Ocean Sci., 13,199-ill, 2017 
https://doi.org/10.5194/os-13-799-2017 
© Author(s) 2017. This work is distributed under 
the Creative Commons Attribution 3.0 License. 
Ocean Science 
Surface drifters in the German Bight: model validation considering 
windage and Stokes drift 
Ulrich Callies 1 , Nikolaus Groll 1 , Jochen Horstmann 1 , Hartmut Kapitza 1 , Holger Klein 2 , Silvia MaBmann 2 , and 
Fabian Schwichtenberg 2 
institute of Coastal Research, Helmholtz-Zentrum Geesthacht, Max-Planck-Str. 1, 21502 Geesthacht, Germany 
2 Federal Maritime and Hydrographic Agency (BSH), Bernhard-Nocht-Str. 78, 20359 Hamburg, Germany 
Correspondence to: Ulrich Callies (uhich.callies@hzg.de) 
Received: 12 May 2017 - Discussion started: 24 May 2017 
Revised: 9 August 2017 - Accepted: 14 August 2017 - Published: 26 September 2017 
Abstract. Six surface drifters (drogued at about 1 m depth) 
deployed in the inner German Bight (North Sea) were tracked 
for between 9 and 54 days. Corresponding simulations were 
conducted offline based on surface currents from two inde 
pendent 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 sim 
ulated with a third-generation wave model (WAM) was tested 
as an alternative approach. Results resembled each other dur 
ing most of the time. Successful simulations based on TRIM 
surface currents (lm depth) suggest that both approaches 
were mainly needed to compensate insufficient vertical reso 
lution of hydrodynamic currents. 
The study suggests that the main sources of simulation er 
rors were inaccurate Eulerian currents and lacking represen 
tation of sub-grid-scale processes. Substantial model errors 
often occurred under low wind conditions. A lower limit of 
predictability (about 3-5 km day -1 ) 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 or less for about 10 days. In most cases, errors in 
simulated 25 h drifter displacements were of similar order of 
magnitude. 
1 Introduction 
Lagrangian particle tracking is a natural choice when origins 
or destinations of drifting objects (or water bodies) need to 
be known. Such methods have been developed for a wide 
range of applications (see Mariano et ah, 2002). Examples 
from oceanography are simulations of physical dispersion 
(Schônfeld, 1995; Sentchev and Korotenko, 2005), possibly 
augmented by specific source and sink terms (e.g. Puls et ah, 
1997). In ecosystem modelling, Lagrangian transport mod 
els have been employed to better understand the process of 
non-indigenous species invading an ecosystem (Brandt et ah, 
2008), the risk of toxic algae blooms (Havens et ah, 2010) 
or larval transport and connectivity being crucial to spatial 
fishery management (e.g. Nicolle et al., 2013; Robins et al., 
2013). Lagrangian transport simulations also provide a ba 
sis for more comprehensive individual-based models of fish 
recruitment (e.g. Daewel et ah, 2015). 
Obviously, the quality of Lagrangian drift simulations has 
a particularly high practical relevance in the context of emer 
gency operations like search and rescue (Breivik et ah, 2013) 
or organization of efficient combating of oil spills (Brostrôm 
et ah, 2011; MaBmann et ah, 2014). Modelling of surface 
drifter trajectories is particularly challenging as many of the 
input factors needed are poorly known. Often drift proper 
ties of search objects can only be estimated (Breivik et ah, 
2013). The present study refers to a drifter experiment con 
ducted in the inner German Bight (North Sea) during May- 
July 2015. Corresponding offline drift simulations based on 
archived currents from two different models were undertaken 
to assess the degree of uncertainty that must reasonably be 
expected in this region. 
The surface drifters deployed are ideal in the sense that 
their exposure to a direct aerodynamic force from wind (lee 
way or windage; Breivik and Allen, 2008) seems negligible. 
However, also Eulerian surface currents used can be a ma- 
Published by Copernicus Publications on behalf of the European Geosciences Union.