Geography Reference
In-Depth Information
not distributed uniformly in the atmosphere. Rather, horizontal temperature gra-
dients tend to be concentrated in baroclinic zones associated with tropospheric jet
streams. Not surprisingly, the development of baroclinic waves is also concentrated
in such regions.
We showed in Section 8.3 that the energetics of baroclinic waves require that
they remove available potential energy from the mean flow. Thus, on average baro-
clinic wave development tends to weaken the meridional temperature gradient (i.e.,
reduce the mean thermal wind). The mean pole to equator temperature gradient is
of course restored continually by differential solar heating, which maintains the
time-averaged temperature gradient pattern. In addition there are transient dynam-
ical processes that produce zones with greatly enhanced temperature gradients
within individual baroclinic eddies. Such zones, which are particularly intense at
the surface, are referred to as fronts . Processes that generate fronts are called fron-
togenetic . Frontogenesis usually occurs in association with developing baroclinic
waves, which in turn are concentrated in the storm tracks associated with the time-
mean jetstreams. Thus, even though on average baroclinic disturbances transport
heat down the mean temperature gradient and tend to weaken the temperature dif-
ference between the polar and tropical regions, locally the flow associated with
baroclinic disturbances may actually enhance the temperature gradient.
9.2.1
The Kinematics of Frontogenesis
A complete discussion of the dynamics of frontogenesis is beyond the scope of
this text. A qualitative description of frontogenesis can be obtained, however, by
considering the evolution of the temperature gradient when temperature is treated
as a passive tracer in a specified horizontal flow field. Such an approach is referred
to as kinematic ; it considers the effects of advection on a field variable without
reference to the underlying physical forces, or to any influence of the advected
tracer on the flow field.
The influence of a purely geostrophic flow on the temperature gradient was given
in terms of the Q vector in (6.46). If for simplicity we focus on the meridional tem-
perature gradient, then from (6.46b) neglecting ageostrophic and diabatic effects
gives
∂T
∂y
∂u g
∂y
D g
Dt
∂T
∂x
∂u g
∂x
∂T
∂y
=−
(9.1)
where we have used the fact that the geostrophic wind is nondivergent so that
∂v g /∂y
∂u g /∂x. The two terms within the brackets on the right in (9.1) can
be interpreted as the forcing of the meridional temperature gradient by horizontal
shear deformation and stretching deformation, respectively.
Horizontal shear has two effects on a fluid parcel; it tends to rotate the parcel
(due to shear vorticity) and to deform the parcel through stretching parallel to the
shear vector (i.e., along the x axis in Fig. 9.1a) and shrinking along the horizontal
=−
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