Geoscience Reference
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the eastward advance of the cold upper trough act
to sharpen the baroclinic zone, strengthening the
upper jet stream through the thermal wind
mechanism (see p. 170). The vertical relationship
between jet stream and front has already been
shown (see
Figure 7.8
); a model depression
sequence is demonstrated in
Figure 9.23
. The
actual relationship may depart from this idealized
case, although the jet is commonly located in the
cold air. Velocity maxima (core zones) occur
along the jet stream and the distribution of vertical
motion upstream and downstream of these cores
is known to be quite different. In the area of the
jet entrance (i.e., upstream of the core), divergence
causes lower-level air to rise on the equatorward
(i.e., right) side of the jet, whereas in the exit
zone (downstream of the core) ascent is on the
poleward side.
Figure 9.24
shows how precipita-
tion is more often related to the position of the jet
stream than to that of surface fronts; maximum
precipitation areas are in the right entrance sector
of the jet core. This vertical motion pattern is also
of basic importance in the initial deepening stage
of the depression. If the upper-air pattern is
unfavorable (e.g., beneath left entrance and right
exit zones, where there is convergence) the
depression will fill.
The development of a depression may also be
considered in terms of energy transfers. A cyclone
requires the conversion of potential into kinetic
energy. The upward (and poleward) motion of
warm air achieves this. The vertical wind shear
and the superimposition of upper tropospheric
divergence drive the rising warm air over a baro-
clinic zone. Intensification of this zone further
strengthens the upper winds. The upper diver-
gence allows surface convergence and pressure
fall to occur simultaneously. Modern theory
relegates the fronts to a subordinate role. They
develop within depressions as narrow zones of
intensified ascent, probably through the effects
of cloud formation.
Recent research has identified a category of
mid-latitude cyclones that develop and intensify
100°
80°
100°
80°
L
H
H
40°
40°
40°
40°
L
H
1012
L
L
30°
30°
30°
30°
H
20 Sept 1958
21 Sept 1958
110°
100°
90°
110°
100°
90°
> 45m s
-1
> 25mm
≤ 25mm
Figure 9.24
The relations between surface fronts and isobars, surface precipitation (
25mm vertical hatching;
>25mm cross-hatching), and jet streams (wind speeds in excess of about 45m s
-1
shown by stipple) over the United
States on 20 September 1958 and 21 September 1958. This illustrates how the surface precipitation area is related
more to the position of the jets than to that of the surface fronts. The air over the south-central United States was
close to saturation, whereas that associated with the northern jet and the maritime front was much less moist.
Source: After Richter and Dahl (1958). Courtesy of the American Meteorological Society.
