Upper Level Low - Key Parameters

by ZAMG and FMI

Height contours 500 hPa:
In the initial stage there is an upper trough. The bottom of the trough forms an inverse omega shape and is detached from the main stream forming a separate low. In the final stage the low weakens and merges with another trough or low.
Temperature 500 hPa:
The air within the Upper Level Low is colder than in the surroundings. The isotherms shows a life cycle similar to that of the geopotential.
Height contours 1000 hPa:
Usually loose field with no remarkable features. In some cases some weak cyclonic circulation may appear.
Equivalent thickness:
There is a thickness ridge ahead of the low and a trough or a distinct minimum behind or in the centre of the low.
Thermal front parameter:
There are two baroclinic zones:
- A frontal-like cloud band ahead of the low
- A Baroclinic Boundary (see Baroclinic Boundary ) behind the low.
Potential vorticity:
Within the Upper Level Low there is colder air with lower tropopause than in its surroundings. Consequently, in the area of the low there is a local maximum of PV.

Height contours and temperature 500 hPa

1. Trough stage

	14 September 2005/06.00 UTC - Meteosat 8 IR 10.8 image; cyan: height contours 500 hPa, dark green: temperature 500 hPa

2. Tear-off stage

	14 September 2005/12.00 UTC - Meteosat 8 IR 10.8 image; cyan: height contours 500 hPa, dark green: temperature 500 hPa

3. Cut-off stage

	15 September 2005/12.00 UTC - Meteosat 8 IR 10.8 image; cyan: height contours 500 hPa, dark green: temperature 500 hPa

4. Final stage

	16 September 2005/12.00 UTC - Meteosat 8 IR 10.8 image; cyan: height contours 500 hPa, dark green: temperature 500 hPa

	15 September 2005/12.00 UTC - Meteosat 8 IR 10.8 image; magenta: height contours 500 hPa, cyan: height contours 1000 hPa

	15 September 2005/12.00 UTC - Meteosat 8 IR 10.8 image; green: equivalent thickness, blue: thermal frontal parameter

	15 September 2005/12.00 UTC - Meteosat 8 WV 6.2 image; magenta: height of PV=1 unit