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Some results are summarized in Fig. 10.21. The heavy solid line separates the
axially symmetric Hadley regime from the wavy Rossby regime. These results
can best be understood qualitatively by considering an experiment in which the
thermal Rossby number is increased slowly from zero by gradually imposing a
temperature difference T
b
−
T
a
the motion is of the Hadley
type, with weak horizontal and vertical temperature gradients in the fluid. As the
horizontal temperature contrast is increased, however, the mean thermal wind must
also increase, until at some critical value of Ro
T
the flow becomes baroclinically
unstable. According to the theory presented in Chapter 8, the wavelength of maxi-
mum instability is proportional to the ratio of the static stability to the square of the
rotation rate. Thus, as can be seen in Fig. 10.21 the wave number observed when
the flow becomes unstable decreases (i.e., wavelength increases) as the rotation
rate is reduced. Furthermore, because baroclinic waves transport heat vertically as
well as laterally, they will tend to increase the static stability of the fluid. Therefore,
as the thermal Rossby number is increased within the Rossby regime, the increased
heat transport by the waves will raise the static stability, and hence increase the
wavelength of the wave of maximum instability. The flow then undergoes transi-
tions in which the observed wave number decreases until finally the static stability
becomes so large that the flow is stable to even the largest wave that can fit the
tank. The flow then returns to a symmetric Hadley circulation stabilized by a high
static stability, which is maintained by a vigorous direct meridional circulation.
This regime is usually called the
upper
symmetric regime to distinguish it from
the
lower
symmetric regime, which occurs for very weak heating.
Laboratory studies, despite their many idealizations, can model the most impor-
tant of those features of the general circulation that are not dependent on the
topography of the earth or continent-ocean heating contrasts. Specifically, we
find, perhaps surprisingly, that the β effect (i.e., the planetary vorticity gradient)
is not essential for the development of circulations that look very much like tropo-
spheric synoptic systems. Thus, observed midlatitude waves should be regarded
essentially as baroclinic waves modified by the β effect, not as Rossby waves in a
baroclinic current.
In addition to demonstrating the primacy of baroclinic instability, the experi-
ments also confirm that internal diabatic heating due to condensation or radiative
processes is not an essential mechanism for simulation of large-scale midlati-
tude circulations. Laboratory experiments thus enable us to separate the essential
mechanisms from second-order effects in a manner not easily accomplished by
observation of the atmosphere itself. Also, because laboratory simulation exper-
iments have typical rotation rates of 10 rpm, it is possible to model many years
of “atmospheric” flow in a short time so that it is feasible to accumulate accurate
statistics even for very low-frequency variability. In addition, because temperatures
and velocities can be measured at uniform intervals, the experiments can provide
excellent sets of data for testing numerical weather prediction models.
T
a
. For small T
b
−
