Front Decay describes a substructure in Cold Front cloud bands. Satellite images show a temporary or complete meso scale to subsynoptic scale dissipation of cloudiness within a frontal cloud band. The life cycle of this mechanism ranges from 6 to 24 hours. The investigation was based on 48 cases, where 27 cases were adressed in detail.

Appearance in METEOSAT imagery:

• IR imagery:
  • The cloud decay generally starts from the colder and dryer rear side of the frontal band, showing an increasing dark or even black area in the IR image
  • At an early stage dissipation takes place within high and middle-level clouds, leaving a remaining dark grey stripe associated with lower cloudiness;
  • Sometimes a clear bright jet cloud fibre forms at the edge of the cloud gap;
  • physical processes, indicated by relative streams, leading either to dissipation of low clouds during the mature stage of the process, or the cloud gap will fill again.
• WV imagery:
  • The light grey WV band is darkening from the rear edge;
  • The greyshade of the dissolution zone in WV differ depending on the characteristics of individual cases, but a weak WV band structure remains at least in all investigated cases.
• VIS imagery:
  • The VIS image shows in most cases low cloudiness remaining even in an advanced state of FD.
  • Complete dissolution in a later stage of the life cycle takes place only in 14% of the cases.

The set of IR images shown below is characteristic for Front Decay. To the left the initial stage of the process is shown, whereas in the right picture a more developed stage 6 hours later can be seen.

The loop of the IR images shows that the dissolution of high and middle cloudiness starts at the rear side of the frontal cloudband, and the gap increases with every timestep. Only a low narrow cloudband (dark grey) remains. In this case the development of a bright jet cloud fibre at the rear edge over the dissolution zone can be observed. After 02.00 UTC in the IR image loop of 08 October 1999 the cloud gap is closing again. This mechanism can often be observed and will be explained in the chapter of meteorological physical background.

The images below show the typical appearance of Front Decay in WV imagery.

In the left image the cloud disspiation from the rear side of the front leads to a darkening of the WV band. At the mature stage, shown below right, a darker sub synoptic scale area indicates the dissipation zone, but the band structure remains in WV image. The bright fibre indicating the jet axis can also be seen in the WV images.

In the VIS image loop below (Case study 03 June 2000) a complete dissipation of frontal cloudiness over Poland can be seen. The loop starts with an already advanced stage of decay. A distinct cloud gap to the rear of the former frontal cloudband can be observed, which leads to a complete cloud dissipation at 12.00 UTC.