UPPER LEVEL LOW - METEOROLOGICAL PHYSICAL BACKGROUND

by ZAMG and FMI


UUpper Level Lows are closed cyclonically circulating eddies. Colder air is involved in the upper level low than in the surroundings (see Typical appearance in vertical cross sections ), which develop in the upper- and mid-levels of the troposphere. The scale of the Upper Level Low is generally smaller than that of extratropical cyclones during its mature stage. This cyclonic circulation is the consequence of its own life cycle and in the end stage is isolated from the main western stream. As these lows are only upper and mid-tropospheric features, they do not have a corresponding low within the lower levels of the troposphere, at least in the beginning (see Key parameters ).

The development of an Upper Level Low depends on the existence of unstable waves within the general flow in the upper levels of the troposphere. A characteristic feature for these unstable waves is the temperature wave (red dashed lines) being situated behind the potential wave (cyan solid lines). Due to this phase shift, two characteristic features can be observed:

The classical process of the life cycle of an Upper Level Low can be separated into four stages:

Upper Level Trough:

The assumption for the development of an upper level trough is unstable potential waves within this layer of the troposphere. As already described above, the temperature field is characterized by the temperature wave being situated behind the potential wave. Therefore CA can be found within the area of the upper level trough. During this stage of development the field of the absolute topography is characterized by an increase of the amplitude of the potential wave and sometimes also by a decrease of the wavelength. The same development takes place for the temperature wave. In the northern hemisphere a southward deviation of the isohypses and isotherms of the upper level trough can be observed leading to a deepening of the trough.

Tear-Off:

This stage of development is characterized by the development of an inverse omega-shape of the isohypses within the mid-levels of the troposphere (for instance within the height field at 500 hPa). The main meteorological process during this stage involves the trough starting to detach from the meridional stream. As a consequence of the further increase in the amplitude of the waves (further deepening of the trough) the cold air from the north streaming to southern regions will be cut from the general polar flow and the warm air from the south streaming to northern regions will be cut from the general subtropical flow. The consequence of this process is the development of a cold Upper Level Low within the southern part of the trough. The circulation of the low is characterized by closed isohypses and an eddy in the wind field at 500 hPa. But as this low is within its initial stage and therefore only weak, the main upper level flow still follows the inverse omega shape of the isohypses.

Cut-Off:

In contrast to the previous stage, the tie-off is finished and the Upper Level Low is now much more pronounced. The wind field at 500 hPa shows a well-developed closed circulation in the area of the former trough which in the ideal case is cut off from the general meridional flow.
23 January 1998/12.00 UTC - Meteosat IR image; cyan: height contours 500 hPa, dark green: temperature 500 hPa
24 January 1998/00.00 UTC - Meteosat IR image; cyan: height contours 500 hPa, dark green: temperature 500 hPa
24 January 1998/12.00 UTC - Meteosat IR image; cyan: height contours 500 hPa, dark green: temperature 500 hPa
The images above show the life cycle of an Upper Level Low. The first image (left image top) shows the upper level trough stage, the second image (right image top) the tear-off stage and the third image (left image bottom) the cut-off stage.

The change of temperature during the cut-off process is not only caused by horizontal advection of colder temperature but also by diabatic warming through the sinking motion of the cold air. In the case of a mature Upper Level Low the temperature of the cold air is constant or even decreases.

Diabatic heat transmission from the surface (for instance from the warmer sea surface) as well as from the lower to the upper levels of the upper level low causes two processes:

The displacement of the fully developed upper level low is very slow and therefore can be interpreted to be quasi-stationary. Its average life time is affected by diabatic heating, which destroys the thermal structure, and is limited to three or four days.

Final stage:

Within the Upper Level Low there is convection, unless the surface is very cold. The air near the surface is warm and the circulation is slowed down by the friction. The convection brings warm air and the effect of the friction upwards. Consequently, the Upper Level Low weakens slowly. In most cases the Upper Level Low merges with the main stream before it has completely dissolved by the convection. Usually a large trough in the main stream approaches from the rear and catches the upper level low. After that the Upper Level Low appears as a small Wave within the trough for a short while, and soon disappears.

The Upper Level Low can also merge with another Upper Level Low, and the main stream then catches up this combination of the Upper Level Lows.

If the Upper Level Low is far from the main stream, it can dissolve solely by convection. This kind of development occurs mostly in southern areas; in Europe they can sometimes be found over the Mediterranean.

Over a very cold surface the convection is not triggered. In this case the Upper Level Low will not be dissolved until it is either caught up by the main stream or drifts over a warm surface.

Example of the caught-up process: An Upper Level Low over England is moving slowly westwards and is caught up by a large trough over the Atlantic.

08 May 2001/12.00 UTC - Meteosat IR image; cyan: height contours 500 hPa, dark green: temperature 500 hPa
09 May 2001/12.00 UTC - Meteosat IR image; cyan: height contours 500 hPa, dark green: temperature 500 hPa
10 May 2001/12.00 UTC - Meteosat IR image; cyan: height contours 500 hPa, dark green: temperature 500 hPa
If the Upper Level Low is large, the baroclinicity is strong and the surface is warm, there is baroclinic development in the leading edge of the Upper Level Low. A low deepens also on the surface and the cyclone moves counterclockwise around the large Upper Level Low. Usually there are several cyclones developing after each other. The large Upper Level Low weakens gradually, and finally the main stream catches up its remains.
This kind of development takes place in about 15% of Upper Level Low cases. The development lasts typically 3-10 days.

Example of the baroclinic development:

On 16 September 2001/12.00 UTC a large Upper Level Low over Germany and Denmark is cut off from the main stream. On 18 September 2001/00.00 UTC there is strong baroclinic development along the edges of the low: three cyclones can be seen over France, Denmark and Poland. The development goes on for several days and the Upper Level Low weakens slowly. On 25 September 2001/06.00 UTC it has almost dissolved and the remains over England are caught up by another upper level low over the Atlantic.

16 September 2001/12.00 UTC - Meteosat IR image; cyan: height contours 500 hPa, dark green: temperature 500 hPa
18 September 2001/01.00 UTC - Meteosat IR image; cyan: height contours 500 hPa, dark green: temperature 500 hPa
23 September 2001/12.00 UTC - Meteosat IR image; cyan: height contours 500 hPa, dark green: temperature 500 hPa
25 September 2001/06.00 UTC - Meteosat IR image; cyan: height contours 500 hPa, dark green: temperature 500 hPa

SUB-MENU OF UPPER LEVEL LOW
CLOUD STRUCTURE IN SATELLITE IMAGES
KEY PARAMETERS