Geography Reference
In-Depth Information
type of disturbance at the levels where the advection is occurring, but only acts to
propagate the disturbance horizontally and (as shown in the next section) to spread
it vertically.
The major mechanism for amplification or decay of midlatitude synoptic sys-
tems is contained in term C of (6.23). This term involves the rate of change with
pressure of the horizontal thickness advection. (If we had retained the diabatic
heating term, it would have contributed in a similar fashion.) The thickness advec-
tion tends to be largest in magnitude in the lower troposphere beneath the 500-hPa
trough and ridge lines in a developing baroclinic wave. Now because
∂/∂p is
proportional to temperature, the thickness advection is proportional to the temper-
ature advection. Thus, term C in (6.23) is proportional to minus the rate of change
of temperature advection with respect to pressure (i.e., plus the rate of change with
respect to height). This term is sometimes referred to as
differential temperature
advection
.
Differential temperature advection enhances upper level height anomalies in
developing disturbances. Below the 500-hPa ridge there is strong warm advection
associated with the warm front, whereas below the 500-hPa trough there is strong
cold advection associated with the cold front. The former increases thickness, thus
builds the upper level ridge; the latter decreases thickness, thus deepens the upper
level trough. Above the 500-hPa level the temperature gradient is usually weaker,
and the isotherms often become nearly parallel to the height lines so that thermal
advection tends to be small. Thus, in contrast to term B in (6.23), the forcing term
C is concentrated in the lower troposphere, but again, the geopotential tendency
response will not be limited to the levels of forcing, but is spread in the vertical so
that, as noted above, in developing waves differential temperature advection will
deepen upper level troughs and build upper level ridges.
In the region of warm advection -
V
g
·∇
−
∂/∂
p
)>0, as
V
g
has a component
down the temperature gradient. However, as explained above, the warm advection
(
−
decreases with height (increases with pressure) so that ∂
−
∂/∂p)
/∂p
> 0. Conversely, beneath the 500-hPa trough where there is cold advection decreas-
ing with height, the opposite signs obtain. Thus, along the 500-hPa trough and ridge
axes where the vorticity advection is zero, the tendency equation states that for a
developing wave
V
g
·∇
(
−
∂
∂p
∂
∂t
∼
∂
∂p
> 0 at the ridge
< 0 at the trough
χ
=
−
V
g
·∇
−
Therefore, as indicated in Fig. 6.9, the effect of cold advection below the 500-hPa
trough is to
deepen
the trough in the upper troposphere, and the effect of warm
advection below the 500-hPa ridge is to
build
the ridge in the upper troposphere.
Hence, differential temperature advection, even if limited to the lower troposphere,
intensifies the upper-level troughs and ridges in a developing system.
