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Appearance in Satellite Data

  • In the satellite image the Warm Conveyor Belt looks like an anticyclonically curved cloud band usually in front of, but separated from, the cloud band of the Cold Front.
  • In the IR and WV images the grey shades of the cloud band of the Warm Conveyor Belt vary from grey to white.
  • The fibrous character dominates but there may be bigger areas of smooth high cirrus cloudiness.
  • In the VIS image the cloudiness of the Warm Conveyor Belt is much less; if there is any appreciable cloudiness it consists of small single cells.
  • As a consequence of the ascending Warm Conveyor Belt the grey shades in the IR image become continuously brighter from south to north or north-east, from there on decreasing again while it turns to south-east where it comes under the influence of sinking.
  • During the life cycle some interactions between Warm Conveyor Belt and frontal cloud band can be observed:
    • higher reaching cells may develop at the rear edge which is oriented to the approaching Cold Front;
    • the anticyclonic part of the Warm Conveyor Belt may merge with the frontal cloudiness of the Cold Front;
    • sometimes the southern boundary of Cold Front bands show superimposed high cloud patches.

13 September 2004/12.00 UTC - Meteosat 8 IR 10.8 image
13 September 2004/12.00 UTC - Meteosat 8 WV 6.2 image
13 September 2004/12.00 UTC - Meteosat 8 VIS 0.6 image

The satellite image shows an anticyclonically curved cloud band of a Warm Conveyor Belt extending from Portugal (approximately 40N/6W) across the Bay of Biscay over France to Germany where it merges with the cloudiness of the Warm Front Band (approximately 54N/13E). As can be seen in the WV image cloudiness exists in a broad wet area and consists of vertically extended cells as well as isolated high Cloud Fibres. In the VIS image only the thick parts of the cloudiness can be recognized indicating high level ice clouds. Dark grey shades in this image represent the middle level cloudiness.

In the above example the Warm Conveyor Belt was clearly connected to the frontal system. An other example shows the Warm Conveyor Belt more isolated over northern Africa

01 July 2005/00.00 UTC - Meteosat 8 IR 10.8 image

The cloud band of the Warm Conveyor Belt extends from Morocco (31N/4W) to Tunis (36N/10E). Within the cloud band some convective cells can be recognised along the northern edge. Within the eastern part of the band over Tunis even a larger convective cell is recognised. The Warm Conveyor belt appears as grey to white in IR imagery.

01 July 2005/00.00 - Meteosat 8 RGB image (3.9, 6.2 and 7.3)

This example shows the typical cloud band enhanced using an RGB build from METEOSAT 8 channels NIR 3.9, WV6.2 and WV7.3. The red areas indicate the dry areas. The Warm Conveyor Belt is marked as blue which indicates that high amounts of water vapour are found at lower levels of the atmosphere. The light blue area over Tunis indicates that high amounts of water vapour are found throughout all layers of the atmosphere.

Meteorological Physical Background

A Warm Conveyor Belt is defined as a layer of air which:

  • originates in a relative easterly to south-easterly flow within the lower levels of the troposphere in the ridge area ahead of the front;
  • generally ascends pole ward (north to north-eastern direction) forming cloudiness due to the condensation of moist air;
  • after turning to south-eastward directions ascending motion changes to descending motion, where cloud dissolution takes place.

As already mentioned Warm Conveyor Belts are directly involved in the frontal processes, but typical cloud configurations can also be observed separately from the frontal cloudiness. Different states of the atmosphere are responsible for these two types:

  • The cloudiness of a front is developed within two relative streams: the Warm Conveyor Belt and a less wet relative stream from the trough area behind; the Warm Conveyor Belt is connected at least with the leading part of the cloudiness, therefore the relative stream lines approach or even overrun the surface front (or the TFP).
  • In the case of separated Warm Conveyor Belt cloudiness the relative stream lines are in front of the frontal cloud band and the relative stream from behind crosses the frontal line in a forward direction, consequently approaching the stream lines of the Warm Conveyor Belt.

warm_conveyor_belt

  • These different situations show up in the vertical cross section of equivalent potential temperature in the case of a Warm Conveyor Belt being involved in the front. The upper troposphere in front of the Cold Front surface shows either an indistinct or a trough configuration of isentropes. In the case of a Warm Conveyor Belt being separated from the front, a second downward inclined crowding zone of isentropes can be observed in high and middle levels. The air in the Warm Conveyor Belt moves on these isentropic surfaces.

