Geoscience Reference
In-Depth Information
THE POLAR FRONT THEORY OF CYCLONES box 9.1
significant
20th-c. advance
The most significant and lasting contribution to synoptic meteorology in the twentieth century was made by the 'Bergen
school of meteorologists' led by Vilhelm Bjerknes working in Norway during the First World War. Isolated by the war
from other sources of information, they focused on careful, systematic analysis of synoptic weather maps and time cross-
sections of weather systems.
There were three components to the theory published between 1919 and 1922: a cyclone model (Jacob Bjerknes),
the idea of a cyclone life-cycle and frontal occlusion (Tor Bergeron) and the concept of cyclone families developing
along the polar front (Halvor Solberg). It was proposed that mid-latitude cyclones develop in conjunction with
frontogenesis as airstream convergence leads to boundaries developing between adjacent airmasses. The term front and
the concept of frontal occlusion were introduced into the meteorological vocabulary. They also outlined a cross-sectional
model of the distribution of clouds and precipitation in relation to frontal zones that is still widely used. In the 1930s,
Bergeron distinguished between ana- and kata-types of fronts, but these ideas were not used widely until the 1960s.
Although recent work has modified many aspects of the work of the Bergen school, several essential attributes have
been clarified and reinforced. For example, in the occlusion process, the warm front may become bent back in the form
of a T-bone, as noted originally by Bergeron. Theoretical and observational studies indicate that major cyclone elements
are conveyor belts that transport heat and moisture within the system and lead to cellular precipitation structures.
It is well recognized that not all mid-latitude cyclones develop in frontal wave families like those forming over the
oceans. Petterssen and Smeybe (1971) drew attention to the differences between waves that form in a frontal zone
over the North Atlantic (type A) and those forming over North America (type B). Continental development usually
involves cold air, possibly with an Arctic cold front, in an upper-level trough moving eastward over a zone of low-level
warm advection. Cyclogenesis can develop from a dry trough in the lee of the Rocky Mountains.
References
Petterssen, S. and Smeybe, S. J. (1971) On the development of extratropical cyclones. Quart. J. Roy. Met. Soc ., 97: 457-82.
Friedman, R. M. (1989) Appropriating the Weather. Vilhelm Bjerknes and the Construction of a Modern Meterology . Cornell
University Press, Ithaca, NY
1 Frontal waves
lation of the upper troposphere plays a key role in
providing appropriate conditions for their development
and growth, as shown below.
The typical geometry of an airmass interface, or front,
resembles a wave form (Figure 9.7). Similar wave
patterns are, in fact, found to occur on the interfaces
between many different media; for example, waves on
the sea surface, ripples on beach sand, eolian sand-
dunes, etc. Unlike these wave forms, however, the
frontal waves in the atmosphere are usually unstable;
that is, they suddenly originate, increase in size, and
then gradually dissipate. Numerical model calculations
show that in middle latitudes waves in a baroclinic
atmosphere are unstable if their wavelength exceeds
a few thousand kilometres. Frontal wave cyclones are
typically 1500 to 3000 km in wavelength. The circu-
2 The frontal-wave depression
A depression (also termed a low or cyclone) (see Note
2) is an area of relatively low pressure, with a more or
less circular isobaric pattern. It covers an area 1500 to
3000 km in diameter and usually has a life span of four
to seven days. Systems with these characteristics, which
are prominent on daily weather maps, are referred to as
synoptic-scale features. The mid-latitude depression is
usually associated with the convergence of contrasting
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