800 
U. Callies et al.: Surface drifters in the inner German Bight 
Ocean Sci., 13, 799-827, 2017 
www.ocean-sci.net/13/799/2017/ 
jor source of uncertainty (Hufnagl et al., 2017). The circula 
tion model BSHcmod, which this study mainly focuses on, 
is run operationally. In cases of necessity, drifter simulations 
will be based on a regridded archived version of model pre 
dictions with near-surface currents representative of a 5 m 
deep top layer. Therefore, even for an ideal surface drifter, 
introducing a direct wind drag can be helpful as a means 
of compensating insufficient vertical resolution of hydrody 
namic currents. The second hydrodynamic model employed 
in this study, TRIM, was set up with aim deep top layer. 
Comparing drift simulations based on outputs from the two 
different models helps assess uncertainties possibly related 
to the vertical resolution of near-surface currents. 
More complex impacts of winds on surface currents may 
be mediated via waves (Perrie et ah, 2003; Ardhuin et ah, 
2009). Rôhrs et ah (2012) found evidence that predictabil 
ity of drift trajectories can be improved by the inclusion of 
numerical wave modelling. On the other hand, Stokes drift 
and other wave effects are often neglected in operational sys 
tems. According to Breivik and Allen (2008), the main rea 
son for this is that wave processes are already taken into ac 
count by empirically tuned windage coefficients that summa 
rize changes of an object’s trajectory induced by combined 
impacts of both winds and waves. The situation can differ 
in near-shore regions, where wave refraction directs wave- 
induced transports towards the coast (Sobey and Barker, 
1997). 
A key objective of this study is checking whether explicit 
inclusion of Stokes drift calculated with a state-of-the-art 
wave model (WAM) improves drift simulations. Assessing 
the necessity to distinguish between effects of direct wind 
drag and Stokes drift is essential to avoid overparametriza- 
tion. Waves and resulting Stokes drift were calculated using 
the wind forcing also employed for hydrodynamic simula 
tions with TRIM. However, we did not explore effects of in 
cluding wave-current interactions into hydrodynamic simu 
lations (Staneva et ah, 2017). 
Horizontal grid resolutions of the two hydrodynamic 
data sets (900 m in BSHcmod and 1.6 km in TRIM) allow 
for a proper representation of mesoscale eddies in the re 
gion of interest. However, simulations may miss relevant 
sub-mesoscale processes. According to Kjellsson and Dôôs 
(2012) the underestimation of eddy kinetic energy by Eule- 
rian flows is a common finding of many model validation 
studies. This deficiency could be fixed by a transition to 
an advection-diffusion equation, introducing an additional 
stochastic random walk term. In this context, specification 
of the proper eddy diffusivity as function of grid resolution 
poses a major problem. There are, however, also concerns 
regarding the simple theoretical concept. For the advection- 
diffusion approach to be valid, a spectral gap should sepa 
rate processes on the scale resolved from sub-grid-scale pro 
cesses. Such a gap may often not exist (see, De Dominicis 
et al., 2012, for instance). 
Garraffo et al. (2001) compared the statistics of drifter ob 
servations in the North Atlantic with those of drift simula 
tions based on Eulerian velocities from a model with about 
6 km horizontal resolution. Without a stochastic model of 
sub-grid-scale actions, they found simulations to underesti 
mate eddy energy. Simulated absolute dispersion being too 
low was also reported by Kjellsson and Doos (2012) evalu 
ating drifters deployed in the Baltic Sea. Referring to global 
ocean data, Doos et al. (2011) tuned random turbulent veloc 
ity in their drift model to achieve better agreement between 
relative dispersion of simulated trajectories and correspond 
ing observations. However, they found this approach was too 
simple for a reasonable reproduction of Lagrangian proper 
ties. 
More sophisticated analyses of the relative dispersion of 
pairs of particles try to distinguish the regimes of “local dis 
persion” driven by eddies comparable in size to the distance 
between two drifters and of “non-local dispersion” driven by 
eddies with scales much larger than this distance (e.g. Kosza- 
lka et al., 2009). Beron-Vera and LaCasce (2016) conducted 
such an analysis for data from the Grand Lagrangian Deploy 
ment experiment (GLAD), in which more than 300 drifters 
were deployed in the Gulf of Mexico. Drifter launch posi 
tions spaced from 100 m to 15 km apart allowed to study sub- 
mesoscale dispersion characteristics in great detail. However, 
referring to experimental data in the south-western Gulf of 
Mexico, Sanson et al. (2017) show that for large initial dis 
tances the probability density functions of pair separations 
get dependent on prevailing mesoscale circulation patterns. 
This aspect seems particularly relevant for the present study. 
Variations of the residual current regime in the inner German 
Bight can very well be approximated in terms of only 2-3 
degrees of freedom, depending on prevailing winds (Callies 
et al., 2017). Tidal currents dominate short-term transports. 
The data available for this study (six drifters, tracked be 
tween 9 and 54 days) are insufficient for studying features of 
oceanic turbulence. Therefore, in the present model valida 
tion study, stochastic simulation of sub-grid-scale processes 
will not be considered. Ohlmann et al. (2012) provide an ex 
ample that even an accurate reproduction of mean drifter pair 
separation does not necessarily imply good agreement be 
tween observations and corresponding simulations. Accord 
ing to Coelho et al. (2015), models used in the aforemen 
tioned GLAD experiment in the Gulf of Mexico had limited 
success capturing the observed drift patterns. Barron et al. 
(2007) provide a list of typical separation rates in different 
regions worldwide. For an experiment in the Ria de Vigo es 
tuary in north-west Spain, Huhn et al. (2012) reported sim 
ulation errors that were relatively small compared to those 
typically found in the open ocean. This study tries to pro 
vide a realistic estimate of how reliable operational forecasts 
in the German Bight, another shelf sea region, can be ex 
pected to be. This includes gaining preliminary indications 
for regions where the deterministic part of a model needs im 
provement.