Upper Level Low - Meteorological Physical Background

by ZAMG and FMI

Upper Level Lows are closed cyclonically circulating eddies in the middle and upper troposphere. They are sometimes also called "cold drops", because the air within an Upper Level Low is colder than in its surroundings.

The development of a typical Upper Level Low goes through four stages, during which a bottom of an upper trough is detached from the main stream, until it finally fills up or merges with another trough:

  1. Upper level trough
  2. Tear-off
  3. Cut-off
  4. Final stage

1. Upper Level Trough stage

The prerequisite of the forming of the Upper Level Low are unstable waves within the main stream, where the temperature wave is behind the geopotential wave.
Cyan: 500 hPa geopotential height, green: 500 hPa temperature

2. Tear-off stage

3. Cut-off stage

4. Final stage

Within an 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 friction upwards. Consequently, the Upper Level Low weakens slowly.
For an example see Key Parameters.

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 be found over the Mediterranean.

Upper Level Lows can be divided into two classes according to their size and lifetime:

Big lows are slightly more common than small ones.

Note that over land an Upper Level Low can also form when a surface low of an extratropical cyclone disappears due to friction. This is just a late stage of a cyclone development and the upper low fills up relatively quickly.

Menu Of Upper Level Low
Cloud Structure In Satellite Images
Key Parameters