Chapter II: The Life Cycle of Upper-Level Lows
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.
Rossby Waves
The polar jet stream is a good indicator of the location of the polar front that separates the cooler polar air mass from warmer subtropical air masses. Jet streams are found in places with a strong horizontal temperature gradient. The meandering band of the jet around the globe follows the pattern of Rossby waves closely.
Rossby waves and thus the jet stream show varying wave amplitudes in space and time. They can have a strong zonal component as indicated in Figure 1a or show a strong meridional component as seen in Figure 1c.
Figure 1: Planetary Rossby waves with increasing amplitude (from a to c). Blue represents the polar air mass, and orange the warmer air masses. The pink ribbon marks the position of the jet stream. © Fred the Oyster, CC-BY-SA.
The shape of Rossby waves has a direct impact on the propagation speed of low-pressure systems that form along the polar front. Low pressure systems move faster with westerly winds and tend to remain stationary in the case of high amplitude Rossby waves.
Why Do Rossby Waves Form?
The Coriolis force is a pseudo force that acts on any air parcel that does not move parallel to the axis of rotation of the earth. This force increases with wind speed and is null when the air parcel is immobile. Hence, the Coriolis force acts in particular on the ribbon of the jet stream that circles the Earth. Additionally, pressure gradient forces also affect the path of the jet stream. Differential heating of the Earth's surface influences the repartition of high and low-pressure areas. Finally, orographic features such as mountain ranges can influence the flow of the jet around the globe.
All these factors act together and possibly lead to an amplification of Rossby waves as indicated in Figure 1.