Middle Latitude Cyclones

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Chapter 13 Middle Latitude Cyclones

• This chapter discusses
• The location, vertical structure, and
  developmental stages of middle latitude cyclones
• How upper level convergent winds, abrupt
  topographic features, and planetary longwaves may
  enhance cyclonic development at the surface

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Polar Front Theory (Norwegian)
Figure 13.1C-Open Wave
Figure 13.1A-Cylconic Shear
Figure 13.1D- Mature Cyclone
Figure 13.1B-Frontal Wave
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Mid-latitude Cyclone Life Cycle

• Begins with a stationary front with warm and cold
  winds in opposite directions (e.g. wind shear).
• A wavelike kink, such as a low pressure system,
  then creates a frontal wave, or incipient
  cyclone.
• As the storm develops into an open wave, a broad
  band of precipitation forms ahead of the warm
  sector.
• As cyclone matures, the cold air catches up
  with the warm sector, and low becomes occluded
• The mature cyclone is now at its strongest, and
  begins to slowly weaken
• Eventually, the low becomes cut-off and
  dissapates

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Triple Point
Figure 13.1E-Occlusion
Figure 13.1F-Dissapation
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Wave Cyclone Development
A series, or family, of cyclones, at various
stages of development, may extend across North
America. Energy originates from kinetic sources
such as rising warm air, sinking cold air, and
converging air, as well as latent heat of
condensation.
Figure 13.2
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Cyclone Anticyclone Paths
Generally NE
Lee Side lows
Generally SE
Figure 13.3A
Figure 13.3B

• Many well-known paths for low and high pressure
  systems extend across North America
• Cyclone development most common in a few
  specific areas
• Cyclogenesis describes the strengthening of
  these storms into huge unstable waves.

