UPPER LEVEL LOW

written by ZAMG

UNDER CONSTRUCTION

CLOUD STRUCTURE IN SATELLITE IMAGE

Upper level lows are long-lived phenomena which show a typical life cycle. The cloud structure of an upper level low in the IR, VIS and WV images is indicative of the present stage of this life cycle (compare meteorological physical background).

• Upper level trough stage:
  This is the stage where a pronounced upper level trough behind a frontal cloud band still exists. The satellite image shows a well-developed frontal cloud band at the leading branch of the upper level trough. In the VIS and IR image this cloud band is white, indicating thick and multilayered cloudiness. The rearward branch of the upper level trough might be accompanied by some fibrous cloudiness which normally is connected to the following warm front. In contrast to the frontal cloud band these cloud fibres are high level cloudiness and can therefore only be seen in the IR and the WV image. The WV image shows two wet zones (light grey WV bands) which are situated within the leading and rearward part of the upper level trough. The centre of the trough around the axis is characterized by dry air, which appears in black in the WV images. Cold air cloudiness may also exist, but does not show in the WV image.
  
  
  

• Tear-off stage:
  This is the stage where the upper level trough becomes more and more pronounced, ultimately to be torn off from the more frontal stream. Consequently this stage of development is characterized by the frontal cloud band being torn off in the north-easternmost part of the trough (area which is called "tear off" in the schematics below). That part of the frontal cloud band which remains at the leading part of the upper level trough starts to curve cyclonically. In some cases a cyclonically curved cloud band develops also at the rearward branch of the upper level trough. This cloudiness is caused predominantly by convergence within the mid- and upper levels of the troposphere. This cloudiness appears in the IR image as light grey to white, while in the VIS image it is dark grey to grey. The WV image shows a distinct cyclonically curved band of moist air. Besides the cold air features the centre of the trough is dark grey to black, indicating dry and sometimes even stratospheric air (compare meteorological physical background). While the cloud structure in the IR image is restricted to the area of the torn off upper level trough, the WV features extend beyond this area indicating the humidity sources.
  
  
  

• Cut-off stage:
  This is the stage where the former upper level trough has been cut-off and become a separate upper level low. In the cut-off stage the cyclonic curvature of both cloud bands becomes stronger. In the WV image often a cyclonic spiral develops from the edge in the centre of the upper level low.
  
  
  

(compare also chapter WATER VAPOUR EYE EY )

The images above show the first stage of the development of an upper level low, the upper level trough stage. In the IR as well as in the WV image a cyclonically curved cloud band can be found within the eastern edge of the trough extending from Belgium across Germany to the Baltic Sea.


Left: 24 January 1998/06 UTC - IR image
Right: 24 January 1998/06 UTC - WV image

Six hours later the tear-off stage is reached. The IR image (left image) clearly shows that the cloud band is torn off from the north-eastern situated cloud band above the south-east coast of Sweden (at approximately 57°N/16°E). The WV image (right image) shows a moist band extending from the Atlantic across France, Germany, the Baltic Sea to north-west Russia. Within the area where the tear-off process takes place the moist cloud band is characterized by a wave structure. The lower pixel values above south Sweden (at approximately 57°N/15°E) indicate the beginning of cyclonic circulation of the upper level low.


Left: 24 January 1998/12 UTC - IR image
Right: 24 January 1998/12 UTC - WV image

At 12 UTC the upper level low has reached its cut-off stage. The centre of the low is situated above France at approximately 52°N/11°E. The cyclonically curved cloud band extending from France across the Czech Republic to Poland is mostly characterized by high pixel values in all three channels (IR, WV and VIS images) indicating thick and multilevel cloudiness. In contrast to this the centre of the low is overcast by mostly low level cloudiness which appears white in the VIS and grey in the IR image.


Left: 5 August 1998/06 UTC - IR image
Right: 5 August 1998/12 UTC - VIS image

The images show a well-developed upper level low which has already reached the cut-off stage above the Mediterranean Sea between Sardinia and Tunisia. The IR (left image top) and VIS image (right image top) show a cyclonically curved cloud spiral extending from Tunisia across the Tyrrhenian Sea and the Mediterranean Sea to the south coast of Sardinia. Embedded within the frontal cloud band and in the centre of the low Cb cloudiness (core convection) can be observed. The centre of the low can best be localized in the WV image above the Mediterranean Sea at approximately 38°N/09°E). Within this area cellular cloud clusters can be found in the WV image.


Left: 24 September 1998/00 UTC - IR image; lines: cyan: height contours at 500 hPa
Right: 24 September 1998/06 UTC - IR image; lines: cyan: height contours at 500 hPa

The satellite image at 00 UTC shows the main upper level low above northern Spain and the west coast of France. The centre of the low, which is also indicated by the development of convective cells, can be found above the Bay of Biscay at approximately 44°N/03°W. A secondary cyclonic circulation has developed at the edge of the main cyclonic circulation above north-west France at approximately 48°N/04°W).
Six hours later this secondary has moved above south-west England producing mid-level cloudiness above the English Channel.

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