CoAuthors

Salvatore Dinardo (CLS, France); Christopher Buchhaupt (University of Bonn, Germany); Bernd Uebbing (University of Bonn, Germany); Matthias Gärtner (University of Bonn, Germany); Joanna Staneva (Helmholtz Zentrum Geesthacht (HZG), Germany); Sebastian Grayek (Helmholtz Zentrum Geesthacht (HZG), Germany); Matthias Becker (TU Darmstadt, Germany); Anna Klos (Faculty of Civil Engineering and Geodesy, Military University of Technology, Poland); Jürgen Kusche (University of Bonn, Germany)

A significant part of the World population lives in the coastal zone, which is affected by sea level rise and extreme events. Consistent and precise new measurements are needed to assess and predict these changes. New altimeter missions equipped with Synthetic Aperture Radar (SAR) mode provide more accurate sea level heights. In this work we analyse their impacts on the estimation of the coastal sea level variability in the last 10 km.

First, by analysing various standard and improved SAR altimetric products we found that SAR altimetry reduces the minimum usable distance to coast from five to three kilometres. The best performance is achieved with the SAR coastal retrackers SAMOSA+ and SAMOSA++. A comparable performance is obtained in low resolution Reduced SAR (RDSAR) mode with the Spatio-Temporal Altimeter sub-waveform Retracker (STAR). In both cases, sea level heights are recovered with 4 cm accuracy up to 3 km from the coast at the Helgoland station, and the accuracy does not decrease with the distance to coast. However, in estuaries and coastal zones with hight tidal regimes, the discrepancy between altimetry and in-situ observations remains large (40 cm standard deviation difference (stdd) with SAR-SAM+). CryoSat-2 and Sentinel-3A have similar accuracy, despite their different repeat cycle and sampling which suggest alternative ways to build the sea level time series.

The monthly coastal variability from the SAR-SAM+ product agrees most favourably with the high resolution NEMO-WAM model, with stdd 3.9 cm and correlation 0.90 for Sentinel-3A. For comparison, the maximum stdd between two altimeter products (between SAR-SAM+ and RDSAR-TALES CryoSat-2) is half of that value (stdd 2.3 cm and correlation 0.96).

An average trend of about 3 +/-1.3 mm/yr has been detected using low resolution altimetry at ninetheen in-situ stations along the German coast over time intervals longer than 15 years. The difference of the trends of the sea level measured by altimetry and by in-situ data is denoted here al-tg and is interpreted as a measure of the Vertical Land Motion (VLM). The al-tg value agrees with the GNSS rate within 1.5 mm/yr at half of the co-located locations, the error of al-tg is larger than the error of the GPS rate by a factor bigger than two. The larger departure between the monthly tide gauge and LRM altimetric time-series compared to the smaller departure between the new constructed tide gauge and SAR altimetric time-series confirms the higher accuracy of SAR altimetry data. Finally, the sea level trends of the merged LRM and SAR altimetry time-series are consistent with the LRM trends over the complete altimetry period 1993-2019.