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https://doi.org/10.5194/os-2017-40
© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License.
Research article
24 May 2017
Review status
This discussion paper is under review for the journal Ocean Science (OS).
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 1Helmholtz-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 14 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 the 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 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 per day) 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 and 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. Discuss., https://doi.org/10.5194/os-2017-40, in review, 2017.
Ulrich Callies et al.
Ulrich Callies et al.

Data sets

German Bight surface drifter data from Heincke cruise HE 445, 2015
R. Carrasco and J. Horstmann
https://doi.org/10.1594/PANGAEA.874511
coastDat-2 Hydrodynamic Model TRIM-NP Principal Component Analysis Residual Currents. World Data Center for Climate
U. Callies
https://doi.org/10.1594/WDCC/TRIM-NP-2d-PCA_ResCurr
Ulrich Callies et al.

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Short summary
Six surface drifters were tracked in the inner German Bight for between 14 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 14 and 54 days....
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