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Convergence Divergence
Deepening of cyclones through explosive
cyclogenesis is prohibited when low pressure
aloft is directly above the surface low. In this
scenario, convergence at the surface low builds
up mass and fills in the low (converging air has
nowhere to go). The same stacking of high
pressure, with divergence at the surface, will
weaken the anticyclone.
Figure 13.4
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Vertical Storm Structure
Divergence of air aloft occurs as contour
interval widen (the inverse happens when height
contours become more packed together) Low
pressure systems deepen and intensify
(cyclogenesis) when upper-level divergence is
stronger than the surface convergence This which
requires a vertical offset of the surface low and
low pressure above it.
Figure 13.5
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Upper Level Waves
Earth's poles are encircled by 3 to 6 longwaves,
or Rosby waves, directing upper level winds
around lows at the 500 mb surface. Small
disturbances (shortwaves) in these larger waves
can trigger storms Cyclones move in the
direction of the 500mb flow, at about ½ the speed
Figure 13.6
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Shortwave Disturbance
Shortwave kinks within the Rossby waves move
faster, and propagate downwind into the Rossby
troughs and cause them to deepen. Barotropic
conditions, where isobars and isotherms are
parallel, then degenerate into a baroclinic state
where the lines cross and cold or warm air is
advected downwind.
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Cyclone Development Upper Winds
Figure 13.8A-Barotropic
Figure 13.8C-Barotropic
Differential temperature advection intensifies
the wave. Existing surface frontal boundary
upper level shortwave moving over the
boundary--cyclogenesis occurs
Figure 13.8B-Barclinic Instability
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Atmospheric conditions at the surface and aloft
affect cyclogenesis. An upper level shortwave
can trigger baroclinic instability. -Convergent
flow aloft helps strengthen surface high
pressure, and supports cold air advection
-Downwind of the upper level trof, divergent
flow aloft deepens the surface low, and warm
advection aids rising air flow
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Finding a Low Center Using Wind Observations
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Jet Streak
Divergence aloft is enhanced by the polar jet
stream, where the jet maximum, or jet streak,
forms in the tightly packed pressure gradients.
Strong C
Strong D
Weak D
Weak C
Figure 13.9
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Jet Convergence Divergence
Figure 13.10A
Figure 13.10B
-The polar jet forces air convergence aloft
upstream of the deepening open wave cyclone, and
then divergence downstream. -When these winds are
gone, the cyclone degrades.
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Summary of Cyclone Weather
Upper and surface maps summarize the role of
convergence and divergence aloft and the pattern
of clouds, precipitation, and temperatures on the
ground.
Figure 13.11
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Conveyor Belt Model
This model describes rising and sinking air along
three conveyor belts, warm conveyor belt rises
with water vapor above the cold conveyor belt
which also rises and turns. Finally the dry
conveyor belt descends brining clearer weather
behind the storm.
(Upper Level)
Figure 13.12
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Comma Cloud Pattern
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Comma Clouds
Rising and turning moist air, illustrated in the
conveyor belt model, condenses into a large
comma-shaped cloud typical of the open wave
cyclone. This March 1993 storm wreaked havoc
along the East Coast.
Figure 13.13
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Mar 1993 Storm of the Century
During the storm of the century, the low
pressure center reached 980 mb, and the storm
extended across several southeastern states.
Figure 13.14
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500mb Temperature Advection
Figure 13.15
Upper level winds flowed along a deep trough with
steep baroclinic cold and warm air advection.
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Storm of Century Path
Low pressure values and location are charted with
time to illustrate the storm track and intensity,
moving from Texas to Maine in 2 days.
Figure 13.16
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Size of Polar Lows
Cyclones that develop north of the polar front,
called polar lows, are smaller in size than
mid-latitude cyclones. They form during the
winter in arctic air, have (relatively) warm
central cores, strong winds, and can generate
snow.
Figure 13.17
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Vorticity Cyclonic Spin
Figure 13.18
Figure 13.19
- Vorticity describes the spin of an air parcel,
which is positive in counterclockwise cyclonic
flow. - Due to the conservation of angular
momentum, vorticity increases with a decrease in
parcel radius (e.g. stretching due to divergence
aloft) and increase in earth's latitude.
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Sources of Vorticity
Figure 13.21
Figure 13.20
- Curvature of upper level isobars and wind, as
well as strong changes in wind speed (shear),
generate the spin of relative vorticity. -
Additional earth vorticity is generated by the
earth's spin, and together they comprise absolute
vorticity.
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Trough to Ridge Vorticity Change
Anticyclonic spin around a ridge reduces absolute
vorticity, but the convergence and cyclonic spin
in the trough enhance the relative and hence
absolute vorticity, as illustrated in this upper
level diagram.
Figure 13.22
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Vorticity Vertical Motion
The 500 mb map vorticity maximum is a signal that
to its east, air is diverging aloft. If there
is also convergence below, then an open wave
cyclone will likely deepen. Hence, 500 mb
charts are useful to find the vort max and
predict potential storms.
Figure 13.23
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500mb Vorticity Chart
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Imaging Shortwave Centers
GOES West satellite infrared imagery of water
vapor are useful in identifying swirling
vorticities, seen off Pacific Northwest coast.
Figure 13.24
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Vertical Storm Profile
Surface, 500mb, and 200mb charts are used to
illustrate the structure of the February 1983
open wave cyclone exploding over North
Carolina. The 500mb chart shows a shortwave
dashed line moving into the longwave trough and
baroclinic cold air advection.
Figure 13.25
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February 1983 Vorticity
Lines of equal vorticity are plotted on the 500
mb chart for the February 1983 open wave cyclone
that buried the east coast in snowfall. Note
that the vort max is west of the storm center,
strengthening the cyclogenesis.
Figure 13.26
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Key Facts and Concepts

• The polar front theory is a model of how an ideal
  storm progresses through the stages of birth,
  maturity, and dissipation
• For a surface mid-latitude cyclone to develop or
  intensify, the upper-level low must be located to
  the west (or behind) the surface low
• Convergence is the piling up of air, divergence
  is the spreading out of air
• Upper-level divergence above the developing
  surface cyclone must be greater than surface
  convergence
• Upper-level convergence above developing surface
  anticyclone must be greater than surface
  divergence

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• As the jet stream develops into a looping wave
  pattern, it produces areas of convergence and
  divergence
• The jet stream tends to direct anticyclones to
  the southeast and cyclones to the NE
• When upper-level flow is disturbed by a
  shortwave, horizontal and vertical air motions
  begin to enhance the formation of a surface storm
• Rising warm air and sinking cold air provide
  energy for storm formation
• As air flows over a mountain range, wind flow
  causes storms to form or intensify
• Polar lows are storms that develop over water in
  polar regions behind the main polar front (on the
  arctic front)
• Vorticity is a measure of the spin in a column of
  air
• An area of positive relative vorticity at 500mb
  can initiate surface mid latitude cyclogenesis