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Chapter 12. Mid-latitude Cyclones. Polar Front Theory. Polar front is a semi-continuous boundary separating cold, polar air from more moderate mid-latitude air Mid-latitude cyclone (wave cyclone) forms and moves along polar front in wavelike manner

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polar front theory
Polar Front Theory
  • Polar front is a semi-continuous boundary separating cold, polar air from more moderate mid-latitude air
  • Mid-latitude cyclone (wave cyclone) forms and moves along polar front in wavelike manner
  • Frontal wave, warm sector, mature cyclone, triple point, secondary low, family of cyclones
where do mid latitude cyclones tend to form
Where do mid-latitude cyclones tend to form?
  • Lee-side lows (cyclogenesis)
  • Nor’easters
  • Hatteras low
  • Alberta Clipper
  • Explosive cyclogenesis, bomb
where do mid latitude cyclones tend to form11
Where do mid-latitude cyclones tend to form?
  • Topic: Northeasters
    • Mid-latitude cyclones that develop or intensify off the eastern seaboard of North America then move NE along coast
vertical structure of deep dynamic lows
Vertical Structure of Deep Dynamic Lows
  • Dynamic low = intensify with height
  • When upper-level divergence is stronger than surface convergence (more air is taken out of the top than the bottom) surface pressure drops and low formation
  • Topic: Convergence and divergence
    • Convergence between ridge and trough, divergence between trough and ridge
upper level waves and mid latitude cyclones
Upper Level Waves and Mid-latitude Cyclones
  • Longwaves and shortwaves
  • Barotropic vs. baroclinic
  • Cold and warm air advection
the necessary ingredients for development of mid latitude storm
The Necessary Ingredients for Development of Mid-latitude Storm
  • Baroclinic instability
  • Upper-Air Support: the overall effect of differential temperature advection is to amplify the upper level wave; cut-off low
  • Role of the Jet Stream: the polar jet stream removes air from surface cyclone and supplies air to surface anti-cyclone
the necessary ingredients for development of mid latitude storm19
The Necessary Ingredients for Development of Mid-latitude Storm
  • Topic: Jet Streaks and Storms
    • Entrance and exit regions associated with divergence and convergence, right exit allows divergence.
  • Conveyor Belt Model: air constantly glides through storm; warm, cold, and dry conveyor belts
  • March Storm of 1993
vorticity divergence and development of cyclones
Vorticity, Divergence and Development of Cyclones
  • Vorticity is a measure of the spin of small air parcels
  • Positive: cyclonic, negative: anticyclonic
  • Divergence aloft causes and increase in the cyclonic vorticity of surface cyclones = cyclogenesis and upward air movement
vorticity on a spinning planet
Vorticity on a Spinning Planet
  • Earth’s vorticity always positive
  • Relative vorticity: curvature + shear, trough: cyclonic, ridge: anticyclonic
  • Absolute vorticity = Earth + relative
  • An increase in absolute vorticity is related to upper level convergence
  • A decrease in absolute vorticity is related to upper level divergence
  • Vorticity maxima/minima
vorticity on a spinning planet34
Vorticity on a Spinning Planet
  • Topic: Vorticity and Longwaves
    • Longwaves develop in upper-levels due to the conservation of absolute vorticity.
  • Putting It All Together
    • Forecasters review 200mb, 500mb, and surface maps to examine pressure, convergence, vorticity, and advection
polar lows
Polar Lows
  • Storms that develop over water behind (poleward of) main polar front.
    • Comma cloud, eye
    • Warm central core, strong winds, heavy showery precipitation.
    • Arctic front = baroclinic instability