A survey by ZAMG has shown that within a period of two years over 57% of the Warm Conveyor Belts that occurred happened over North-Africa and the Mediterranean and could be subscribed to the more isolated and separated type of Warm Conveyor Belt. Of the 467 cases examined less than 70 (15%) occurred over western, northern or eastern Europe and were mostly connected to a frontal system.

13 September 2004/12.00 UTC - Vertical cross section; black: isentropes (ThetaE), orange thin: IR pixel values, orange thick: WV pixel values

The 320K isentrope belongs to the Cold Front, the 326K isentrope to the Warm Conveyor Belt cloudiness.

13 September 2004/12.00 UTC - Meteosat 8 IR 10.8 image; magenta: relative streams 320K - system velocity: 263° 13 m/s, yellow: isobars 308K, position of vertical cross section indicated
13 September 2004/12.00 UTC - Meteosat 8 IR 10.8 image; magenta: relative streams 326K - system velocity: 264° 13 m/s, yellow: isobars 326K, position of vertical cross section indicated

At the 320K isentropic surface, which is relevant for the cloud band of the Cold Front, the whole frontal cloudiness is under the influence of the upper relative stream from behind. The limiting stream line between this stream and the Warm Conveyor Belt can be found at the eastern edge of the cloudless area between Cold Front and Warm Conveyor Belt. The cloudiness with the embedded cells in front of this zone is already under the influence of the Warm Conveyor Belt.

At the 326K surface the Warm Conveyor Belt, which has an ascending poleward direction, forms the cloudiness Warm Conveyor Belt. Above the Atlantic (at approximately 43N/10W) the conveyor belt splits into two branches. The southern branch turns from a north-eastward direction to a south-eastward one and forms the cloudiness which extends to the west coast of Portugal. The northern branch turns from a eastward to a north-eastward direction forming the cloudiness above the Gulf of Biscay and France towards Germany. The limiting stream line between the relative stream from behind the front and the Warm Conveyor Belt can be found above the convective cells. Therefore the cells are under the influence of the warm moist air mass of the Warm Conveyor Belt in lower layers, but in the colder drier air of the upper relative stream in higher layers. This is a typical situation for instability and convective development.

  • At the rear boundary of the Warm Conveyor Belt close to the Cold Front cloud band, sometimes convective cells develop. This is the area where in lower levels the moist air of the Warm Conveyor Belt streams northward, while in higher levels the air in the drier relative stream from behind the front crosses over. This is a typical unstable layer of the troposphere leading, in combination with upward motion, to convective developments (compare the example before).

Key Parameters

The parameters most relevant for a Warm Conveyor Belt are parameters on isentropic surfaces - like relative streams - (see Meteorological physical background ) and not so much parameters on isobaric surfaces.

Typical Appearance In Vertical Cross Sections

In front of the inclined crowding zone of a Cold Front system a second upper level Cold Front - like inclined crowding zone exists. In some cases this crowding zone can also extend into the lower levels. Further it often can be observed that the crowding zones of the upper level Cold Front and the crowding zone of the Warm Conveyor Belt are separated by a small trough of the isolines of equivalent potential temperature.

The relative humidity shows in the area of the Warm Conveyor Belt high values of about 70% and higher.

The distribution of the temperature advection shows a maximum of warm advection within the Warm Conveyor Belt in the mid- and upper levels of the troposphere.

