Meteorological Physical Background

In the case of a Warm Front, warm moist air moves against colder dry air. At the boundary of these two air masses the warm air tends to slide up over the wedge of colder air (see Typical Appearance In Vertical Cross Sections). This process causes the frontal cloud band, and the associated precipitation, found mainly in front of the surface front (or the TFP) (see Weather events).

The idealized structure and physical background of a Warm Front can be explained with the conveyor belt theory as follows:

  • Frontal cloud band and precipitation are in general determined by the ascending Warm Conveyor Belt, which has its greatest upward motion between 700 and 500 hPa. The Warm Conveyor Belt starts behind the frontal surface in the lower levels of the troposphere, crosses the surface front and rises to the upper levels of the troposphere. There the Warm Conveyor Belt turns to the right (anticyclonically) and stops rising, when the relative wind turns to a direction parallel to the front. If there is enough humidity in the atmosphere, the result of this ascending Warm Conveyor Belt is condensation and more and more higher cloudiness.
  • The Cold Conveyor Belt in the lower layers, approaching the Warm Front perpendicularly in a descending motion, turns immediately in front of the surface Warm Front parallel to the surface front line. From there on the Cold Conveyor Belt ascends parallel to the Warm Front below the Warm Conveyor Belt. Due to the evaporation of the precipitation from the Warm Conveyor Belt within the dry air of the Cold Conveyor Belt, the latter quickly becomes moister and saturation may occur with the consequence of a possible merging of the cloud systems of Warm and Cold Conveyor Belt to form a dense nimbostratus.

Discussion

In addition to this idealized structure, the experience from a series of case studies carried out at ZAMG differs somewhat from the one described above, allowing more differentiation. In the case of a band type the Warm Conveyor Belt can be observed within the warm sector up to the Cold and Warm Front line. If there is high cloudiness in front of and parallel to the Warm Front line, it is situated within an (at least in the area of the fronts) ascending conveyor belt from the rear side of the Cold Front extending from south-west to north-east, the so-called upper relative stream. While the high cloudiness can thus be explained, the Warm Front cloudiness in the lower levels of the troposphere develops, as in the ideal case described above, within the Cold Conveyor Belt.

The conveyor belt situation, especially of the Warm Conveyor Belt and the upper relative stream, described above can be found in a thick layer of isentropic surfaces, but there is some tendency for the Warm Conveyor Belt in lower layers to overrun the surface Warm Front to a small extent. The ascending Warm Conveyor Belt in the warm sector is not accompanied by appreciable cloudiness either because of too dry air masses or/and too little lifting. But it can be observed that in the ascending Warm Conveyor Belt cloudiness may develop leading to a second Warm Front type, the Warm Front Shield.

Very often a transition from the Warm Front Band to the Warm Front Shield can be observed by the development of cloudiness within the warm sector. This is described in detail in the chapter of the Warm Front Shield (see Warm Front Shield - Meteorological physical background ).

14 November 2004/00.00 UTC - Vertical cross section; black: isentropes (ThetaE), orange thin: IR pixel values, orange thick: WV pixel values

The 302K isentropic surface chosen for the relative streams in the figure below is very close to the upper boundary of the higher gradient, inclined Warm Front zone.

14 November 2004/00.00 UTC - Meteosat 8 IR 10.8 image; magenta: relative streams 302K - system velocity: 295° 10 m/s, yellow: isobars 300K, position of vertical cross section indicated

The analysis shows two Conveyor Belts: there is one from eastern directions across The Netherlands, Belgium, France, turning northward in an anticyclonic direction over the Atlantic. This is a typical example of a Warm Conveyor Belt extending across the warm sector to the leading edge of the Cold Front and the rear edge of the Warm Front cloudiness. A second relative stream originates from the cold air mass over the Atlantic behind the Cold Front and approaches the Warm Conveyor Belt over Iceland, extending from there on across the Atlantic (approx. 66N/2W) towards Norway parallel to the Warm Conveyor Belt. Generally speaking, the Warm Conveyor Belt rises from about 800 hPa over the Atlantic up to 400 hPa over Norway. The relative stream from the Atlantic exists in a higher layer, rising from about 600 to 450 hPa. The high cloudiness of the Warm Front exists in this latter relative stream.

15 September 2004/06.00 UTC - Vertical cross section; black: isentropes (ThetaE), orange thin: IR pixel values, orange thick: WV pixel values

The isentropic surfaces which are used for the relative streams are 318K, which represents the situation within the lower levels of the troposphere, and 326K which is characteristic for the upper levels.

15 September 2004/06.00 UTC - Meteosat 8 IR 10.8 image; magenta: relative streams 318K - system velocity: 252° 13 m/s, yellow: isobars 318K, position of vertical cross section indicated
15 September 2004/06.00 UTC - Meteosat 8 IR 10.8 image; magenta: relative streams 326K - system velocity: 252° 13 m/s, yellow: isobars 326K, position of vertical cross section indicated

The analysis shows that in the lower layers of the troposphere a Cold Conveyor Belt (left) is originating from northern directions. As described above, the Cold Conveyor Belt approaches the warm front with a descending component (from approximately 750 hPa to approximately 900 hPa). In front of the surface front line it turns parallel to the front where it starts ascending (from approximately 900 hPa to approximately 550 hPa).

The upper levels of the troposphere are as described in the case before within the ascending part of the upper relative stream (right), which originates from the western side of the trough. The Warm Conveyor Belt can only be found within the warm sector, in front of the cloud band of the Cold Front.