Appearance in Satellite Data

As already mentioned in the general remarks the cloud band of the Occlusion described in this chapter develops in the area of a Wave (see Wave ) but shows, in contrast to the Occlusion of the Warm Conveyor Belt type (see Occlusion: Warm Conveyor Belt Type ), clearly different cloud layers.

In a fully developed stage the satellite image shows two synoptic scale cloud bands:

  • a multilayered frontal cloud band of a Cold Front
  • a lower cloud spiral which seems to penetrate from below the higher cloud band
  • both cloud bands seem to be uncoupled
  • In the VIS image the cloud spiral is white indicating deep cloudiness with high albedo.
  • In the IR image the grey shades of a cold conveyor belt cloud band are spoken rather grey but with some increased white areas superimposed. The following points can be summarized:
    • At the transition between the cloud band of the Cold and Warm Front (oriented south-west to north-east) and the cloudiness of the cold conveyor belt type, a distinct gradient from white to (dark) grey can be observed. After some distance from the transition zone often cloud tops gradually become higher (whiter) followed again by a decrease of cloud tops within the innermost part of the cloud spiral. Such a cloud structure can be explained quite well with the conveyor belt theory (see Meteorological physical background ).
    • During the later stages of development, also high cloudiness can develop leading to a similar appearance to the Warm Conveyor Belt type of the Occlusion (see Meteorological physical background ).
  • In the WV image the two cloud bands with different cloud types are seen even more distinctly. A black stripe characterizing the dry air on the cyclonic side of the jet axis extends parallel to the higher cloud band and crosses the cold conveyor belt cloudiness immediately at the transition zone. This effect cannot be detected in the case of an Occlusion of the Warm Conveyor Belt type. The driest air represented by very black pixel values is included in the spiral development. This fact can be observed in both types of Occlusion (see Occlusion: Warm Conveyor Belt Type ). During the later stages of development high and bright pixel values can be observed within the whole area of the cloud spiral which means that the crossing black stripe vanishes.

On the 1st of December a Comma cloud was located over Ireland, and a Cold Front extends from Norway to England and further southwest. The Instant Occlusion process took place during the next 12 hours.
In the following images the initial and final stages are shown in different channels.

21 June 2005/09.00 UTC - Meteosat 8 IR 10.8 image
21 June 2005/09.00 UTC - Meteosat 8 IR 3.9 image
21 June 2005/09.00 UTC - Meteosat 8 Vis 0.6 image
21 June 2005/09.00 UTC - Meteosat 8 HRVIS image
21 June 2005/09.00 UTC - Meteosat 8 WV 7.3 image
21 June 2005/09.00 UTC - Meteosat 8 WV 6.2 image

The penetration of the cloud band of the Cold Conveyor Belt cloudiness from beneath the south-west to east-north-east oriented frontal cloud band is well developed and can be seen at 09.00 UTC above the Atlantic Ocean at approximately 54N/35W.
The IR-images (10.8 and both 3.9 µm) show quite clear the lower temperature of the Cold Conveyor Belt clouds.
In the visible images (Highres Vis and 0.6 µm) the shadows of the Warm and Cold Front clouds on the Occlusion clouds are quite pronounced.
The WV-images (7.3 and 6.2 µm) show a small dark stripe as separation of the Warm and Cold Front cloudiness and the Occlusion.

21 June 2005/00.00 UTC - Meteosat 8 RGB image (3.9, 10.8 and 12.0)
21 June 2005/09.00 UTC - Meteosat 8 RGB image (0.6, 1.6 and 10.8)
21 June 2005/09.00 UTC - Meteosat 8 RGB image (0.6, 0.8 and 10.8)

During nighttime the RGB-image, RGB 3.9 - 10.8 - 12.0 µm will enhance the difference between low warm clouds (brownish) and the high cold cloudiness (whitish).
The daytime RGB-image, RGB 0.6 - 1.6 - 10.8 µm, shows especially the different water and ice clouds (yellow and pink).
The daytime RGB-image, RGB 0.6 - 0.8 - 10.8 µm, enhances the contrast between the low (yellow) and the high cold clouds (bluish white).

20 June 2005 22.00 UTC - Meteosat 8 RGB image (3.9, 10.8 and 12.0)
21 June 2005 06.00 UTC - Meteosat 8 RGB image (0.6, 1.6 and 10.8)
  Loop: 01/12.00 - 02/12.00 UTC three-hourly image   Loop: 01/12.00 - 02/12.00 UTC three-hourly image

In the loop of the RGB-images (0.6-0.8-10.8 Ám) the low cloudiness (yellow) in the center of the low and beneath the high cloudiness of the occlusion can be seen quite good.

21 June 2005 06.00 UTC - Meteosat 8 RGB image (0.6, 0.8 and 10.8)
  Loop: 01/12.00 - 02/12.00 UTC three-hourly image