Upper level lows are closed cyclonically circulating eddies. 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 westerly stream. As they are upper and mid-tropospheric features, they do not necessarily have a corresponding low within the lower troposphere, at least in the beginning stages.
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.

The classical process of the life cycle of an upper level low can be separated into four stages:

• upper level trough
• tear-off
• cut-off
• final stage

Upper level trough

The assumption for the development of an upper level trough is the existence of unstable potential waves within this layer of the troposphere. The temperature field is characterized by the temperature wave being situated behind the geopotential 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 development of the temperature wave is similar. In the northern hemisphere a southward deviation of the isohypses and isotherms of the upper level trough can be observed leading to the deepening of the trough.

Tear-off

This stage of life-cycle is characterized by the development of an inverse omega-shape of the isohypses within the mid-levels of the troposphere (for instance within the 500 hPa height field). The main meteorological process during this stage involves detachment of the trough 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 still in its initial stage and therefore very weak, the main upper level flow still follows the inverse omega shape of the isohypses.

Cut-off

In contrast to the previous stage, the tear-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 is, in the ideal case, cut off from the general meridional flow. 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 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 lower to higher levels of the upper level low causes two processes:

• first of all, the diabatic heat transmission causes instability within the core of the upper level low, which often leads to the development of Cb cloudiness. Unstable regions appear, mostly within two areas which are predestined for strong convective activity: in the centre of the low - cold air cloudiness, core convection and in front of the low - centre within the area of the thickness ridge.
• on the other hand, the colder air within the core becomes continuously warmer. As a consequence, the temperature gradient of the air between the core and the surroundings diminishes. As the result, the cyclonic circulation first weakens and then after some time dissipates. But often redevelopment takes place and the low is incorporated again into the general flow.

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 convection appears, 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 is 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 low. 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.

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References

• Manual of Synoptic Satellite Meteorology