Geoscience Reference
In-Depth Information
9.1 The polar front theory of cyclones
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
World War I. 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 during 1919-1922: 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 air masses. 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 widely used until the 1960s. Although recent work has modified many aspects of
the ideas 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, with possibly 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
Friedman, R.M. (1989) Appropriating the Weather. Vilhelm Bjerknes and the Construction of a Modern Meterology, Cornell
University Press, Ithaca, NY, 251pp.
Petterssen, S. and Smeybe, S.J. (1971) On the development of extratropical cyclones. Quart. J. Roy. Met. Soc. 97, 457-82.
gradually dissipate. Numerical model calculations
show that in mid-latitudes waves in a baroclinic
atmosphere are unstable if their wavelength
exceeds a few thousand kilometers. Frontal wave
cyclones are typically 1500-3000km in wave-
length. The circulation of the upper troposphere
plays a key role in providing appropriate
conditions for their development and growth, as
shown below.
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 1500-3000km in diameter and usually
has a lifespan of four to seven days. Systems with
these characteristics, which are prominent on
daily weather maps, are referred to as
synoptic-
