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Figure 9.22 Schematic represen-
tation of the relationship between
surface pressure (H and L), airflow and
frontal systems, on the one hand, and
the location of troughs and ridges in
the Rossby waves at the 300-mb level.
The locations of maximum (cyclonic)
and minimum (anticyclonic) relative
vorticity are shown, as are those of
negative (anticyclonic) and positive
(cyclonic) vorticity advection.
Sources : Mostly after Musk (1988), with
additions from Uccellini (1990).
Figure 9.23 Model of the jet stream
and surface fronts, showing zones of
upper tropospheric divergence and
convergence and the jet stream cores.
flow. The basic theory relates to the vorticity equation,
which states that, for frictionless horizontal motion, the
rate of change of the vertical component of absolute
vorticity (d Q /d t or d( f +
favouring surface convergence and low-level cyclonic
vorticity (see Figure 9.23). Once the surface cyclonic
circulation has become established, vorticity production
is increased through the effects of thermal advection.
Poleward transport of warm air in the warm sector and
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.
131). 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 (Plate 18). 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.
)/d t ) is proportional to airmass
convergence (- D , i.e. negative divergence):
ζ
d Q 1 Q
—— = DQ or D = — - ——
d t
Q d t
The relationship implies that a converging (diverging)
air column has increasing (decreasing) absolute vor-
ticity. The conservation of vorticity equation, discussed
above, is in fact a special case of this relationship.
In the sector ahead of an upper trough, the decreasing
cyclonic vorticity causes divergence (i.e. D positive),
since the change in
ζ
outweighs that in f , thereby
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