In the satellite image the Warm Conveyor Belt is characterized by high pixel values in the IR as well as in the WV image which indicates mid- and upper level cloudiness. Sometimes it is only possible with help of the vertical cross sections to distinguish between the high pixel values of the frontal system and those of the Warm Conveyor Belt in front of it.

13 September 2004/12.00 UTC - Meteosat 8 IR 10.8 image - Vertical Cross section indicated
13 September 2004/12.00 UTC - Vertical cross section; black: isentropes (ThetaE), blue: relative humidity, orange thin: IR pixel values, orange thick: WV pixel values
13 September 2004/12.00 UTC - Vertical cross section; black: isentropes (ThetaE), red thick: temperature advection - WA, red thin: temperature advection - CA, orange thin: IR pixel values, orange thick: WV pixel values

Weather Events

Usually the Warm Conveyor Belt cloudiness consists only of mid- and high level cloudiness with hardly any or just slight precipitation.

Parameter Description
Precipitation
  • Slight or moderate precipitation in thicker cloudiness.
  • Showers when cloudiness merging with frontal zone
  • Sometimes thunderstorms observed in convective cells at the rear edge of conveyor belt cloudiness.
Temperature
  • No significant change
Wind (incl. gusts)
  • Around embedded Cb's strong gusts are possible
  • Veering of the wind at the front passage.
Other relevant information
  • Precipitation not always reaches the ground
  • In convective cells risk of moderate to severe icing and turbulence.

warm_conveyor_belt

References

General Meteorology and Basics

  • BROWNING K. A. (1985): Conceptual models of precipitation systems; Quart. J. R. Meteor. Soc., Vol. 114, p. 293 - 319
  • BROWNING K. A. (1986): Conceptual models of precipitation systems; Weather&Forecasting, Vol. 1, p. 23 - 41
  • CARLSON T. N. (1987): Cloud configuration in relation to relative isentropic motion; in: Satellite and radar imagery interpretation, preprints for a workshop on satellite and radar imagery interpretation - Meteorological Office College, Shinfield Park, Reading, Berkshire, England, 20 - 24 July 1987, p. 43 - 61
  • CONWAY B. J., GERARD L., LABROUSSE J., LILJAS E., SENESI S., SUNDE J. and ZWATZ-MEISE V. (1996): COST78 Meteorology - Nowcasting, a survey of current knowledge, techniques and practice - Phase 1 report; Office for official publications of the European Communities
  • GREEN J. S. A., LUDLAM F. H. and MCILVEEN J. F. R. (1966): Isentropic relative-flow analysis and the parcel theory; Quart. J. R. Meteor. Soc., Vol. 92, p. 210 - 219

General Satellite Meteorology

  • BADER M. J., FORBES G. S., GRANT J. R., LILLEY R. B. E. and WATERS A. J. (1995): Images in weather forecasting - A practical guide for interpreting satellite and radar imagery; Cambridge University Press

Specific Satellite Meteorology

  • BROWNING K. A. and HARROLD T. N. (1970): Air motion and precipitation growth at a cold front; Quart. J. R. Meteor. Soc., Vol. 96, p. 369 - 389
  • BROWNING K. A. and PARDOE C. W. (1973): Structure of low level jet streams ahead of mid-latitude cold fronts; Quart. J. R. Meteor. Soc., Vol. 99, p. 619 - 638
  • BROWNING K. A. and MONK G. A. (1982): A simple model for the synoptic analysis of cold fronts; Quart. J. R. Meteor. Soc., Vol. 108, p. 435 - 452
  • CARLSON T. N. (1980): Airflow through mid-latitude cyclones and the comma cloud pattern; Mon. Wea. Rev., Vol. 108, p. 1498 - 1509
  • ZWATZ-MEISE V. (1990): Satellite synoptic of warm fronts; Proceedings of 8th Meteosat Scientific Users' meeting, Norrk√∂ping, Sweden, 28 - 31 August 1990, p. 151 - 160