OS Ocean Science OS Ocean Sci. 1812-0792 Copernicus Publications Göttingen, Germany 10.5194/os-6-161-2010 Ensemble perturbation smoother for optimizing tidal boundary conditions by assimilation of High-Frequency radar surface currents – application to the German Bight Barth A. 1 2 Alvera-Azcárate A. 1 2 Gurgel K.-W. 3 Staneva J. 4 Port A. 5 Beckers J.-M. 1 2 Stanev E. V. 4 GeoHydrodynamics and Environment Research (GHER), MARE, AGO, University of Liège, Liège, Belgium National Fund for Scientific Research, Belgium Institute of Oceanography, University of Hamburg, Germany Institute for Coastal Research, GKSS Research Center, Geesthacht, Germany Institute for Chemistry and Biology of the Marine Environment (ICBM), University of Oldenburg, Germany 04 02 2010 6 1 161 178 Copyright: © 2010 A. Barth et al. 2010 This work is licensed under the Creative Commons Attribution 3.0 Unported License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/3.0/ This article is available from https://os.copernicus.org/articles/6/161/2010/os-6-161-2010.html The full text article is available as a PDF file from https://os.copernicus.org/articles/6/161/2010/os-6-161-2010.pdf

High-Frequency (HF) radars measure the ocean surface currents at various spatial and temporal scales. These include tidal currents, wind-driven circulation, density-driven circulation and Stokes drift. Sequential assimilation methods updating the model state have been proven successful to correct the density-driven currents by assimilation of observations such as sea surface height, sea surface temperature and in-situ profiles. However, the situation is different for tides in coastal models since these are not generated within the domain, but are rather propagated inside the domain through the boundary conditions. For improving the modeled tidal variability it is therefore not sufficient to update the model state via data assimilation without updating the boundary conditions. The optimization of boundary conditions to match observations inside the domain is traditionally achieved through variational assimilation methods. In this work we present an ensemble smoother to improve the tidal boundary values so that the model represents more closely the observed currents. To create an ensemble of dynamically realistic boundary conditions, a cost function is formulated which is directly related to the probability of each boundary condition perturbation. This cost function ensures that the boundary condition perturbations are spatially smooth and that the structure of the perturbations satisfies approximately the harmonic linearized shallow water equations. Based on those perturbations an ensemble simulation is carried out using the full three-dimensional General Estuarine Ocean Model (GETM). Optimized boundary values are obtained by assimilating all observations using the covariances of the ensemble simulation.

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