Mid-latitude cyclone
What
is a mid-latitude cyclone?
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The
mid-latitude cyclone is a synoptic scale low pressure system that has cyclonic
(counter-clockwise in northern hemisphere) flow that is found in the middle
latitudes (i.e., 30�N-55�N)
-
IT
IS NOT A HURRICANE OR TROPICAL STORM
o There is a location (tropics vs.
mid-latitudes) and size difference between hurricane and mid-latitude cyclone
�
Typical
size of mid-latitude cyclone = 1500-5000km in diameter
�
Typical
size of a hurricane or tropical storm = 200-1000km in diameter
Here is a picture of a typical mid-latitude
cyclone and hurricane. Notice the size difference.
How does the mid-latitude
cyclone form (for a longer description read pages 219-228)
1.
From
polar front theory, we know that in the mid-latitudes there is a boundary between
cold dry (cP) air to the north and warm moist (mT) air to the south
2.
Along
this boundary a counter-clockwise circulation can set up at the surface, which
acts to take warm air up from the south and cold air down from the north. This
is called cyclogenesis.
3.
In
the center of this circulation, there is mass convergence (all the air is
moving in toward the center of the circulation much like when you make a whirl-pool
in a pool, all the leaves and stuff floating around moves toward the center).
When all that air hits the center, we have rising motion because it has nowhere
else to go.
4.
If
the upper levels are favorable for cyclone development, then there is a region
of divergence aloft above the developing Low-pressure center. This will help
pull the air that is converging at the surface upward and continue to develop the
surface cyclone. (The upper levels also steer the system and make it progress
east (like we learned earlier)).
a.
If
the upper levels aren�t favorable for cyclone development, the cyclone won�t
grow and the mass convergence into the Low at the surface will just pile up and
fill in the Low and it will decay.
b.
Refer
to the second figure below for the optimal situation.
5.
If
the upper levels are favorable, then the mid-latitude cyclone will continue to
develop and bring up mT air in the warm sector and bring down cP air in the
cold sector.
a.
The
mT air rises as it moves out ahead of the low helping to deepen the Low
pressure center to the east and help it move along
b.
The
cP air sinks behind the system and fills in the Low on the backside also
helping to move the system along
i.
It
is this transfer of energy that both strengthens and propagates the
mid-latitude cyclone!
6.
Once
the mid-latitude cyclone is fully developed, well-defined fronts appear.
7.
As
the mid-latitude cyclone reaches maturity, the central pressure will be at its
lowest and the occluded front will begin to form (as the cold front catches up
to the warm front).
8.
Once
the system is occluded (all the warm air is above the cold air) the mass convergence
acts to fill in the Low and therefore the pressure increases in the Low and the
system decays.
Evolution of a mid-latitude cyclone. Each figure
follows the description above 1-8
The
above figures were taken from www.u.arizona.edu/
~korine/230/chap10_figs.htm
Where
do mid-latitude cyclones typically form (in North America in winter)? (Remember that cyclones
like to develop along air mass boundaries)
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Lee
side of the Rockies = Lee Cyclones
o Alberta Clippers
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FAST
MOVING and usually don�t have too much precip associated with them because they
are far from a moisture source
o
Colorado
Low
�
Intense
Low, with strong warm air advection in the warm sector, very cold temps in the
cold sector. If there is a lot of gulf moisture to work with, they there is
usually sleet, freezing rain and rain associated with the warm front, strong thunderstorms
along the southern edge of the cold front and snow along the backside and to
the NW of the Low (even BLIZZARDS)
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Along
the East Coast
o Gulf Low
�
Form
along the southern coast where there is a thermal boundary between the warm
ocean and cool land. Usually have a lot of precip associated with them because they
are so close to the ocean
o
Hatteras
Low and Nor�easters (aka ��bomb�
cyclones)
�
These
are the MOST INTENSE systems and they form along the thermal boundary between
the warm Gulf Stream and the cold Atlantic coast. They can bring flooding rains
along the coast and several feet of snow further inland as they use the ocean
as a vast source of the moisture. These also develop very quickly and sometimes
have pressure drops of 24mb in a single day. With a pressure drop of this
magnitude you can imagine how fast the winds are around these things.
Precip patterns (and types),
winds, temperatures, fronts, upper level flow and clouds that are around a typical
mid-latitude cyclone in winter.