Geoscience Reference
In-Depth Information
1.5
1.0
0.5
0.0
-0.5
-1.0
-1.5
2.0
1.5
1.0
0.5
0.0
-0.5
-1.0
-1.5
0
10
20
30
40
50
60
Time (seconds)
Figure 3.6
Time series of luctuations of vertical wind speed (top) and temperature
(bottom). A sub-sample of 1 minute from
Figure 3.4
. Each dot represents one obser-
vation. Sampling rate is 20 Hz. The ramp structure visible in the lower panel is an
expression of the gradual heating of the air by the surface heat lux, followed by an
abrupt removal of this warmed air by a sweep that brings down colder air from aloft.
This pattern is exploited in the surface renewal method to estimate surface lux (Paw
U et al.,
1995
).
10 seconds, from 20 to 30 seconds and around 40 seconds). During those periods of
upward motion, the air is also relatively warm (positive
θ
luctuation). Thus,
warm
air
is transported
upward.
There are also periods with negative vertical motion (around 5,
35 and 55 seconds). Those are roughly accompanied by negative temperature luctua-
tions. Thus
cool
air is trans
porte
d
downward.
The net effect of those motions is that
heat is transported upward (
w
′
θ
>0
): a positive sensible heat lux. But the picture is
not ideal: there are periods in
Figure 3.6
where the
w
and
θ
signals are not well corre-
lated. The degree to which the signals are correlated can be expressed by the correla-
tion coeficient
R
wθ
. For the general combination of two signals
X
and
Y
:
′′
σσ
XY
R
≡
(3.4)
XY
XY
If the time series of
w
and
θ
, as shown in
Figure 3.4
, would have been perfectly
correlated,
R
wθ
would be equal to 1 (or -1 for perfect anti-correlation). However,
the actual correlation coeficient for this time series is only 0.54. This value for the
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