Geography Reference
In-Depth Information
Fig. 9.3
(a) Horizontal streamlines, isotherms, and
Q
vectors in a frontogenetic confluence. (b)
Vertical section across the confluence showing isotachs (solid), isotherms (dashed), and
vertical and transverse motions (arrows). (After Sawyer, 1956.)
The mechanisms of horizontal shear and horizontal stretching deformation
discussed above operate to concentrate the pole-equator temperature gradient on
the synoptic scale (
1000 km). The time scale on which these processes operate
can be estimated with the aid of (9.1). Suppose that the geostrophic wind consists
of a pure deformation field so that u
g
∼
=
Kx, and v
g
=−
Ky, and let T be a
function of y only. Then (9.1) simplifies to
∂T
∂y
D
g
Dt
K
∂T
∂y
=
so that following the geostrophic motion
e
Kt
∂T
∂y
∂T
∂y
=
t
=
0
10
−
5
s
−
1
so that the temperature gradient amplifies by a factor
of 10 in about 3 days. This is much slower than observed rates of atmospheric
frontogenesis.
Thus, geostrophic deformation fields alone cannot cause the rapid frontogenesis
often observed in extratropical systems, in which the temperature gradient can
become concentrated in a zone of
Typically,
K
∼
∼
50 km width on a time scale of less than a day.
This rapid reduction in scale is caused primarily by the frontogenetic character
of the secondary circulation driven by the quasi-geostrophic synoptic-scale flow
(moist processes may also be important in rapid frontogenesis).
The nature of the secondary flow may be deduced from the pattern of
Q
vectors
illustrated in Fig. 9.3a. As discussed in Section 6.4.2, the divergence of
Q
forces a
secondary ageostrophic circulation. For the situation of Fig. 9.3 this circulation is
in the cross-frontal plane as illustrated in Fig. 9.3b. Advection of the temperature
