The Conveyor Belt Model


The conveyor belt theory is a very convenient way to visualize the three-dimensional airflows involved in weather systems and therefore a useful method to explain the physical background of Conceptual Models. This theory is especially suited for CMs like fronts and cyclogenesis. The conveyor belt theory does not only provide a new perspective on atmospheric processes such as cyclogenesis or frontogenesis, but it also allows a better insight into the phenomenon of the Occlusion process. It shows a higher variety of possible developments and clearly indicates that the Occlusion process as described by the Norwegian model only represents a special case.

Conveyor Belts are:

Even though Carlson first presented the conceptual model in 1980, parts of this concept were developed earlier by Harrold (1973), modified later by Young et al (1987), and summarized by Browning (1999). This concept was intended to describe the storm-relative flow around a mature frontal wave.

Relative Streams

The concept of relative streams on isentropic surfaces have gained more and more importance with the introduction of conveyor belt theory. However, current work with relative streams is mostly restricted to research work and is not that common in operational weather forecasting. Relative streams are computed on isentropic surfaces (i.e. on layers with constant potential or equivalent potential temperature). The wind speed and direction of a relative stream is calculated as a function of the system velocity. This system velocity is subtracted from the absolute velocity:

The determination of the system velocity is one of the critical steps in the computation process; it can be derived from the propagation of cloud systems accompanying a weather system. For instance, the propagation of a frontal cloud band can be used for the system front and upper level trough.

The subtraction of the system velocity means that an observer would move with the system and that all the air streams and their changes occur relative to this system.

The next schematics explain the connection between presentations on isobaric and isentropic surfaces.

Figure 1: The connection between presentations on isobaric and isentropic surfaces

There are two main advantages when using relative streams:

As already mentioned, the conveyor belt theory has been derived from an approach using relative streams. In the above schematics two typical conveyor belts associated with a cold front situation are marked: A warm conveyor belt and a dry intrusion.

Conveyor Belts

Three types of conveyor belts have revealed useful for the description of the processes involved in cyclogenesis (described here for the northern hemisphere):

The Warm Conveyor Belt (WCB), the Cold Conveyor Belt (CCB), the Dry Intrusion (DI).

Schematics below:

Typical configurations of the three conveyor belts for two different stages of occlusion cloud bands

Figure 2: Two different stages of occlusion cloud bands

A good way to look into Conveyor Belts is the example of a highly developed frontal cloud system presented with the air mass RGB (14 November 2018/ 06 - 12 UTC) in two stages of development.

WCB: Warm conveyor belt:
(red arrow)
Rising from S,SW -> N,NE
Sinking from N,NE -> S, SE
Transports warm, moist air
Air mass RGB: Green, yellowish

CCB: Cold conveyor belt:
(blue arrow)
Rising from E,SE -> NW
Sinking from N,NW -> S,SW
Emerging from below the WCB
Transports moist air but less warm than in the WCB
Air mass RGB: blue

DI: Dry intrusion:
(yellow arrow)
Sinking from NW -> SE
Splitting in branch to NW and to NE
Transports: very cold dry air
Air mass RGB: brown to dark brown

Figure 3: 14 November 2018/ Airmass RGB - Upper image: 06 UTC; lower image: 12 UTC

Conveyor Belts are primarily connected with the Conceptual Models of Fronts and Cyclogenesis. The table below gives a summary.

Figure 4: Conveyor belts in fronts