Geography Reference
In-Depth Information
CHAPTER 6
Synoptic-Scale Motions I:
Quasi-Geostrophic Analysis
A primary goal of dynamic meteorology is to interpret the observed structure
of large-scale atmospheric motions in terms of the physical laws governing the
motions. These laws, which express the conservation of momentum, mass, and
energy, completely determine the relationships among the pressure, temperature,
and velocity fields. As we saw in Chapter 2, these governing laws are quite compli-
cated even when the hydrostatic approximation (which is valid for all large-scale
meteorological systems) is applied. For extratropical synoptic-scale motions, how-
ever, the horizontal velocities are approximately geostrophic (see Section 2.4).
Such motions, which are usually referred to as
quasi-geostrophic
, are simpler to
analyze than are tropical disturbances or planetary scale disturbances. They are
also the major systems of interest in traditional short-range weather forecasting,
and are thus a reasonable starting point for dynamical analysis.
This chapter shows that for extratropical synoptic-scale systems, the twin require-
ments of hydrostatic and geostrophic balance constrain the baroclinic motions so
that to a good approximation the structure and evolution of the three-dimensional
velocity field are determined by the distribution of geopotential height on iso-
baric surfaces. The equations that express these relationships constitute the quasi-
geostrophic system. Before developing this system of equations, it is useful to
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