Geography Reference
In-Depth Information
Fig. 3.9
Relationship between turning of geostrophic wind and temperature advection: (a) backing
of the wind with height and (b) veering of the wind with height.
Thus, as is illustrated in Fig. 3.9a, a geostrophic wind that turns counterclockwise
with height (backs) is associated with cold-air advection. Conversely, as shown in
Fig. 3.9b, clockwise turning (veering) of the geostrophic wind with height implies
warm advection by the geostrophic wind in the layer. It is therefore possible to
obtain a reasonable estimate of the horizontal temperature advection and its vertical
dependence at a given location solely from data on the vertical profile of the wind
given by a single sounding. Alternatively, the geostrophic wind at any level can be
estimated from the mean temperature field, provided that the geostrophic velocity
is known at a single level. Thus, for example, if the geostrophic wind at 850 hPa
is known and the mean horizontal temperature gradient in the layer 850-500 hPa
is also known, the thermal wind equation can be applied to obtain the geostrophic
wind at 500 hPa.
3.4.1
Barotropic and Baroclinic Atmospheres
A barotropic atmosphere is one in which the density depends only on the pressure,
ρ
ρ(p), so that isobaric surfaces are also surfaces of constant density. For
an ideal gas, the isobaric surfaces will also be isothermal if the atmosphere is
barotropic. Thus,
=
0 in a barotropic atmosphere, and the thermal wind
equation (3.30) becomes ∂ V g /∂ln p
p T
=
=
0, which states that the geostrophic wind
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