802 
U. Callies et al.: Surface drifters in the inner German Bight 
Ocean Sci., 13, 799-827, 2017 
www.ocean-sci.net/13/799/2017/ 
Figure 1. Drifter types MD03i (panels a and b) and ODi (panel c) used during the experiment. Both drifter types were photographed shortly 
after launch so that the drogues had not yet settled. 
The following equation is used for simulating drifter loca 
tion x as function of time t : 
àx 
— = w E + aws+ytfwiom- (1) 
at 
Here, me denotes Eulerian marine surface currents calculated 
with either BSHcmod (Sect. 2.2.1) or TRIM (Sect. 2.2.2), u$ 
is the surface Stokes drift obtained from wave model WAM 
and Miom is the 10 m height wind vector. Coefficients a and fi 
are weighting factors (see Sect. 2.2.3). Equation (1) describes 
windage (or leeway) as a drag in downwind direction, ne 
glecting any cross-wind lift component. Such lift component 
depending on the specific overwater structure of a drifting 
object is crucial for search and rescue (Breivik and Allen, 
2008). For surface drifters used in experiments, however, 
these effects should be negligible. 
Throughout this study, Stokes drift and wind drag will not 
be considered in combination but rather as alternative op 
tions. Therefore, at least one of the two weighting factors (a 
or /3) in Eq. (1) will always be set to zero. Drift paths were 
calculated offline based on archived data. Sub-grid-scale tur 
bulence effects implemented in PELETS-2D in terms of ran 
dom movements were deactivated. 
2.2.1 BSHcmod 
BSHcmod is run operationally by the Federal Maritime and 
Hydrographic Agency (BSH) on a two-way nested grid for 
the North Sea and Baltic Sea. A description of the 3-D model 
can be found in Dick et al. (2001). Horizontal resolution in 
the German Bight is about 900 m; the vertical coordinate is 
dynamical (Dick et ah, 2008). Atmospheric forcing of BSHc 
mod is taken from the regional model COSMO-EU (Consor 
tium for Small-Scale Modelling; Schulz and Schattler, 2014). 
This operational atmospheric model of the German Meteoro 
logical Service (Deutscher Wetterdienst - DWD) has a spa 
tial resolution of 7 km; output is stored on a hourly basis. 
For BSHcmod, winds are interpolated to a 15 min model 
time step. The parametrization by Smith and Banke (1975) 
is used to include wind stress. The option to include Stokes 
drift from surface wave models (as described in Dick et ah, 
2001) is not activated operationally so that effects of Stokes 
drift are also not included in archived model output. 
Archived surface current data represent approximately the 
upper 5 m of the water column. Higher-resolution output of 
the operational model BSHcmod (version 4) was regridded 
accordingly, conserving transport rates. Time resolution of 
archived data is 15 min. Although operationally BSHcmod is 
run in combination with its own Lagrangian transport mod 
ule (Mal.imann et ah, 2014); for the present study, this mod 
ule was replaced by PELETS-2D, which provides convenient 
interfaces to both BSHcmod and TRIM. 
2.2.2 TRIM 
TRIM solves the hydrodynamic equations on a Cartesian 
grid, allowing for coastal regions that are falling dry. Casulli 
and Stelling (1998) provide a description of the numerical 
implementation; extensions with regard to parallelization and 
nesting can be found in Kapitza (2008). After three refine 
ments nested one way into a coarse grid with 12.8 km res 
olution covering the north-eastern Atlantic, North Sea and