818 
U. Callies et al.: Surface drifters in the inner German Bight 
Ocean Sci., 13, 799-827, 2017 
www.ocean-sci.net/13/799/2017/ 
Magnitudes of drift velocities (hourly values) 
Observations: coloured BSHcmod+W: Windage in BSHcmod+W: Stokes drift in BSHcmod+S: 
Figure 12. Magnitudes of drift velocities on an hourly basis, considering drifter nos. 5 (a) and 6 (b). As in Fig. 6, magnitudes of observed 
velocity vectors (coloured) are compared with simulations based on BSHcmod + W. In addition, magnitudes of windage (in BSHcmod + W) 
and Stokes drift (in BSHcmod + S) are shown. All model values are specified from either atmospheric or marine fields interpolated to 
observed (not simulated) drifter locations. For full time series, see the Supplement (SM5). 
had no drogue presence sensor and could also not be col 
lected at the end of their journey to check the conditions of 
the devices. 
5 Conclusions 
Trajectories of six surface drifters deployed in the German 
Bight were compared with corresponding offline simula 
tions based on hydrodynamic data from two independent 
models. Successful simulations based on BSHcmod currents 
archived for a 5 m depth surface layer needed inclusion of ex 
tra wind (or wave) effects, which was not the case for simula 
tions based on TRIM currents for aim depth surface layer. 
This suggests the assumption that the extensions in BSHc 
mod+W or BSHcmod+S primarily acted to compensate 
insufficient vertical resolution in archived data. There was no 
convincing evidence that the drifters deployed experienced 
an appreciable direct wind drag. In a similar way, Ullman 
et al. (2006) attributed a bias of trajectories predicted based 
on HF radar currents not to a drifter leeway but rather to the 
fact that effective depth of HF radar measurements exceeded 
that of surface layer drifters. 
On the other hand, it is striking that often errors in simula 
tions based on TRIM and BSHcmod + W (or BSHcmod + S) 
closely resembled each other (e.g. day 8 - see Fig. 7d and h; 
or day 18 - see Fig. 8d and h). This points to problems shared 
by both models, explanation of which probably requires anal 
yses considering also other aspects of hydrodynamic model 
output. 
The present study focused on a synoptic assessment of 
(mainly four) drifter trajectories overlapping in time. Expect 
edly, differences between synchronous drift trajectories were 
much larger in observations than in simulations, due to un 
resolved sub-grid-scale processes. Simulated fields of wind 
(not including sub-grid-scale weather phenomena and gusti 
ness as important drivers for drifter dispersion) and Stokes 
drift are even more smooth than simulated current fields. 
Small-scale model data misfits can therefore obviously not 
be remedied by employing windage or Stokes drift. 
Although the small number of drifters does not enable an 
in depth analysis, it seems that major deficiencies of sim 
ulations often manifest themselves under low or moderate 
wind speeds. For instance, data from days 7 to 9 (see panels 
in Fig. 7) suggest that simulations underestimate currents in 
coastal areas at that time. Insufficient resolution of intertidal 
areas could be one aspect contributing to this model defi 
ciency. Also, on days 15 and 16, observed drifters moving 
much faster than simulated (Fig. 6) coincides with low wind 
conditions (e.g. Fig. 8c and g). However, all instances also 
correspond with changes in wind conditions and transitions 
between different residual current regimes (Fig. 3). 
On an hourly basis, contributions from windage in BSHc 
mod + W are often much smaller than discrepancies between 
simulated and observed drifter velocities (Fig. 12 or SM5), in 
particular under low wind conditions. When averaging over