Geography Reference
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Fig. 10.20
Time exposures showing the motion of surface tracer particles in a rotating annulus. The
four photographs illustrate various stages of a five-wave-tilted trough vacillation cycle.
The period of the vacillation cycle is 16.25 revolutions, and the photographs are at intervals
of four revolutions. (Photographs by Dave Fultz.)
is an example of internally generated low-frequency variability, which may be
analogous to some atmospheric climate variability.
In order to determine whether the annulus experiments are really valid ana-
logues of the atmospheric circulation or merely bear an accidental resemblance to
atmospheric flow, it is necessary to analyze the experiments quantitatively. Math-
ematical analysis of the experiments can proceed from essentially the same equa-
tions that are applied to the atmosphere, except that cylindrical geometry replaces
spherical geometry and temperature replaces potential temperature in the heat
equation. (Because water is nearly incompressible, adiabatic temperature changes
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