Appearance in Satellite Data

Upper Level Lows are relatively long-lived (mostly from 2 to 10 days) phenomena. Their life cycle consists of three stages:

  • Upper level trough
  • Tear-off
  • Cut-off
  • Final stage

1. Upper level trough stage

There is a pronounced upper level trough behind a frontal zone.

  • In IR10.8 images white or light grey cloud band connected to the frontal zone on the leading side of the trough, some thin white cloud stripes due to Cloud Fibres on the rear side and possibly some convective white cells around the axis of the trough.
  • In VIS0.6 images white or light grey cloud band connected to the front on the leading side of the trough and possibly some convective white cells around the axis of the trough.
  • In WV6.2 images light grey bands on the leading and rear part of the trough.

2. Tear-off

The bottom of the upper trough is detached from the main stream resulting in a closed circulation.

  • In IR10.8 images white to light grey cyclonically curved cloud band on the leading side of the trough, possibly also on the rear side. Some white convective shells may occur between the bands.
  • In VIS0.6 images grey cyclonically curved cloud band on the leading side of the trough, possibly also on the rear side. Some white convective shells may occur between the bands.
  • In WV6.2 images grey cyclonically curved cloud band around the detaching low.

3. Cut-off stage

The Upper Level Low is separated from the main upper stream.

  • In IR10.8 and VIS0.6 images white to light grey cyclonically curved cloud bands on the leading and rear side of the low, later forming a spiral. Possibly some white convective cells within it.
  • In WV6.2 images round area or a spiral of grey, with some white cells on the leading side.
  • In the WV6.2-WV7.3;IR9.7i-IR10.8i;WV6.2 combination ("airmass") the descending stratosperic air in the centre of the Upper Level Low appears dark red, and the cold pool appears bluish
  • In WV6.2-WV7.3;IR3.9i-IR10.8i;NIR1.6i-VIS0.6 combination ("convective storms") convective clouds appear red; strong, growing cells are yellow.

4. Final stage

The Upper Level Low merges with the main stream, or dissolves slowly while being almost stationary.

  • In IR10.8 and VIS0.6 images there are light grey cyclonically curved stripes that merge with a white cloud band of a frontal zone.
  • In WV6.2 images there is a dark grey area that soon disappears under the light grey of the frontal zone.

1. The cloudiness belonging to the main stream is approaching
2. The ULL cloudiness merges with the frontal cloudiness

On the 14th of September 2005 at 06.00 UTC there is an elongated upper trough over Southern Spain and Gibraltar.

14 September 2005/06.00 UTC - Meteosat 8 IR 10.8 image
14 September 2005/06.00 UTC - Meteosat 8 WV 6.2 image

6 hours later tear-off occurs over Southern Spain.

14 September 2005/12.00 UTC - Meteosat 8 IR 10.8 image
14 September 2005/12.00 UTC - Meteosat 8 WV 6.2 image
14 September 2005/12.00 UTC - Meteosat 8 HIRVIS image

On the 15th of September 2005 at 12.00 UTC the separate Upper Level Low is clearly seen with core convection.

15 September 2005/12.00 UTC - Meteosat 8 IR 10.8 image
15 September 2005/12.00 UTC - Meteosat 8 WV 6.2 image
15 September 2005/12.00 UTC - Meteosat 8 HIRVIS image
15 September 2005/12.00 UTC - Meteosat 8 Airmass RGB image
15 September 2005/12.00 UTC - Meteosat 8 Convection RGB image

On the 16th of September 2005 at 12.00 UTC the Upper Level Low is merging with an upper trough coming from the Atlantic.

16 September 2005/12.00 UTC - Meteosat 8 IR 10.8 image
16 September 2005/12.00 UTC - Meteosat 8 WV 6.2 image
16 September 2005/12.00 UTC - Meteosat 8 HIRVIS image

Six hours later there is only a trough left.

16 September 2005/12.00 UTC - Meteosat 8 IR 10.8 image
16 September 2005/12.00 UTC - Meteosat 8 WV 6.2 image

The air below an Upper Level Low is potentially unstable, which leads to the so-called core convection and convective cloudiness. Over warm sea this convective development can be intensive. Contrary to this, the centre of an Upper Level Low over land is often overcast with low or middle level cloudiness with some convective cells embedded.

Other conceptual models that may look like an Upper Level Low in satellite images are Comma (see Comma ) and Polar Low (see Polar Low ). These can be separated from each other with the help of numerical fields, especially on the 500 hPa level.