Geoscience Reference
In-Depth Information
1
3.5
3
0.8
2.5
0.6
2
1.5
0.4
1
0.2
0.5
0
0
0
0.2
0.4
0.6
0.8
1
0
5
10
15
u
*
, m/s, profile measurements
U
10
, m/s
Figure 9.2 Comparison of friction velocity
u
∗
obtained by means of stress measurements (sonic
anemometer) and measurements of the mean wind profile in the boundary layer (vertical array of cup
anemometers). (left panel)
u
∗
sonic
versus
u
∗
profile
. (right panel) Ratio of
u
∗
sonic
to
u
∗
profile
versus
U
10
.
Figure is reproduced from
Babanin & Makin
(
2008
) by permission of American Geophysical Union
effects were small and the Monin-Obukhov-scale function
can be neglected in
(3.19)
.
In any scenario, the stratification is expected to be insignificant as air-water temperature
differences in the 1m-deep Lake George were usually small, within a few degrees, with
water temperature of the shallow lake fast-tracking any atmospheric changes.
In the bottom panel, one-minute-average wind speeds at the 10m height for the four
records are shown which exhibit both trends of the mean and the gustiness. Gustiness is
the largest for the lightest wind and smallest for the strongest winds, as is generally the
case (see
Babanin & Makin
,
2008
, for details).
The selection of records in
Figure 9.1
, therefore, illustrates a variety of small-scale wind
unsteadiness. This does not appear to cause deviations from the logarithmic-boundary-
layer wind profile
(3.19)
, but has the potential to affect sea drag in a serious way as will be
outlined below.
The friction velocity
u
ψ
obtained from the wind profiles
(3.19)
can also be measured
using the sonic anemometer:
∗
τ
ρ
a
= −
U
w
u
∗
=
(9.2)
where
U
and
w
are oscillations of the horizontal (i.e. length of vector sum of the down-
wind and cross-wind components) and vertical velocities, correspondingly. Thus, the con-
stancy of
and adequacy of the wind profile measurements, can be verified.
In
Figure 9.2
,
u
τ
measurements by the sonic anemometer (
u
sonic
) and by the anemome-
∗
∗
ter mast (
u
profile
) are compared (for the data, see Table 2 in
Babanin & Makin
(
2008
)). As
seen in the left panel of
u
∗
sonic
versus
u
∗
profile
, the scatter is significant, but overall matching
in terms of absolute values of
u
∗
is quite good, with a correlation coefficient of 95% and a
sampling standard deviation of 0
∗
.
06m
/
s.
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