Cloud Structure In Satellite Images

The term Baroclinic Boundary is used to describe a front - like cloud band which is located differently when compared to that associated with a front. It can also be distinguished from stationary fronts and air mass boundaries. A Baroclinic Boundary has no significant propagation. The investigation is based on 60 cases.

The appearance of Baroclinic Boundary in different channels:

  • IR imagery:
    • Dark to light grey low and middle level cloud band
    • The Baroclinic Boundary usually appears darker (i.e. warmer cloud tops) than frontal cloud
    • Sometimes a bright jet cloud fibre forms or fibre clouds are embedded within the cloudband
  • WV imagery:
    • The Baroclinic Boundary appears as a dark grey to grey cloud band with sometimes bright high fibres embedded
    • The Baroclinic Boundary generally appears weaker in WV than a frontal cloud band
  • VIS imagery:
    • Depending on the type of cloud the VIS image shows low white patches or cloud bands. If existing, thin high cloud fibres appear grey.

The investigation showed Baroclinic Boundaries at several typical locations (e.g. indicated by the height field at 500 hPa):

  • At the rear of a synoptic scale trough (most frequent type, ~ 50 cases, schematic below left)
  • Baroclinic Boundary associated with ULL (6 cases, schematic below middle)
  • Baroclinic Boundary associated with a deformation zone at a saddle point of upper height fields (3 cases, schematic below right)

 

At the rear of a synoptic scale trough

The set of images below shows a representative case of a Baroclinic Boundary at the rear of a synoptic scale trough extending from the Bay of Biscay to the Tyrrhanian Sea. The position at the rear of a synoptic scale trough is clearly indicated by the heights at 500hPa (below left).

19 October 2002/06.00 UTC - Meteosat IR image; cyan: height contours 500 hPa
19 October 2002/06.00 UTC - Meteosat IR image; SatRep overlay: names of conceptual models
19 October 2002/06.00 UTC - Meteosat WV image; names of conceptual models
19 October 2002/09.00 UTC - Meteosat VIS image; names of conceptual models

The WV and the VIS images above clearly show the lower cloud tops in the Baroclinic Boundary compared to the CF over the Atlantic. High white fibre clouds can be seen which are reflected in both - WV and IR - images, indicating the jet axis. This type of Baroclinic Boundary can be distinguished as two types, according to development:

 

"In situ" development (~15 cases)

  • Initially low clouds and Stratocumulus sheets exist, which are merging and intensifying (see schematics below, stage 1 - 3)
  • Further development may show either dissipation of the Baroclinic Boundary or a merging with the succeeding frontal system
  • The Baroclinic Boundary cloud band appears either as connected to a WF upstream and/or a CF downstream or can even be a separated cloud feature

18/07.00 - 19/07.00 UTC 3-hourly image loop
18 October 2002/07.00 UTC - Meteosat IR image

The image loop above shows the in situ development of a Baroclinic Boundary starting over N. Spain and protruding into the western Mediterranean.

 

Modification of former frontal cloud (~ 35 cases)

The initiation of this type of Baroclinic Boundary is the modification of a former frontal zone. Such modifications have also been observed from former Occlusion (19 cases), from former elongating Cold Front (12 cases) and from former Warm Fronts or Detached Warm Fronts (4 cases).

The schematics below (stage 1 - 3) outline a typical process: A new frontal system is appproaching the rear of a dissolving and/or elongating old cloud band. The old cloud system splits in two or more parts, where the northern part merges with the new system or is elongating and weakening. The southern part of the old cloud band redevelops to the rear of the trough in the gradient of equivalent thickness. The further development shows a general merging with the succeeding system or weakening and dissolution of the Baroclinic Boundary region.

The set of IR images shown below is characteristic of a development of a Baroclinic Boundary by frontal modification. Below left the initial stage of the process is shown, whereas below right a more developed stage can be seen 12 hours later.

The loop of the IR images shows an old system stretching from Great Britain to Iceland and a newly developed system over the Atlantic. In the loop the splitting of the old cloudband can be observed as well as the redevelopment and the positioning of the southern part at the rear of the upwind trough. The newly developed Baroclinic Boundary is finally positioned over Great Britain and northern France (image below right).

30/06.00 - 31/06.00 UTC 3-hourly image loop
30 January 2001/06.00 UTC - Meteosat IR image
31 January 2001/06.00 UTC - Meteosat IR image; SatRep overlay: names of conceptual models

 

Baroclinic boundary associated with ULL

Synoptic scale cloud bands often appear to the rear of and at the leading edge of an Upper Level Low (see Upper Level Low ). They mostly consist of multilayered low and middle clouds and are cyclonically curved. Furthermore, a third zone with high baroclinicity is likely to appear at the north-easternmost part of the Upper Level Low ("Tear - off point"). This part is usually the remains of a former CF and shows as a highly deformed and elongated fibrous cloud band. According to its appearance this cloud band resembles that of the CM of the Deformation Band (see Deformation Band ).

The two IR images below show a case of an Upper Level Low with two well developed Baroclinic Boundaries associated with it, and a Deformation Band near the Tear - off point. The centre of the Upper Level Low is situated over the Aegean Sea and Greece (see Height at 500 hPa, left image below).

05 February 2002/06.00 UTC - Meteosat IR image; cyan: height contours 500 hPa
05 February 2002/06.00 UTC - Meteosat IR image; SatRep overlay: names of conceptual models

The formation of the Baroclinic Boundary takes place in parallel with the development of the Upper Level Low.

 

Baroclinic Boundary associated with a deformation zone at a saddle point of upper height fields

This type is the least common, and can be associated with a "cut - off" process within upper level troughs and ridges. Contrary to the other types described above, the Baroclinic Boundary exists in the frontogenetic/frontolytic areas at the saddle point of four pressure systems. Nearly all cases show a modification of former frontal cloud bands consisting of multilayered low and middle cloudiness which is elongating and gradually dissipating. The two IR images below show a Baroclinic Boundary within a characteristic deformation zone at the saddle point of height fields at 500 hPa (lower left image). It extends from S. France to Ireland and consists of low and middle level multilayered grey clouds. It can clearly be distinguished from all surrounding frontal cloud bands, by its brightness and curvature. The northern part of the old system from which the Baroclinic Boundary is now torn - off can still be seen in front of the trough north of Scotland.

04 April 2002/06.00 UTC - Meteosat IR image; cyan: height contours 500 hPa
04 April 2002/06.00 UTC - Meteosat IR image; SatRep overlay: names of conceptual models

he development of this type of Baroclinic Boundary normally takes place during the "cut - off" process of a trough into an upper level depression.