Geography Reference
In-Depth Information
In fact, observations show that the transient planetary scale flow amplitude is
comparable to that of the time-mean. As a result, monthly mean charts tend to
smooth out the actual structure of the instantaneous jetstream since the position
and intensity of the jet vary. Thus, at any time the planetary scale flow in the region
of the jetstream has much greater baroclinicity than indicated on time-averaged
charts. This point is illustrated schematically in Fig. 6.4, which shows a latitude-
height cross section through an observed jetstream over North America. Fig. 6.4a
shows the zonal wind and potential temperature, whereas Fig. 6.4b shows the
potential temperature and Ertel potential vorticity. The 2 PVU contour of potential
vorticity approximately marks the tropopause.
As illustrated in Fig. 6.4a, the axis of the jetstream tends to be located above
a narrow sloping zone of strong potential temperature gradients called the
polar-
frontal
zone. This is the zone that in general separates the cold air of polar origin
from warm tropical air. The occurrence of an intense jet core above this zone of large
magnitude potential temperature gradients is, of course, not mere coincidence, but
rather a consequence of the thermal wind balance.
The potential temperature contours in Fig. 6.4 illustrate the strong static stability
in the stratosphere. They also illustrate the fact that isentropes (constant θ surfaces)
cross the tropopause in the vicinity of the jet so that air can move between the
troposphere and the stratosphere without diabatic heating or cooling. The strong
gradient of Ertel potential vorticity at the tropopause, however, provides a strong
resistance to cross-tropopause flow along the isentropes. Note, however, that in the
frontal region the potential vorticity surfaces are displaced substantially downward
so that the frontal zone is characterized by a strong positive anomaly in potential
vorticity associated with the strong relative vorticity on the poleward side of the
jet and the strong static stability on the cold air side of the frontal zone.
It is a common observation in fluid dynamics that jets in which strong velocity
shears occur may be unstable with respect to small perturbations. By this is meant
that any small disturbance introduced into the jet will tend to amplify, drawing
energy from the jet as it grows. Most synoptic-scale systems in midlatitudes appear
to develop as the result of an instability of the jetstream flow. This instability, called
baroclinic instability
, depends on the meridional temperature gradient, particularly
at the surface. Hence, through the thermal wind relationship, baroclinic instability
depends on vertical shear and tends to occur in the region of the polar frontal zone.
Baroclinic instability is not, however, identical to frontal instability, as most
baroclinic instability models describe only geostrophically scaled motions,
whereas disturbances in the vicinity of strong frontal zones must be highly ageo-
strophic. As shown in Chapter 9, baroclinic disturbances may themselves act to
intensify preexisting temperature gradients and hence generate frontal zones.
The stages in the development of a typical baroclinic cyclone that develops
as a result of baroclinic instability are shown schematically in Fig. 6.5. In the
stage of rapid development there is a cooperative interaction between the upper
