U. Callies et al.: Surface drifters in the inner German Bight
801
www.ocean-sci.net/13/799/2017/
Ocean Sci., 13, 799-827, 2017
Table 1. Drifters deployed in May 2015.
No.
Type
Start
End
Length
(km)
Dist
(km)
AT
(days)
Time (UTC)
° E
° N
Time (UTC)
° E
° N
1
MD03i
19 May (12:31)
7.5216
54.2160
2 Jun (21:12)
8.8338
54.5180
1032.1
91.7
14.4
2
MD03i
21 May (17:13)
7.1484
55.0752
25 May (09:47)
7.3080
55.1360
87.4
12.2
3.7
3
MD03i
21 May (17:13)
7.1480
55.0750
25 May (09:59)
7.2526
55.1160
85.7
8.1
3.7
4
MD03i
21 May (17:36)
7.1426
55.0786
24 May (15:00)
7.2960
55.0626
66.6
10.0
2.9
5
MD03i
27 May (09:49)
5.9126
54.3752
15 Jul (01:28)
8.4680
55.1232
1264.0
184.4
48.7
6
MD03i
27 May (16:01)
6.0446
54.2024
20 Jul (23:15)
8.0944
55.1930
1467.7
172.1
54.3
7
ODi
30 May (08:36)
6.7516
54.6712
8 Jun (09:59)
8.2360
55.7702
273.2
154.6
9.1
8
ODi
30 May (12:09)
6.7476
54.2554
9 Jul (19:15)
8.5282
55.2812
1203.0
161.8
40.3
9
ODi
31 May (07:46)
7.8816
54.0842
24 Jun (03:28)
8.8360
54.1316
844.3
62.6
23.8
Type: two drifter types used (see Fig. 1). Length: sum of the lengths of linear segments connecting observed drifter locations. Dist: linear distance between the first
and last drifter locations observed. AT: days between the first and the last observation. Drifter nos. 2, 3 and 4 travelling for only few days were ignored for this study.
The paper is organized as follows: Sect. 2 documents how
observations were taken (Sect. 2.1) and how corresponding
model simulations were performed (Sect. 2.2). Section 2.3
describes two data sets used for characterizing residual cur
rent variability in the German Bight on a daily basis. Re
sults (Sect. 3) are presented in two parts: Sect. 3.1 provides
a synoptic description of all drifters deployed and places
observations into the context of ambient atmospheric and
marine conditions; Sect. 3.2 provides the analysis of how
corresponding model simulations match observations. First,
full simulated trajectories are presented using currents from
TRIM or BSHcmod, the latter also combined with wind drag
and Stokes drift, respectively. A more detailed evaluation of
model performance is then based on subdividing drift trajec
tories into segments of 25 h length. Results are discussed in
Sect. 4; main conclusions are provided in Sect. 5.
2 Material and methods
2.1 Drifter observations
In May 2015, a total of nine drifters were deployed at dif
ferent locations in the German Bight (North Sea) during the
FS Heincke cruise HE 445. The raw data are freely acces
sible at Carrasco and Horstmann (2017). Table 1 specifies
each drifter’s launch position and launch time as well as its
last position, the total length of its trajectory and the simple
linear distance between its initial and final locations. Drifter
nos. 2, 3 and 4, travelling for only few days, were ignored
for this study. All drifters obtained their positions via the
Global Positioning System (GPS) and communicated them
to the lab via the satellite communication network Iridium.
Three drifters could successfully be tracked for between 40
and 54 days. In order to conserve battery power, an initial
sampling rate of about once every 15 min was later reduced
to once every 30 min.
Two different drifter types were utilized (see Table 1). The
first drifter type, MD03i, from Albatros Marine Technolo
gies (Fig. la) is cylinder shaped with a diameter of 0.1m
and a length of 0.32 m. Only ~ 0.08 m of the drifter protrude
from the water surface when deployed (Fig. lb). The sec
ond drifter type, ODi from the same manufacturer (Fig. lc),
has a spherical shape with 0.2 m diameter, with about half
of it protruding from the water surface. The ratio of drag
area in the water to drag area outside the water was 33.2
for the MD03i and 16.9 for the ODi model, respectively. To
both drifters, a drogue with 0.5 m length and diameter (e.g.
Fig. la) was attached 0.5m below the sea surface. Due to
this drogue and the small sail area exposed to winds above
the water surface, drifter movements are supposed to be rep
resentative of currents in a surface layer of about 1 m depth. It
must be noted, however, that drifters deployed had no drogue
presence sensors.
2.2 Drifter simulations with PELETS-2D
For drifter simulations, we used the Lagrangian transport
module PELETS-2D (Program for the Evaluation of La
grangian Ensemble Transport Simulations; Callies et al.,
2011) developed at Helmholtz-Zentrum Geesthacht (HZG).
The PELETS algorithm was designed for particle tracking on
two-dimensional unstructured triangular grids. As both mod
els underlying this study use regular grids, the grid topology
was preprocessed, splitting each rectangular grid cell into
two triangles. Neither the number of nodes nor the informa
tion content of underlying hydrodynamic fields is affected
by this formal procedure. The integration algorithm used is
a simple Euler forward method. Particle velocities are up
dated (linear interpolation between two neighbouring nodes)
each time a particle leaves a cell of the triangular grid. If no
edge is reached within the maximum time step of 15 min,
velocities are updated based on linear interpolation between
three nodes.