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Volume 13, issue 5 | Copyright
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

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 Ulrich Callies et al.
  • 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 1m 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 10m wind) was needed for successful simulations based on BSHcmod currents archived for a 5m 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 (1m 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 3–5km day−1) was estimated from two drifters that were initially spaced 20km apart but converged quickly and diverged again after having stayed at a distance of 2km or less for about 10 days. In most cases, errors in simulated 25h drifter displacements were of similar order of magnitude.

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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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