Geoscience Reference
In-Depth Information
′
A--A
warm air
cold air
cold air
′
B--B
warm air
cold air
cold air
Vertical cross sections along the lines A-A
and B-B
indicated in Figure 3.3c. (Vertical scale is
exaggerated.)
Fig. 3.4
warm air
C--C
′
cool air
cold air
Vertical cross section along the line C-C
indicated in Figure 3.3d, showing a cold-front occlusion.
Fig. 3.5
and 3.5. In the initial stage the winds on both sides of a stationary front are blowing in
parallel but opposite directions. As a result of the shear and small disturbances, surface
roughness or heating irregularities, the front may gradually assume a wave-like shape,
which may persist and increase in amplitude, and eventually evolve into a counterclock-
wise (in the Northern Hemisphere) flow pattern, called a
frontal wave
. By now there
is a well-defined cold front and a warm front, and the cyclonic circulation continues
to intensify. The cold front section usually moves faster and eventually overtakes the
warm front. At this point, which is the time of maximal intensity of the cyclone, the
combined front is referred to as an
occlusion
or an
occluded front
. In the later stages of
the occlusion, the intensity of the cyclone and the frontal movement gradually decrease;
finally the occlusion vanishes while a new stationary front may be formed. Observe that,
in contrast to the cold and warm fronts sketched in Figures 3.1 and 3.2, those shown in
Figure 3.4 are assumed to involve a stable warm air mass, so that the clouds shown are
of the stratiform type. Many different factors control the evolution of occluded fronts. It
has been shown (Stoelinga
et al
., 2002) that, more than the temperature contrasts, it is
the stability contrasts across the fronts which govern the dynamics of occlusions.
