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