Geography Reference
In-Depth Information
CHAPTER 8
Synoptic-Scale Motions II:
Baroclinic Instability
Chapter 6 showed that the quasi-geostrophic system can qualitatively account
for the observed relationships among the vorticity, temperature, and vertical veloc-
ity fields in midlatitude synoptic-scale systems. The diagnostic approach used in
that chapter provided useful insights into the structure of synoptic-scale systems;
it also demonstrated the key role of potential vorticity in dynamical analysis. It did
not, however, provide quantitative information on the origins, growth rates, and
propagation speeds of such disturbances. This chapter shows how linear perturba-
tion analysis can be used to obtain such information.
The development of synoptic-scale weather disturbances is often referred to as
cyclogenesis
, a term that emphasizes the role of relative vorticity in developing
synoptic-scale systems. This chapter analyzes the processes that lead to cycloge-
nesis. Specifically, we discuss the role of dynamical instability of the mean flow
in accounting for the growth of synoptic-scale disturbances. We show that the
quasi-geostrophic equations can, indeed, provide a reasonable theoretical basis for
understanding the development of synoptic-scale storms, although as discussed in
Section 9.2, ageostrophic effects must be included to model the development of
fronts and subsynoptic-scale storms.
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