Geography Reference
In-Depth Information
normalized height tendency
Lx = 2000 km
Lx = 6000 km
100
100
200
200
300
300
400
400
500
500
600
600
700
700
800
800
900
900
1000
1000
0
500
1000
0
1000
2000
3000
x (km)
x (km)
Fig. 6.11
Vertical spread of geopotential height tendencies forced by advection of a potential vorticity
anomaly that is confined above the 250-hPa level. The displayed height tendencies are
normalized by their value at the 100-hPa level. Solutions for zonal wavelength 2000 km
(left) and 6000 km (right). Here, l
10
−
4
s
−
1
, and σ
=
0,k
=
2π/L
x
,f
0
=
=
2
×
10
−
6
m
2
Pa
−
2
s
−
2
.
6.4
DIAGNOSIS OF THE VERTICAL MOTION
As shown in the previous section, prediction of the evolution of the geopoten-
tial distribution for adiabatic flow does not depend explicitly on the ageostrophic
motion. Nevertheless, the vertical velocity, which is strongly correlated with dis-
turbed weather conditions, is an important diagnostic field.
6.4.1
The Traditional Omega Equation
Because ζ
g
and
V
g
are both defined in terms of (
x, y, p, t
), the vorticity equation
(6.19) can be used to diagnose the ω field provided that the fields of both and
∂/∂t are known. The former is a primary product of operational weather anal-
ysis. However, because upper level analyses are generally available only twice
per day, the latter can only be crudely approximated from observations by taking
differences over 12 h. Despite this limitation, the vorticity equation method of esti-
mating ω is usually more accurate than the continuity equation method discussed
