The Life Cycle of Upper-Level Lows
The life cycle of an upper-level low is usually divided into four stages (see also the Manual of Synoptic Satellite Meteorology):
- The upper-level trough
- The tear-off
- The cut-off
- The dissipation stage
The following chapters of this module will discuss this upper-air phenomenon stage by evolution stage. The physical processes that lead to each stage of development will be taken into account, as will their manifestation in satellite and NWP data. Note that over land an upper-level low can also form when the surface low of an extratropical cyclone disappears due to friction. In that case the ULL is merely a product of the final (dissipation) stage of a cyclone and the upper part of the system fills up relatively quickly.
As a starting point for our observations on upper-level lows, let's have a look at the polar front, and more precisely the strong winds that accompany such a strong meridional temperature gradient zone: the jet stream.
The jet stream and its meandering path around the globe plays an important role in the creation of upper-level lows. The path of the jet stream is closely linked to the pattern specified by Rossby waves, also called planetary waves.