OCCLUSION: WARM CONVEYOR BELT TYPE - METEOROLOGICAL PHYSICAL BACKGROUND

by ZAMG


As has been explained in the general remarks the starting viewpoint for the conceptual model Occlusion: Warm Conveyor Belt Type is the typical configuration in the satellite imagery and its development up to an eventually longer lasting mature stage.

The classical Norwegian model

The classical development of an Occlusion is described in the well-known polar front theory after Bergeron as a development from a Wave stage (see Wave) with a well developed Cold Front (see Cold Front, Cold Front In Cold Advection, Cold Front In Warm Advection, Split Front and Arctic Cold Front) and Warm Front (see Warm Front Band, Warm Front Shield and Detached Warm Front). The basic idea is that the Cold Front moves faster than the Warm Front. Therefore the warm sector continously becomes more narrow until finally the Cold Front overtakes the Warm Front completely, thereby lifting the warm air. This is the typical situation for the Occlusion band which turns cyclonically around the core of the cyclone.
According to this classical theory of the Occlusion band, this front separates the cold air mass, which is situated in front of the former Warm Front, from that behind the former Cold Front with a tongue of warm air within the higher levels of the troposphere (see Typical appearance in vertical cross section). Following the distribution of the temperature in front of and behind the Occlusion, two sub-types can be separated:
Warm Occlusion
Cold Occlusion
During this process, the displacement of the low becomes smaller, until it becomes quasi-stationary.

Deviations between cloud configurations in satellite images and the classical Norwegian model

Very soon after the use and study of satellite images it became clear that this idealized theory cannot be observed in every step and detail in reality. In particular, the overtaking of the Warm Front by the Cold Front with very narrow warm sectors can never be seen (left schematic). Instead of this a mergence of Cold and Warm Front cloudiness in the centre of the surface low takes place followed by a westward extension of the Occlusion cloud spiral, while the Warm Front cloudiness becomes shorter (right schematic).
Beside this there are Occlusion spirals which contradict the overtaking mechanism completely. Those developments show a lower cloud spiral penetrating westward from below the Cold Front and Warm Front.
As already mentioned for Cold Fronts and Warm Fronts, those contradictions between the typical polar front theory and the appearance in the satellite imagery were also one reason for the conveyor belt theory.

Conveyor Belt model

This theory is not an independent new model, but allows a different insight into the same phenomena of the Occlusion process. It shows a much bigger variety of development possibilities and clearly indicates that the Occlusion development described by the Norwegian model represents only a special case.

The conveyor belt theory for the Cold Front deals predominantly with two relative streams: the warm conveyor belt and the dry intrusion. The warm front deals predominantly with the relative streams of the warm conveyor belt and the cold conveyor belt. For the Occlusion all three conveyor belts are relevant:

The vertical relation of these three conveyor belts to each other results in the different cloud types of the Occlusion: the multilayered Warm Conveyor Belt Type of the cloud spiral and the Cold Conveyor Belt Type.

The structure and formation of the cloud spiral of the warm conveyor belt Occlusion type can also be described with the conveyor belt theory as follows:

Discussion

A comprehensive study at ZAMG of all oOclusion events during one year shows many of the mentioned points described above, but also some deviations. These facts can be summarized as follows:
21 October 1996/18.00 UTC - Vertical cross section; black: isentropes (ThetaE), blue: relative humidity, orange thin: IR pixel values, orange thick: WV pixel values
21 October 1996/18.00 UTC - Meteosat IR image; magenta: relative streams 305K - system velocity: 206° 13 m/s, yellow: isobars 305K, position of vertical cross section indicated
21 October 1996/18.00 UTC - Meteosat IR image; magenta: relative streams 310K - system velocity: 206° 13 m/s, yellow: isobars 310K, position of vertical cross section indicated
21 October 1996/18.00 UTC - Meteosat IR image; magenta: relative streams 320K - system velocity: 206° 13 m/s, yellow: isobars 320K, position of vertical cross section indicated
In the vertical cross section, which extends in the Atlantic from approximately 44N/28W to approximately 60N/25W, the isentropic surface of 310K is a surface within the occlusion cloud band. Both relative streams on this surface cut the cross section perpendicularly at about 650 hPa. This is an area of high humidity for the cold conveyor belt, but an area of high humidity gradient for the dry intrusion.
On the isentropic surface of 305K the cold conveyor belt can be observed within the cloud spiral.
A cold conveyor belt can still be observed on the isentropic surface of 310K but the innermost parts of the cloud spiral are already overrun by the dry intrusion.
On the isentropic surface of 320K the hook form of the upper relative stream accompanies a large part of the Occlusion cloudiness. For this the assumption has to be made that the zero line of shear vorticity does not cut the cloud band as computed by the numerical model, but follows the rear cloud edge and is connected to the western zero line of the shear vorticity.

SUB-MENU OF OCCLUSION: WARM CONVEYOR BELT TYPE
CLOUD STRUCTURE IN SATELLITE IMAGES
KEY PARAMETERS