Journal cover Journal topic
Ocean Science An interactive open-access journal of the European Geosciences Union
Ocean Sci., 13, 799-827, 2017
https://doi.org/10.5194/os-13-799-2017
© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License.
Research article
26 Sep 2017
Surface drifters in the German Bight: model validation considering windage and Stokes drift
Ulrich Callies1, Nikolaus Groll1, Jochen Horstmann1, Hartmut Kapitza1, Holger Klein2, Silvia Maßmann2, and Fabian Schwichtenberg2 1Institute of Coastal Research, Helmholtz-Zentrum Geesthacht, Max-Planck-Str. 1, 21502 Geesthacht, Germany
2Federal Maritime and Hydrographic Agency (BSH), Bernhard-Nocht-Str. 78, 20359 Hamburg, Germany
Abstract. Six surface drifters (drogued at about 1 m depth) deployed in the inner German Bight (North Sea) were tracked for between 9 and 54 days. Corresponding simulations were conducted offline based on surface currents from two independent models (BSHcmod and TRIM). Inclusion of a direct wind drag (0.6 % of 10 m wind) was needed for successful simulations based on BSHcmod currents archived for a 5 m depth surface layer. Adding 50 % of surface Stokes drift simulated with a third-generation wave model (WAM) was tested as an alternative approach. Results resembled each other during most of the time. Successful simulations based on TRIM surface currents (1 m depth) suggest that both approaches were mainly needed to compensate insufficient vertical resolution of hydrodynamic currents.

The study suggests that the main sources of simulation errors were inaccurate Eulerian currents and lacking representation of sub-grid-scale processes. Substantial model errors often occurred under low wind conditions. A lower limit of predictability (about 3–5 km day−1) was estimated from two drifters that were initially spaced 20 km apart but converged quickly and diverged again after having stayed at a distance of 2 km or less for about 10 days. In most cases, errors in simulated 25 h drifter displacements were of similar order of magnitude.


Citation: Callies, U., Groll, N., Horstmann, J., Kapitza, H., Klein, H., Maßmann, S., and Schwichtenberg, F.: Surface drifters in the German Bight: model validation considering windage and Stokes drift, Ocean Sci., 13, 799-827, https://doi.org/10.5194/os-13-799-2017, 2017.
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Short summary
Six surface drifters were tracked in the inner German Bight for between 9 and 54 days. Corresponding simulations were conducted based on currents from two hydrodynamic models. Effects of including either a direct wind drag or simulated Stokes drift were similar during most of the time. Results suggest that main sources of simulation errors were inaccurate Eulerian currents and lacking representation of sub-grid-scale processes. Substantial model errors often occurred under low wind conditions.
Six surface drifters were tracked in the inner German Bight for between 9 and 54 days....
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