Geography Reference
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0
250
χ < 0
χ > 0
500
750
V g T > 0
V g T < 0
1000
A
B
Fig. 6.9
East-west section through a developing synoptic disturbance showing the relationship of
temperature advection to the upper level height tendencies. A and B designate, respectively,
regions of cold advection and warm advection in the lower troposphere.
Qualitatively, the effects of differential temperature advection are easily under-
stood since the advection of cold air into the air column below the 500-hPa trough
reduces the thickness of that column, and hence lowers the height of the 500-hPa
surface unless there is a compensating rise in the surface pressure. Obviously warm
advection into the air column below the ridge has the opposite effect.
To summarize, in the absence of diabatic heating the horizontal temperature
advection must be nonzero in order that a midlatitude synoptic system intensifies
through baroclinic processes. As shown in Chapter 8, the temperature advection
pattern described above indirectly implies conversion of potential energy to kinetic
energy.
6.3.2 The Quasi-Geostrophic Potential Vorticity Equation
The geopotential tendency equation stated in the form (6.23) is useful for phys-
ical motivation of processes leading to geopotential changes (and hence upper
level troughing and ridging), as the tendency χ is related to the easily compre-
hended processes of vorticity and temperature advection. However, this form of
the equation actually conceals its true character as a conservation equation for a
field commonly referred to as quasi-geostrophic potential vorticity . To put (6.23)
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