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3
3
corr = 0.89
2
2
corr = 0.78
1
1
0
0
0
5
10
15
20
25
0
0.5
1
U
10
, m/s
b
T
Figure 9.3 Drag coefficient
C
D
versus (left) wind speed
U
10
and (right) breaking probability of
dominant waves
b
T
(2.3)
- for wave fields with
b
T
≥
0
.
02
The right panel, however, identifies a potential problem. In this subplot, the ratio of
u
∗
sonic
to
u
∗
profile
is plotted versus
U
10
.Atthelow
U
10
wind speeds, the ratio can be as large
as 3. This considerable scatter of the relative values of
u
∗
shows that the physics of the
constant-flux layer could have been affected by background processes in the atmosphere
and at
U
10
s the concept of the constant flux may not be valid over the ocean
as mentioned in
Komen
et al.
(
1994
). This can be due to the non-stationarity and non-
homogeneity of the mean wind, which is very likely for fields of light winds (see, for
example, Figure 8 in
Babanin & Makin
,
2008
), or perhaps the height of the constant-
flux layer can be less than 10m. In any case, the light-wind records were excluded from
the analysis below, as well as cases with very low breaking rates when plotting
C
D
as a
function of breaking occurrence in
Figure 9.3
.
One of the findings of
Babanin & Makin
(
2008
), based on the Lake George data, were
the limiting dependences of sea drag. These dependences envelope the Lake George data
from below:
<
4m
/
10
−
7
U
10
+
C
D
=
1
.
92
·
0
.
00096
(9.3)
and
10
−
7
U
10
c
p
4
C
D
=
9
.
33
·
+
0
.
00096
,
(9.4)
whereas in open-sea observations sea-drag values below these limiters are a regular occur-
rence. Dependences
(9.3)
and
(9.4)
provide some Lake George “ideal” relationships for
C
D
. At this stage, it is not obvious what physical properties constitute the ideal conditions.
But almost any deviation from such conditions, at a given wind speed
U
10
, causes the drag
at Lake George to increase.
Babanin & Makin
(
2008
) suggested that decrease of the drag
with respect to the ideal conditions, which exhibits itself in a number of known open-ocean
data sets, would be caused by a momentum flux back from the waves to the wind due to
long waves outrunning the wind. In Lake George, swell and such waves do not exist.
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