WARM FRONT SHIELD - METEOROLOGICAL PHYSICAL BACKGROUND

by ZAMG


The classical physical Warm Front model as well as the conveyor belt theory concerning Warm Fronts do not discriminate between the Warm Front Band and Warm Front Shield type. But the main difference between the Warm Front Band and the Warm Front Shield, the cloudiness within the warm sector (see Cloud structure in satellite image), is not easy to understand with the classical model. The conveyor belt theory is more qualified to explain this feature.

The cloudiness within the warm sector is connected with a more or less pronounced upper level front (see Typical appearance in vertical cross section) and there is a pronounced ascending Warm Conveyor Belt on the various surfaces of these frontal zones. It overruns the Cold and Warm Front surface lines (a fact which is different to the band type) and mostly exists in a deep layer of the troposphere. This may be the reason for thick cloudiness and precipitation in the warm sector behind the surface front.

Beside this upper level frontal zone there exists also a surface level frontal zone, which is comparable to the band type. Cloudiness on this frontal zone mostly can not be differentiated in the satellite image, but can be explained in a similar way as for the band type (see Warm Front Band - Meteorological physical background ). With this type a different relative stream from behind the Cold Front has been described forming the high Warm Front Band cloudiness. In the shield type this relative stream can only be seen in the area of the leading edge of the shield. Therefore this stream is either overrun or shifted forward by the dominant Warm Vonveyor Belt.

In the case of the band type it was stated that there is not enough ascending motion connected with the Warm Conveyor Belt, which is the reason that no cloudiness develops. In the shield type the ascent is much stronger: either this is the characteristic of the special situation or it is the consequence of a further development of the frontal system. If the frontal surfaces approach, then the isentropic surfaces become more and more inclined leading to a stronger ascent of the relative stream. This process can often be followed in the satellite images by the development of cloudiness in the warm sector of a Warm Front band culminating in the formation of the complete cloud shield (see Cloud structure in satellite image).

26 January 1995/12.00 UTC - Vertical cross section; black: isentropes (ThetaE), orange thin: IR pixel values, orange thick: WV pixel values
There is a distinct surface level and a developing upper level front. This configuration remains for the next 12 hours and is therefore also true for 27 January 00.00 UTC. The 306K isentropic surface chosen for the relative streams in the first three figures below belongs to the upper level frontal zone. The 296K isentropic surface chosen for the relative streams in the subsequent two figures belongs to the pronounced surface frontal zone.
26 January 1995/12.30 UTC - Meteosat IR image; blue: relative streams 296K - system velocity: 257° 20 m/s, green: isobars 296K, red: wind vectors 296K, position of vertical cross section indicated
26 January 1995/18.00 UTC - Meteosat IR image
27 January 1995/00.00 UTC - Meteosat IR image; blue: relative streams 296K - system velocity: 257° 20 m/s, green: isobars 296K, red: wind vectors 296K
This isentropic surface is characteristic of the situation and cloud development at the surface frontal zone.
The most striking difference to the upper level surface is the northward shift of the ascending Warm Conveyor Belt. This causes an overrunning of the frontal line (represented by the TFP) at a height layer of about 700 to 500 hPa.
The second relative stream from behind the Cold Front cannot be observed on this isentropic surface.
26 January 1995/12.30 UTC - Meteosat IR image; blue: relative streams 306K - system velocity: 257° 20 m/s, green: isobars 306K, red: wind vectors 306K
27 January 1995/00.00 UTC - Meteosat IR image; blue: relative streams 296K - system velocity: 257° 20 m/s, green: isobars 296K, red: wind vectors 296K
This isentropic surface is characteristic of the situation at the upper level front as well as for the warm sector.
Although this situation is rather difficult, representing the development and merging of several synoptic scale systems, a lot of features described in the meteorological physical background above can be detected.
Both satellite images with superimposed relative streams show a pronounced ascending warm conveyor belt (ascent from about 600 to 400 hPa). While at 12.00 UTC the warm sector is only partly cloudy (from Italy and Austria north- to eastward) it is already overcast at 00.00 UTC (from east Poland across the west Ukraine to Romania). The increase of cloudiness within the warm sector is manifested in the 18.00 UTC image in the relevant area from Poland to Hungary. While at 12.00 UTC the relative stream from south-west behind the Cold Front (France) accompanies the biggest part of the northern cloudiness (Denmark, south Sweden and the Baltic Sea) it is shifted north-north-eastward in the 00.00 UTC image and accompanies by now only the leading part of the shield.
18 February 1996/06.00 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 290K, representing the situation within the lower levels of the troposphere, and 298K for the upper levels.
18 February 1996/06.00 UTC - Meteosat IR image; magenta: relative streams 290K - system velocity: 305° 18 m/s, yellow: isobars 290K, position of vertical cross section indicated
18 February 1996/06.00 UTC - Meteosat IR image; magenta: relative streams 298K - system velocity: 305° 18 m/s, yellow: isobars 298K
The upper figure shows an ascending Cold Conveyor Belt from Switzerland across Germany and the North Sea to north Scotland and the Orkneys forming the cloudiness within the lower layers of the troposphere.
The lower figure shows within the Warm Front Shield an ascending Warm Conveyor Belt originating in front of the Cold Front over France (at approximately 750 hPa). Above south Denmark and north-east Germany (at approximately 550 hPa) it turns from south-western to south-eastern directions where it stops ascending above the Czech Republic (approximately between 530 to 500 hPa).

SUB-MENU OF WARM FRONT SHIELD
CLOUD STRUCTURE IN SATELLITE IMAGES
KEY PARAMETERS