Geoscience Reference
In-Depth Information
Time period (hours)
1000
100
10
1
0.1
0.01
0.001
Synoptic scale
Spectral gap
Turbulent scales
Weather
systems
Microscale eddies
1 day
1 minute
Figure 15.3
A typical
frequency spectrum for the
variability in atmospheric
variables measured above the
ground in the turbulent field
in the atmospheric boundary
layer.
0.001
1000
0.01
0.1
1
10
100
Eddy frequency (cycles per hour)
ured value around a gradually changing background. These are symptomatic of a
field of turbulence above the ground that involves parcels of air of variable size and
longevity moving horizontally and vertically in an apparently haphazard way in
the atmospheric boundary layer. Such turbulence is generated partly by friction as
the moving air stream moves across the rough surface and, in daytime conditions,
partly also by buoyancy.
Time series of measurements such as those shown in Fig. 15.1 can be analysed
using Fourier analysis to define the frequency spectrum of component
contributions to the variations in weather variables. The results of such an analysis
were described in Chapter 1 for a much wider range of frequencies, see Fig. 1.3.
Here we consider the more restricted range of frequencies from fractions of a
second to about 100 days shown in Fig. 15.3. In this figure three distinct peaks are
visible, as follows:
a.
at around 100 hours there is variability that is associated with weather
systems which typically influence local conditions for a few days;
b.
at around 24 hours there is variability that is associated with difference in
conditions between day and night; and finally,
c.
there is variability across the range of frequencies less than about 10 to
15 minutes.
A feature that is apparent in Fig. 15.3, which is crucially important from the
standpoint of describing turbulence in the atmosphere, is the distinct lack of
variability for frequencies with periodicity between about 30 and 90 minutes.
