Geoscience Reference
In-Depth Information
50
40
30
20
10
Figure 1.3
Approximate
spectrum of the contributions
to the variance in the
atmosphere for frequencies
between 1 second and 1000
days.
1 hour
1 min.
1 sec.
1 year
1 month
1 day
0.01
0.1
1
10
100
1000
10 000
Frequency (day
−
1
)
perturbations generated by changes in the inputs that drive the atmosphere
typically decay with time scales on the order of days to weeks.
Differential heating by the Sun causes movement in the atmosphere that is
complicated by the rotation of the Earth, the Earth's orbit around the Sun, and
inhomogeneous surface conditions. Consequently, the air in the troposphere
undergoes large-scale circulation which, on average, is organized at the global
scale. There are substantial perturbations within this circulation associated with
weather systems at mid-latitudes, and also pseudo-random turbulent motion in
the atmospheric boundary layer and near 'jet streams' higher in the atmosphere.
Figure 1.3 shows how contributions to the variance of atmospheric movements in
the atmospheric boundary layer change as a function of frequency. Most movement
occurs at low frequencies. The first peak in this figure is associated with movement
linked to the annual cycle and is in response to seasonal changes in solar heating,
while the third peak is linked to the daily cycle of heating. The large contribution
to variance at the time scales of days to weeks is the result of the large-scale
disturbances associated with transient weather systems. At lower frequencies
atmospheric variance is therefore mainly associated with horizontal features
within the atmospheric circulation. The fourth maximum in variance, which
occurs at timescales of an hour or less, is different because it is due to small-scale
turbulent motions. Such turbulent variations occur in all directions, but their
influence on the vertical movement of atmospheric properties and constituents is
particularly important. Understanding this influence on the vertical movement is
a critical aspect of hydrometeorology.
