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1.5 m. Moreover, two adjacent keels have to
satisfy the Rayleigh criterion: the minimum points must be separated by a point
whose depth is less than half of the depth of the keel in order to be resolved as
separate entity (Hibler
1975
; Wadhams and Horne
1980
; Wadhams and Davy
1986
). An example of a
finally choose a cut-off depth of
−
le is shown in Fig.
2
. The depth of the keels that
are detected and shown in Fig.
2
are then multiplied by a factor of 2 in order to
account for the systematic underestimation of the maximum draft by a factor of 2
due to the EM technique (Haas
2004
; Martin
2007
).
final pro
3 Calculation of Oceanic Drag Coef
cients
In order to calculate the oceanic drag coef
cients we make use of a parameterization
presented in Lu et al. (
2011
). This parameterization is based on a partitioning
concept that was already introduced for atmospheric drag coef
cients in Arya
(
1973
,
1975
). The parameterization in Lu et al. (
2011
) distinguishes between the
in
uence of small scale roughness (skin drag) and larger obstacles such as the keels
associated with ridges and the edges of the ice
fl
floes (form drag). Since we focus our
analysis on areas with 100 % sea-ice cover, the contribution of
fl
fl
oe edges can be
neglected.
The oceanic drag coef
cient c
w
is then the sum of the skin drag c
s
w
and the form
drag due to ridges c
r
w
:
c
w
¼
c
s
w
þ
c
r
w
:
ð
1
Þ
The drag contributions, in the case of sea-ice concentration A equal to 1, are
calculated as following:
m
H
r
D
r
c
s
w
¼
C
s
1
;
ð
2
Þ
"
#
2
1
=
2
C
r
H
r
p
D
r
H
r
D
r
c
r
w
¼
1
:
ð
3
Þ
H
r
and D
r
are the mean depth of the keels and the mean separation between them
respectively. The remaining constants are: m
¼
10
3
.
1, C
r
¼
0
:
5 and C
s
¼
2
cients we need to compute the mean depth
of the keels H
r
and the mean separation D
r
between them as obtained from the
available data. We focus on the Central Arctic region (see Fig.
1
). There, we have
320 pro
For the calculation of the drag coef
le varies between ca. 10 km and 30 km for a
total of more than 700 km of data. For each pro
les. The length of each pro
le we detect the keels as described
in Sect.
2
and we calculate the mean depth and the mean spacing between them.
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