Geoscience Reference
In-Depth Information
This portion of the spectrum is called the
spectra gap
in variability and its
presence provides an opportunity to divide the representation of atmospheric
variability into two halves.
To the left of the spectral gap, variations are primarily associated with synoptic
scale features and with the more gradual change that occurs through the daily
cycle, such as the change in the mean value of wind speed between 12:00 and 14:00
already noticed in Fig. 15.1. To the right of the spectral gap, variations correspond
to the haphazard variability at higher frequency that are apparent in Fig. 15.1 and
are associated with turbulence. They reflect the movement of the parcels of air that
are continuously being created and destroyed in the turbulent field which, as they
move, provide the primary mechanism by means of which water vapor, heat,
momentum and chemical constituents are transported between the surface and
the atmosphere. Some important characteristics of turbulence to bear in mind in
what follows are:
it is irregular and appears random, or at least pseudo-random, and because
of this a deterministic description of turbulence at all spatial scales is not
feasible;
●
it is diffusive, and in most situations where turbulence occurs, the turbulent
transfer of energy, water vapor, momentum and atmospheric entities is much
more effective than transfer by molecular diffusion;
●
it is three-dimensional, and entities such as plumes and vortexes can play a
significant role; and finally,
●
it is continually being created and destroyed.
●
Mean and fluctuating components
Separating variations in atmospheric entities into low-frequency and high-
frequency variations is used as the basis for representing the turbulent atmos-
phere mathematically. Variations to the left of the spectral gap are described by
equations which are firmly based on physical principles such momentum, mass
and energy conservation, and which explicitly represent, or '
resolve
, changes in
the '
mean flow
' of atmospheric entities. Describing the haphazard turbulent vari-
ations that occur at higher frequencies that are to the right of the spectral gap in
Fig. 15.3 is less tractable. The most common approach is to represent their net
effect in the form of less well-grounded empirical equations, which are often
called '
parameterizations
. In later chapters it is shown that such parameterizations
are sometimes written in the form of empirical functions of the mean values of
atmospheric entities, these functions having been derived and calibrated by field
studies.
The first step toward writing a mathematical description of atmospheric changes
and movement is to re-write the value of each of the several atmospheric variables
in a form that explicitly recognizes that the variable has poorly described turbulent
