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Subject: A7) What is an extra-tropical cyclone ?
Contributed by Stan Goldenberg
An extra-tropical cyclone is a storm system that primarily gets
its energy from the horizontal temperature contrasts that exist
in the atmosphere. Extra-tropical cyclones (also known as
mid-latitude or baroclinic storms) are low pressure systems with
associated cold fronts, warm fronts, and occluded fronts.
Tropical cyclones, in contrast, typically have little
to no temperature differences across the storm at the surface and
their winds are derived from the release of energy due to cloud/rain
formation from the warm moist air of the tropics (
Holland 1993, Merrill
1993).
![](trop-extra.jpg)
The top schematics show horizontal maps of the surface temperature
and wind fields associated with a tropical cyclone (left) and an
extratropical cyclone (right). Colors indicate temperature (blue
15C=59F, blue green 20C=68F, green 25C=77F). Solid lines indicate
surface windspeeds - 34 kt=39 mph=63 kph and 64 kt=74 mph=117 kph .
The bottom schematics show vertical maps of the pressure surfaces
and circulation at the surface and tropopause.
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Structurally, tropical cyclones have their strongest winds near the
earth's surface , while extra-tropical cyclones have their strongest
winds near the tropopause - about 8 miles (12 km) up. These
differences are due to the tropical cyclone being "warm-core" in the
troposphere (below the tropopause) and the extra-tropical cyclone
being "warm-core" in the stratosphere (above the tropopause) and
"cold-core" in the troposphere. "Warm-core" refers to being relatively
warmer than the environment at the same pressure surface ("pressure
surfaces" are simply another way to measure height or altitude).
Often, a tropical cyclone will transform into an extra-tropical cyclone
as it recurves poleward and to the east. Occassionally, an
extra-tropical cyclone will lose its frontal features, develop
convection near the center of the storm and transform into a
full-fledged tropical cyclone. Such a process is most common in
the North Atlantic and Northwest Pacific basins. The transformation
of tropical cyclone into an extra-tropical cyclone (and vice versa)
is currently one of the most challenging forecast problems
(i.e., Jones et al. 2003).
References:
Jones, S.C., Harr, P.A., Abraham, J., Bosart, L.F., Bowyer, P.J.,
Evans, J.L., Hanley, D.E., Hanstrum, B.N., Hart, R.E., Lalaurette, F.,
Sinclair, M.R., Smith, R.K., Thorncroft, C. 2003: The Extratropical
Transition of Tropical Cyclones: Forecast Challenges, Current
Understanding, and Future Directions. Weather and Forecasting,
18, 1052-1092.
Merrill, R. T., (1993): "Tropical Cyclone Structure" - Chapter 2,
Global Guide to Tropical Cyclone Forecasting, WMO/TC-No. 560,
Report No. TCP-31, World Meteorological Organization; Geneva,
Switzerland
Web version of Guide
Last updated August 13, 2004
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