Midlatitude Cyclones

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Midlatitude Cyclones

• EAS 211
• Spring 2005
• 02/18/05

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Midlatitude Cyclones

• Also known as Extratropical Cyclones (ETCs)
• Often form lee of the Rockies
• Describe a low pressure system
• Cyclonic flow low pressure leads to sfc con
  which leads to UVM which in turn leads to clouds
  and precip
• This is the principle wx-maker in the
  midlatitudes
• Covers 1000s of square miles, and has a lifespan
  of 3-10 days

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• Cyclogenesisthe birth of a cyclone, originates
  along a polar front.
• Polar Front TheoryTheory of how a cyclone
  developsfrom the Norwegians in the early 1900s.

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Before Storm

• System originates as stationary front b/w cold,
  polar air and warmer air
• The front represents a trough of low pressure
  with High pressure on either side
• The cold air to the north and warm air to the
  south flow parallel to the front but in opposite
  directions. This type of flow sets up cyclonic
  wind shear.

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Wave Develops

• This is known as a frontal wave or incipient
  cyclone
• Now we can identify a warm front pushing
  northward and a cold front pushing southward
• The region of lowest pressure is called the
  central pressure and is located at the junction
  of the two fronts
• As the cold air displaces the warm air upward
  ahead of the cold front, and as overrunning
  occurs ahead of the warm front, a narrow band of
  precip. usually occurs.

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Cyclonic-circulation

• Steered by winds aloft, the system typically
  moves E or NE and gradually becomes a fully
  developed open wave in 12-24 h
• Central pressure much lower, several isobars
  fully encircle the waves apex
• Creates a stronger cyclonic flow, as the winds
  swirl counterclockwise and inward toward the
  lows center.
• Precip wide band ahead of the warm front and
  along a a narrow band of the cold front.
• Warm air region is called the warm sector
  (usually partly cloudy, but could have scattered
  showers if air is unstable)

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Storms Energy Source

• As the air masses try to attain equilibrium, warm
  air rises and cool air sinksthereby changing
  potential energy to kinetic energy.
• Condensation supplies energy to the system in the
  form of latent heat.
• As the surface air converges toward the lows
  center, wind speed may increase, producing an
  increase in kinetic energy.
• NOTE Kinetic energy is the energy of motion.

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Occlusion begins

• As the open wave continues to progress eastward,
  its central pressure continues to decrease, and
  the winds move more vigorously
• The cold front continually inches closer to the
  warm front, squeezing the warm sector into a
  smaller area.

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Occluded front developed

• At this time the storm is usually the most
  intense, with clouds and precip covering the
  largest area
• The point of occlusion, where the cold, warm, and
  occluded fronts all come together is called the
  triple point.
• Notice in this region, the cold and warm front
  resembles the open wave cyclone discussed
  previously. It is at the triple point where a
  new wave will occasionally form. This is called
  secondary cyclone, and may move eastward and
  intensify into a cyclonic storm.

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Cyclone dissipates

• Center of the intense storm will now weaken, as
  cold air is on both sides of the occluded front
  (decreases the temp. gradient)
• The warm sector is still present, but is far
  removed from the center of the system
• Without the supply of energy provided by the
  rising warm, moist air, the old system usually
  dies out and gradually dissipates.

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• Total lifespan can last from about 3 days to well
  over a week
• When an ETC deepens rapidly (in excess of 24 mb
  in 24 h), the term explosive cyclogenesis or bomb
  is used.
• Frontal waves that develop into huge storms are
  called unstable waves. These waves form
  suddenly, grow in size then dissipate with the
  entire process typically taking 3-10 days.
• Other waves called stable waves remain small and
  never grow into giant weather producers.

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• Why is it that some waves develop into huge
  storms while other simply dissipate in a day or
  so?
• There are many surface conditions that influence
  the formation of a wave, including mountain
  ranges and land-ocean temperature contrasts.
  However, the real key to the development of a
  wave cyclone lies in the upper-level flow in the
  region of high level westerlies.
• So, before we can really answer the question, we
  need to see how the winds aloft influence the
  surface pressure systems.

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