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Occluded Fronts

Mountain ranges have much the same effect on occluded fronts as they do on warm and cold fronts. Cold type of occlusions behave as cold fronts, and warm type of occlusions behave as warm fronts. The occlusion process is accelerated when a frontal wave approaches a mountain range. The warm front is retarded; but the cold front continues at its normal movement, quickly overtaking the warm front (views A and B of fig. 4-7-3).

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, anew 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. 4-7-3). 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

Figure 4-7-3.—Acceleration of the occlusion process and development of a frontal wave cyclone.

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 4-7-4. 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 4-7-4 either moves against a mountain range or is overtaken by another cyclone. The occlusion then undergoes frontolysis (view B of fig. 4-7-4). The new occlusion forms immediately and soon overshadows its predecessor in both area and intensity (view C of fig. 4-7-4). 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. 4-7-4) about a point somewhere on the axis joining them until the old center has filled and loses its separate identity. This can take place with either a warm or cold occlusion. If it occurs near a west coast in winter, there is a good chance the new occlusion is warm. This formation of a secondary wave cyclone, the dissipation of the original occluded front, and the rapid development of a new occlusion is sometimes called skagerraking, pressure jump, or bent-back occlusion:

Figure 4-7-4.—Stages in the development of a secondary wave cyclone.

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