Geography Reference
In-Depth Information
0
δζ
δ
δζ
δ
250
> 0
< 0
t
t
div
div
500
δζ
δ
t
δζ
δ
t
750
< 0
> 0
div
div
1000
0
π
/2
3
π
/2
2
π
π
Phase (rad)
Fig. 8.6
Vertical cross section showing phase of vorticity change due to divergence-convergence for
an unstable baroclinic wave in the two-level model.
vorticity maxima and minima are less than the advective tendencies at the upper
level and greater than the advective tendencies at the lower level:
adv
Furthermore, vorticity concentration by the divergence effect will tend to amplify
the vorticity perturbations in the troughs and ridges at both the 250- and 750-hPa
levels as required for a growing disturbance.
<
adv
>
∂ζ
1
∂t
∂ζ
1
∂t
∂ζ
3
∂t
∂ζ
3
∂t
,
8.3
THE ENERGETICS OF BAROCLINIC WAVES
The previous section showed that under suitable conditions a vertically sheared
geostrophically balanced basic state flow is unstable to small wave-like perturba-
tions with horizontal wavelengths in the range of observed synoptic scale systems.
Such baroclinically unstable perturbations will amplify exponentially by drawing
energy from the mean flow. This section considers the energetics of linearized
baroclinic disturbances and shows that the potential energy of the mean flow is the
energy source for baroclinically unstable perturbations.
8.3.1
Available Potential Energy
Before discussing the energetics of baroclinic waves, it is necessary to consider
the energy of the atmosphere from a more general point of view. For all practical
