When a cold front associated with an occluded
frontal system passes a mountain range, the cold front
may develop a bulge or wave. In the case of an
occlusion, a new and separate low may form at the peak
of the warm sector as the occluded front is retarded by a
mountain range (view C of fig. 4-46). The low develops
on the peak of the wave because of induced low
pressure that results when air descends on the leeward
side of the mountain and warms adiabatically.
The development of a new low on a frontal wave
and ultimate separation from its original cyclone is a
fairly common occurrence. This can occur over open
oceans but occurs more frequently along the west coast
of mountainous continents and along the west coast of
Japan.
The typical stages of this type of frontal
modification are shown in figure 4-47. Orographic
features play a great role in certain preferred areas of
this phenomena, but over the ocean some other factors
must be operative. In some cases, a rapidly moving
wave overtakes the slow moving occlusion and may be
the triggering mechanism for this cyclogenesis.
Whatever the exact nature of its causes, this type of
cyclogenesis proceeds with great rapidity. Initially, the
old occlusion in view A of figure 4-47 either moves
against a mountain range or is overtaken by another
cyclone. The occlusion then undergoes frontolysis
(view B of fig. 4-47). The new occlusion forms
immediately and soon overshadows its predecessor in
both area and intensity (view C of fig. 4-47). However,
the cold occlusion, having greater vertical extension,
exerts a certain control on the movement of the new
center, which at first follows the periphery of the old
center. Later, the two centers pivot cyclonically (view D
of fig. 4-47) about a point somewhere on the axis
joining them until the old center has filled and loses its
separate identity.
4-49
-10
-30
-15
-30
-15
-15
C
D
A
B
AG5f0447
Figure 4-47.—Stages in the development of a secondary wave cyclone.
