ESCI 241 –
Meteorology
Lesson 16 –
Extratropical Cyclones
Dr. DeCaria
Reading: Meteorology Today, Chapter 13
CYCLONES
l A
cyclone technically refers to any cyclonically rotating circulation.
o
Cyclonic rotation is counter-clockwise in the
Northern Hemisphere and Clockwise in the Southern Hemisphere
l Cyclonic
rotation is associated with low-pressure systems, so that cyclone and
low can be used interchangeably.
l The
term cyclogenesis refers to cyclone formation.
l Cyclones
occurring in the midlatitudes are known as midlatitude or extratropical
cyclones.
l Cyclones
forming in the tropics (such as hurricanes) are known as tropical cyclones
POLAR-FRONT THEORY OF CYCLONE
FORMATION (THE NORWEGIAN MODEL)
l Norwegian
meteorologists studies cyclone formation and development during World War I,
using mainly surface observations. They
formulated what became known as the polar-front theory of cyclone formation.
l Stages
of cyclone development
o
Front develops (known as frontogenesis).
o
A wave develops on the front.
o
A cyclonic circulation (low-pressure) becomes
established.
o
The cold front overtakes the warm front,
beginning the occlusion.
o
The occluded front continues to develop as
the cyclone reaches maturity.
o
The lifetime of the cyclone is roughly 3 – 5
days from the initial wave to the dissipating stage.
CYCLONE
ENERGETICS
l The
energy that drives the cyclone is derived from the temperature contrast across
the front.
o
It is converting potential energy to kinetic
energy.
o
After the cyclone has mixed away the
temperature contrast across the front there is no longer any potential energy
available to keep the cyclone going, so it dissipates.
o
Note that the cyclone is serving to reduce
the latitudinal heat imbalance by transporting warm air toward the pole and
cold air toward the equator.
CLOUDS AND PRECIPITATION
ASSOCIATED WITH MIDLATITUDE CYCLONES
l The
cloud pattern associated with the mature cyclone has a comma or wishbone
shape.
l The
weather associated with the passage of a midlatitude cyclone depends on where
you are relative the center of the low.
l If
you are well to the south you will experience the weather associated with a
warm front, followed by that of a cold front.
l If
you are to the north you will either experience the occluded front, or no
front. The weather will be characterized
by lots of clouds, precipitation, and wind.
This poor weather can last for a day or more, since the low is traveling
slower than the fronts themselves.
l During
the winter it is the northern area of the cyclone that tends to get the most
snow.
VERTICAL STRUCTURE OF
MIDLATITUDE CYCLONES
l The
midlatitude cyclone begins with a wave developing on the polar front. We would like to know what causes the wave to
develop in the first place?
l Often,
the wave develops at the surface because there is a trough in the upper
atmosphere (called an upper-level trough).
l Ahead
of the upper-level trough there is upward vertical motion.
l The
upward motion requires that there be convergence at the surface.
l
The convergence at the surface causes the
low-pressure system to develop at the surface.
l In
order to continue to develop, the surface cyclone must always be ahead
of (downstream from) the upper-level trough.
l Once
the upper-level trough moves directly over the surface cyclone there is no
longer any divergence aloft. This shuts
off any further development, and the surface cyclone begins to weaken and
dissipate.
MOVEMENT OF MIDLATITUDE CYCLONES
l Most
of the so-called bad weather experienced in the U.S. is associated with warm
and cold fronts associated with midlatitude cyclones. Therefore, a key to weather forecasting is
not only to predict when a cyclone will develop, but where it will move once it
has developed.
l Once
a cyclone forms on the polar front it tends to move eastward along the front
(much like a wave on a tight string).
o
One reason that the cyclone travels along the
front is that it is the temperature contrast across the front that feeds energy
to the cyclone.
o
The upper-level winds are sometimes referred
to as steering currents that push the cyclone along. This is a gross oversimplification. It is true that the cyclone tends to follow
the jet stream, but this is because both the cyclone and the jet stream are
linked via the polar front. The jet stream doesn’t push the cyclone along like a
cork in a stream.
ANTICYCLONES
l Anticyclones
are also known as high-pressure systems, or simply highs.
l Because
anticyclones are associated with surface divergence and sinking air they
generally bring good weather with clear skies.
l They
are not always associated with pleasant weather .
l In
the winter they can bring bitterly cold, dry air masses down from Canada and
the Arctic.
o
There can be a tight pressure gradient
associated with them which causes strong winds.
This drastically reduces the wind-chill temperature and causes blowing
snow.
o
The clear, cold nights can produce radiation
fog.
l In
the summer
o
They can cause very hot weather, resulting in
heat stress and other heat related illnesses, as well as draughts.
o
The stagnant air can cause pollution to build
to excess levels.
l When
a large, vertically stacked anticyclone becomes stationary it can disrupt the
entire upper-level wind flow in the atmosphere.
In extreme cases it can block storms from moving eastward. It then becomes known as a blocking high.