Geoscience Reference
In-Depth Information
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Figure 7.4
Streamline analysis of mean January resultant wind field. Dashed lines indicate airstream confluences.
Source: After Wendland and Bryson, 1981
responsible for the sudden swings in temperature from
hot to cold or vice versa as air masses change.
For over seventy years the Bergen model of depres-
sion formation has dominated our views (
Figure 7.5
).
However, subsequent work, especially that involving the
upper atmosphere, has revealed significant deficiencies in
the model. For example, it is now clear that depression
formation does not need a polar front but rather a zone
of strong temperature gradient known as a baroclinic
zone. The process of cyclogenesis (or depression forma-
tion) actually intensifies the thermal gradients to produce
the fronts. In many parts of the world warm fronts,
an important part of the Bergen model, are weak or
limited in extent. Finally, the classic 'catching-up'
occlusion process is difficult to identify and many studies
have shown that ideal occluded frontal structures are
rarely observed in their entirety. Unfortunately no clear
conceptual model of depresson evolution has replaced the
Bergen one, possibly because depression development is
highly variable, depending upon surface conditions,
topography and upper atmospheric flows.
If we follow depressions over a period of several days
we find that many, though by no means all, conform
to the general pattern of the Bergen model. Initially a
