Middle-Latitude Cyclones - I

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Middle-Latitude Cyclones - I
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RECAP Types of Fronts

• Cold fronts cold, dry stable air is replacing
  warm, moist unstable air. Moves fast, showers
  along the leading edge (squall line).
• Warm fonts warm, moist unstable air is replacing
  cold dry stable air. Overrunning warm air rides
  up and over the cold air, widespread cloudiness,
  light-to-moderate precipitation well ahead of the
  front
• Stationary fronts essentially no movement, winds
  blow parallel to the front, in opposite
  directions on both sides
• Occluded fronts when a cold front catches up
  with a warm front (more later)
• The symbols on a map are in the direction of the
  air mass motion.

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Stationary Front

• Stationary front- a front which does not move or
  barely moves.
• Stationary fronts behave like warm fronts, but
  are more quiescent.
• Many times the winds on both sides of a
  stationary front are parallel to the front and
  have opposite direction.
• Typically stationary fronts form when polar air
  masses are modified significantly so as to lose
  their character (e.g., cold fronts which stall).
• Typically there is no strong precipitation
  associated with stationary fronts (why? no big
  contrast in the air mass properties, no air
  uplifting and condensation).

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Occluded fronts.
Cold occlusion

• Cold fronts move faster than warm fronts. They
  can catch up and overtake their related warm
  front. When they do, an occluded front is formed.
   
• Cold occlusion very cold air behind, not so cold
  air ahead of, the warm front
• The upper warm front follows the surface occluded
  front

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Warm Occlusion

• Very cold air ahead of, not so cold air behind,
  the warm front
• The cooler air from the cold front cannot lift
  the very cold air ahead, rides piggyback
• The warm front aloft precedes the surface
  occluded front

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Weakening/Strengthening of the Front

• Frontolysis
• The front weakens and dissipates
• Why?-the air masses start losing their
  identities.
• The temperature (humidity) contrast across the
  front is decreasing.
• Typical for slow moving fronts

• Frontogenesis
• The front intensifies.
• Why? The temperature (humidity) contrast across
  the front is increasing.
• Example cP air mass moves over warm ocean
  water.

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Weather Map

• Shown surface-pressure systems, air masses,
  fronts, isobars, winds and air flow (large
  arrows)
• Green-shaded area precipitation

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Weather Map

• The example from Chapter 1 Fig. 1.14.

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Sample weather maps
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Polar Front Theory

• A model of how mid-latitudes storms develop
  their birth, growth, and decay.
• The model connects the storms with the dynamics
  of the polar front the transition zone between
  the cold air in the polar cell and the warmer air
  at middle-latitude (Ferrel cell).
• The polar front is a region of air conversions
  at the surface, upward motion, and divergence
  aloft. This results in low surface pressure.

Jacob Bjerknes
Vilhelm Bjerknes
Halvor Solberg Tor Bergeron
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The life of a mid-latitude (wave) cyclone
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The life of a mid-latitude (wave) cyclone

• A the polar front is stationary the winds are
  in opposite directions on the two sides of the
  front. This creates a cyclonic wind shear.
• B A local perturbation a region of low pressure
  appears somewhere along the front. The front then
  breaks in two fronts
• warm (moving northward - why? Coriolis force)
• cold (moving southward - why? Coriolis force)
• Central pressure the junction of the two fronts
• A frontal wave is formed.
• The winds aloft set the general direction of
  motion (black arrow)
• The wave starts moving to the east and gradually
  becomes
• C Open wave. The cold front moves faster than
  the warm front -gt polar front bends. Warm sector
  between the two fronts. The central pressure
  keeps dropping isobars now encircle it.

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Weather patterns around a cyclonic wave

• South of the wave
• First a warm front
• Warmer air advancing
• Wide band of precipitation.
• Starts with snow first.
• Then rain and drizzle.
• Cold front
• Cold air advancing
• Sharp drop of pressure
• Strong precipitation at the front.
• Then dry, cold, clear weather.
• North of the wave some clouds associated with
  the low pressure center but no strong
  precipitation because there is no warm moist air
  around.

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The life of a mid-latitude (wave) cyclone

• D Mature cyclone (initial occlusion). Cold front
  closes in on the warm front. Most intense stage
  of the storm. Clouds cover a large area.
• E Advanced occlusion. Triple point where all
  three fronts come together. The center of the
  storm gradually dissipates
• Cold air on both sides of the occluded front
• Warm sector far removed the rising warm and
  moist air provides energy for the storm (kinetic
  energy, latent heat of condensation)
• F Cut-off cyclone plus a stationary front once
  again.

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Family of cyclones
Dying out E Advanced occlusion
C Open wave
Just forming B Frontal wave
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Cyclogenesis

• Any development or strengthening of a
  mid-latitude cyclone
• Some regions have greater propensity for
  cyclogenesis
• Gulf of Mexico
• Eastern slopes of Rockies
• and Sierra Nevada
• Lee-side lows
• Atlantic ocean east of
• Carolinas
• Noreasters

Lee cyclogenesis
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Famous noreasters the Great Blizzard of 2006

• Began on Feb 11 2006
• Developed an eye
• All-time record snowfall (27 in) in New York
  City.

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Where do mid-latitude cyclones form?

• Typical paths of winter mid-latitude
  (anti)cyclones
• Lows towards the east-northeast
• Highs towards the east-southeast
• Explosive cyclogenesis (bomb) when the central
  pressure drops very rapidly (more than 24 mb in
  24 hours)