Download Air Masses, Fronts, and Middle-Latitude Cyclones-II

Air Masses, Fronts, and Middle-Latitude
Cyclones-II
GEOL 1350: Introduction To Meteorology
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Cyclone – an area of low pressure around which the
winds blow counterclockwise in the Northern
Hemisphere and clockwise in the Southern
Hemisphere
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How do middle-latitude cyclone (wave cyclone)
form ? – Polar Front Theory (Norwegian
model, or the wave cyclone model)
1. 
2. 
3. 
4. 
5. 
6. 
Stationary Front
Nascent Stage
Mature Stage
Partially Occluded Stage
Occluded Stage
Dissipated Stage
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Before Birth
N
Cyclone Development
begins with a
stationary front
(usually a segment of
the polar front)
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Birth and
adolescence
N
Nascent stage of
Cyclone or incipient
cyclone
Frontal wave
developed
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Mature stage
of Cyclone
Development
Fully
developed open
wave with cold
and warm front
Adulthood
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Mature Wave Cyclone
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The Partially Occluded
Stage begins
when the cold front
starts to overrun the
warm front
Middle age
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Partially occluded wave cyclone
Cold-occluded front
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Relationship between occluded fronts and midlatitude cyclone
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Over the hill
The Occluded Stage
is characterized by
more warm air
being pushed aloft
and the size of the
warm air wedge
at the surface
decreases significantly
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Relationship between occluded fronts and midlatitude cyclone
Partially
occluded
wave
cyclone
Occluded
wave
cyclone
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The final decay stage
of the cyclone. The warm
air is isolated aloft with
cold air beneath.
Death
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Stationary front
Incipient cyclone
Open wave
Summary of
Cyclone
development
stages
Mature stage
occlusion
dissipating
• Life time of
a typical
cyclone is
several days
to a week
•  Moves
1000’s of km
during
lifecycle 15
A low pressure system may develop into
a cyclone if the low pressure system
intensifies (pressure drops in the center
of the low).
What cause surface low pressure to
intensify?
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Air going out of the column
Air going into
the surface low
When upper air divergence is stronger than surface convergence
(more air is taken out from the top than is brought in at the bottom),
surface pressure drop, and the low intensifies, or deepens.
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500 mb height
What initiates “cyclogenesis?”
Low
Cyclogenesis
region
Convergence
behind trough
High
Divergence
ahead of
trough
When upper-level
divergence is stronger
than lower-level
convergence, more
air is taken out at the
top than is brought in
at the bottom. Surface
pressure drops, and
the low intensifies, or
“deepens.”
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The relationship between upper-level
ridge/trough and surface high and low
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Upper-level convergence/divergence and surface high/low
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Cyclone development:
Strong north south gradient + passage of a shortwave trough
can lead to rapid cyclogenesis via baroclinic instability
(baroclinic means temperature varies on an isobaric surface)
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•  upper level shortwave passes
•  upper level divergence -> surface low
•  cold advection behind the trough and
warm advection ahead of the trough
intensifies the wave and deepens the
trough
• Increase the divergence over the surface
low
•  Eventually a upper-level low right above
surface low, divergence aloft weakens,
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cyclone dissipate
The role of jet stream in the development of midlatitude cyclone
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SUMMARY
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Cloud bands associated with mid-latitude cyclone look
like a giant comma, called comma clouds
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Storm track – the position of the lows
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SUMMARY
1.  Fronts are actually part of the mid-latitude cyclone.
2.  Mid-latitude cyclone goes through a series of stages from birth, to
maturity, to death as an occluded storm.
3.  An important influence on the development of a mid-latitude
cyclonic storm is the upper-air flow, including the jet stream.
4.  When an upper-level low lies to the west of the surface low, and the
polar jet stream bends and then dips south the surface storm, an
area of divergence above the surface low provides the necessary
ingredients for the surface mid-latitude cyclone to develop into a
deep low-pressure area.
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