marine Environment Monitoring Service (CMEMS) Level-3 sea-level products and the Nucleus for European Modelling of the Ocean (NEMO) reanalysis outcomes were compared with measurements from Estonia’s 21 tide gauges and the buoy deployed offshore. Our results showed that the uncertainty of the Sentinel-3 Level-2 altimetry product was below decimetre level for the seacoast and the selected lakes of Estonia. Results from CMEMS Level-3 altimetry products showed a correlation of 0.83 (RMSE 0.18 m) and 0.91 (RMSE 0.27 m) when compared against the tide gauge measurements and the NEMO model, respectively. The overall performance of the altimetry products was very good, except in the immediate vicinity of the coastline and for the lakes, where the accuracy was nearly three times lower than for the open sea, but still acceptably good.

Highlights

Water level monitoring stations or tide gauges (TG) have been widely used to observe sea-level variations on the coast and their time series are considered longer and more reliable than remote sensing measurements
According to the results in multiple locations, the two datasets in the Baltic Sea correspond better than those of inland cases; the mean (MEAN) and standard deviation (STD) were 0.11 ± 0.08 m and 0.14 ± 0.05 m in the Gulf of Finland and the Gulf of Riga, respectively
In Lake Peipus and Lake Võrtsjärv, the MEAN was 0.16 ± 0.13 m and 0.31 ± 0.39 m, respectively. These Global Navigation Satellite System (GNSS) validation results agree with the validation results using the geoid-based water surface height, showing a discrepancy of 0.15 ± 0.14 m and 0.28 ± 0.30 m for the Lake Peipus on 19 June 2019 and for Lake Võrtsjärv on 13 July 2019, respectively (Figure 9)

Summary

Introduction

Water level monitoring stations or tide gauges (TG) have been widely used to observe sea-level variations on the coast and their time series are considered longer and more reliable than remote sensing measurements. Aforenamed problems are the key limitations for their use. To avoid these influences and to transform sea surface heights (SSH) into a common height system, it is recommended to use the tide gauges connected to the Global Navigation Satellite System (GNSS) stations or satellite altimetry (SA) to calculate the absolute SSH above a reference ellipsoid [1,2]. Satellite altimetry has been used in sea-level variation studies for more than 30 years. It is an important tool for understanding the topography of mesoscale eddies and the multidecadal trend of eustatic (global) sea-level rise

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Conclusion