The name Instant Occlusion arises because in the mature stage of 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 (see Occlusion: Warm Conveyor Belt Type ). The process of the Instant Occlusion is characterized by the mergence of a distinct cold air, Comma - like cloud feature (see Comma ) 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.

Consequently three different conceptual models are involved in the process:

• Comma (see Comma )
• Cold Front (see Cold Front )
• Wave (see Wave )

In the initial stage these three conceptual models are relatively independent and their detailed description can be found in the relevant chapters. The detailed description in this chapter is restricted to the process of the approach and mergence of the three features.

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.

In literature the two main different types describing a merging and interaction respectively of Comma - like cloud features and the polar front are named Instant Occlusions.

• The first type describes a Comma - like cloud feature which develops from an ascending cold conveyor belt; in literature this is often called a polar trough conveyor belt. In this manual such a process is not called Instant Occlusion but is described under Occlusion: Cold Conveyor Belt Type (see Occlusion: Cold Conveyor Belt Type ).
• The second type describes the merging of a Comma moving from west to east with the cloud band of a Cold Front. This process of the Instant Occlusion can be separated (McGinnigle et al., 1988; called life-cycle model below) into three stages:
  • pre-merging stage
  • merging stage mature stage

The three stages of development will be summarized in more detail below and will be illustrated with the results of a ZAMG study comprising Instant Occlusions of one year (about 5 to 7 cases).

Pre-merging Stage:

At this stage of 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 (see Cloud structure in satellite image ). 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.

Discussion:

The investigations at ZAMG have shown that the so-called shallow moist zone cannot be observed in every case and that there are also differences in the relative streams.

During the pre-merging stage under discussion the area of the Instant Occlusion is characterized by a warm conveyor belt accompanying the frontal cloud band and a conveyor belt within the low- and mid-levels of the troposphere changing from western directions behind to north-eastern directions in front of the Comma; in this area which would be the area of the shallow moist zone the relative stream is strongly rising. Very often this relative stream from behind shows a splitting at the rear edge of the frontal cloud band in a north-eastward and a south-westward branch. This happens close to the Wave feature in the front for which it is characteristic (see Wave ).

The three isentropic surfaces used for the relative streams are 298K (representing a lower isentropic surface of the Comma - like cloudiness), 302K and 310K which are relevant for the cold front as well as for the Comma - like cloudiness (left image top).
On the isentropic surface of 298K (right image top) the whole area of the Comma - like cloudiness is within the influence of the relative streams from behind, the upper relative stream (moist relative stream) and the dry intrusion (dry relative stream).
The Comma - like cloudiness on the isentropic surface of 302K (left image bottom) is likewise also under the influence of the upper relative stream and the dry intrusion. Within the area of the Wave (approximately 53N/21W) an ascending warm conveyor belt can be observed. The limiting stream line between the warm conveyor belt and the relative streams from behind can be found in the area of the rear cloud edge of the front.
On the higher isentropic surface of 310K (right image bottom) the situation for the Comma - like cloudiness is the same as described for the lower isentropic surfaces. The frontal cloud band is under the influence of the warm conveyor belt. The limiting stream line between the warm conveyor belt and the relative streams from behind can be found within the area of the rear cloud edge of the front.

The comparison of the life-cycle model with the ZAMG investigation reveals some deviations: although relative streams have been computed for several isentropic surfaces, no wet stream from south to north-east in low layers could be observed. As the life-cycle model shown in the left figure is based only on one case study it is not clear if such a relative stream is a single case event or if the ECMWF model is too coarse to resolve the different streams. It is absolutely necessary to look at more cases and with help of the fine mesh model. This will be done in a future version of the manual.

Merging Stage:

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.

These findings of the life-cycle model are fully supported by the case studies carried out at ZAMG.

Although the life-cycle model does not refer explicitly to conveyor belts for this development stage the following consequences can be drawn from the schematics below:

• now a low level dry outflow from eastern directions appears
• the wet low level stream from south-west no longer exists

Some case studies show for this stage of development very similar results as in the stage before:

• a rising warm conveyor belt accompanying the frontal cloud band
• a relative stream from behind strongly rising in the area of the front and the Comma in accordance with the area of cloud development
• a splitting of the relative stream from behind close to the Wave point

A new aspect for this stage of development is a noticeable separation between low and upper levels. The relative stream from behind shows a tendency to override the Wave area of the frontal cloud band.

The two isentropic surfaces used for the relative streams are 302K and 310K (left image top). The isentropic surface for 302K is representative of the lower levels of the troposphere, and the isentropic surface of 310K for the higher levels of the troposphere.
On the lower isentropic surface (right image top) the Comma - like cloudiness is, as described for six hours before under the influence of the relative streams of the dry intrusion and the upper relative stream.
On the higher isentropic surface (left image bottom) the limiting stream line between the warm conveyor belt and the relative streams from behind can be found within the rear part of the frontal cloud band. The area of the merging comma-like cloudiness is within the area of the upper relative stream and the dry intrusion.

Mature Stage:

The life-cycle model (in the image below) shows a cloud configuration very similar to an occlusion of the Cold Conveyor Belt Type (see Occlusion: Cold Conveyor Belt Type ) and indeed the case studies at ZAMG support the possibility of such a development leading to the existence of the following conveyor belts:

• warm conveyor belt
• cold conveyor belt
• upper relative stream from behind
• dry intrusion

However, not every mature stage shows such a configuration as the one case on which the life-cycle model is based on. There are cases where a complete restructure of cloudiness takes place.

The images above show the Instant Occlusion in its mature stage. The cloud spiral has the appearance of an Occlusion of the Cold Conveyor Belt Type.

One important question is also whether the different development of an Instant Occlusion compared to a cold conveyor belt type occlusion has any impact on the mature stage. At the time being major differences cannot be observed in the relative streams, but there are slight differences in the configuration of some key parameters (see Key parameters ).

To summarize, one has to note differences between the relative streams in the life-cycle model and those summarized from several cases of the ZAMG studies. The main difference is a more pronounced fine structure of relative streams in low layers. The reason for the fine structure might be the use of a fine mesh model, but it might also be the consequence of only one case study being involved. More investigations have to be done in this respect.