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ABSTRACT
In this paper , we present a novel Kalman filter approach to combine a hydrodynamic model-derived lowest astronomical tide (LAT) surface with tide gauge record-derived LAT values. In the approach, tidal water levels are assimilated into the model. As such, the combination is guided by the model physics. When validating the obtained “Kalman-filtered LAT realization” at all tide gauges, we obtained an overall root-mean-square (RMS) difference of 15.1 cm. At the tide gauges not used in the data assimilation, the RMS is 17.9 cm. We found that the assimilation reduces the overall RMS difference by ∼ 31% and ∼ 22%, respectively. In the Dutch North Sea and Wadden Sea, the RMS differences are 6.6 and 14.8 cm (all tide gauges), respectively. Furthermore, we address the problem of LAT realization in intertidal waters where LAT is not defined. We propose to replace LAT by pseudo-LAT, which we suggest to realize similarly as LAT except that all water level boundary conditions and assimilated tidal water levels have to be enlarged by a constant value that is removed afterward. Using this approach, we obtained a smooth reference surface for the Dutch Wadden Sea that fits LAT at the North Sea boundary within a few centimeters.
Acknowledgments
This study was performed in the framework of the Netherlands Vertical Reference Frame (NEVREF) project, funded by the Netherlands Technology Foundation STW. This support is gratefully acknowledged. In addition the authors gratefully acknowledge the support of the UK Met Office for providing the results of the North West European Shelf Reanalysis. Tide gauge data were kindly provided by the Vlaamse Hydrografie. Agentschap voor Maritieme Dienstverlening en Kust, Afdeling Kust, Belgium; Danish Coastal Authority; Danish Meteorological Institute; Danish Maritime Safety Administration; Service Hydrographique et Océanographique de la Marine, France; Bundesamt für Seeschifffahrt und Hydrographie, Germany; Marine Institute, Ireland; Rijkswaterstaat, Netherlands; Norwegian Hydrographic Service; Swedish Meteorological and Hydrological Institute; and U.K. National Tidal and Sea Level Facility (NTSLF) hosted by POL. We also acknowledge the valuable comments of two anonymous reviewers.