Instant Occlusion

The name Instant Occlusion arises because in the mature stage of the development the cloud system resembles that of an Occlusion. But the meteorological background of the development is completely different from an Occlusion process where warm air is lifted from the surface. The process of the Instant Occlusion is characterized by the mergence of a distinct cold air, Comma - like cloud feature with a cloud band associated with the polar front (Cold Front). The adjective instant represents the fact that the Occlusion appearance is reached suddenly without the usual development at a front.


Fig 1. The lifecycle of an Instant Occlusion

Consequently three different conceptual models are involved in the process:

The Comma - like cloud feature is associated with a polar air trough within the mid- and upper levels of the troposphere which can be found within the cold air mass behind a Cold Front. The Comma - like cloudiness is transported downstream by a strong flow at the eastern side of the trough until it merges with the cloud band of the polar front. A mergence of the two initially independent systems is possible because of different system velocities. While the frontal system is nearly stationary (partly caused by the ongoing wave development) the mesoscale Comma is steered by the upper level flow.


Fig 2. Development of an Instant Occlusion within an upper level trough

At the pre-merging stage of the development the Comma - like cloudiness within the cold air mass is clearly separated from the front and approaches the cloud band of the Cold Front. The area between the Cold Front and the Comma - like cloudiness is named shallow moist zone. In the satellite image the Comma - like cloudiness is characterized by a series of convective cells which have developed at the rear cloud edge. Within and in front of the Comma an ascending stream of warm and moist air is orientated from southern to northern directions in lower layers. Within the middle and upper levels of the troposphere, dry air moves around the upper level trough overrunning the Comma tail and turning to northern directions parallel to the rear cloud edge of the Cold Front. The vertical stratification of relative stream with warm, moist air and dry air above causes a potential unstable stratification in the shallow moist zone.

During the so-called merging stage the Comma cloudiness appears to rotate rapidly cyclonically. The potential unstable stratification within the area of the shallow moist zone will now be released due to the developing ascending motion. The ascending motion is caused by the combination of WA and PVA which contributes according to the omega equation to upward motion; in particular WA is connected with the Wave feature at the cold front. As a consequence of this process rapid cloud development can be observed in the shallow moist zone. Due to WA ahead of the Comma a thermal gradient is generated on its northern side.


Fig 3. Typical distribution of PVA and TA during the Merging Stage

In the mature stage of the Instant Occlusion the cloud spiral has the appearance of an Occlusion of the Cold Conveyor Belt Type.

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