Geography Reference
In-Depth Information
briefly summarize the observed structure of midlatitude synoptic systems and
the mean circulations in which they are embedded. We then develop the quasi-
geostrophic momentum and thermodynamic energy equations and show how these
can be manipulated to form the
quasi-geostrophic potential vorticity equation
and
the
omega equation
. The former equation provides a method for predicting the
evolution of the geopotential field, given its initial three-dimensional distribution;
the latter provides a method for diagnosing the vertical motion from the known
distribution of geopotential. In both cases, alternative versions of the equations are
discussed to help elucidate the dynamical processes responsible for the develop-
ment and evolution of synoptic-scale systems.
6.1
THE OBSERVED STRUCTURE OF EXTRATROPICAL
CIRCULATIONS
Atmospheric circulation systems depicted on a synoptic chart rarely resemble the
simple circular vortices discussed in Chapter 3. Rather, they are generally highly
asymmetric in form, with the strongest winds and largest temperature gradients
concentrated along narrow bands called
fronts
. Also, such systems generally are
highly baroclinic; the amplitudes and phases of the geopotential and velocity per-
turbations both change substantially with height. Part of this complexity is due to
the fact that these synoptic systems are not superposed on a uniform mean flow,
but are embedded in a slowly varying planetary scale flow that is itself highly
baroclinic. Furthermore, this planetary scale flow is influenced by
orography
(i.e.,
by large-scale terrain variations) and continent-ocean heating contrasts so that it
is highly longitude dependent. Therefore, it is not accurate to view synoptic sys-
tems as disturbances superposed on a zonal flow that varies only with latitude and
height. As shown in Chapter 8, however, such a point of view can be useful as a
first approximation in theoretical analyses of synoptic-scale wave disturbances.
Zonally averaged cross sections do provide some useful information on the
gross structure of the planetary scale circulation in which synoptic-scale eddies
are embedded. Figure 6.1 shows meridional cross sections of the longitudinally
averaged zonal wind and temperature for the solstice seasons of (a) December,
January and February (DJF) and (b) June, July and August (JJA). These sections
extend from approximately sea level (1000 hPa) to about 32 km altitude (10 hPa).
Thus the troposphere and lower stratosphere are shown. This chapter is concerned
with the structure of the wind and temperature fields in the troposphere. The strato-
sphere is discussed in Chapter 12.
The average pole to equator temperature gradient in the Northern Hemisphere
troposphere is much larger in winter than in summer. In the Southern Hemisphere
the difference between summer and winter temperature distributions is smaller,
due mainly to the large thermal inertia of the oceans, together with the greater
fraction of the surface that is covered by oceans in the Southern Hemisphere. Since
the mean zonal wind and temperature fields satisfy the thermal wind relationship
