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Mid-latitude Cyclones

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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Mid-latitude Cyclones

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  1. Chapter 12 Mid-latitude Cyclones

  2. 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

  3. Where do mid-latitude cyclones tend to form? • Lee-side lows (cyclogenesis) • Nor’easters • Hatteras low • Alberta Clipper • Explosive cyclogenesis, bomb

  4. 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

  5. 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

  6. Upper Level Waves and Mid-latitude Cyclones • Longwaves and shortwaves • Barotropic vs. baroclinic • Cold and warm air advection

  7. 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

  8. 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

  9. 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

  10. 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

  11. 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

  12. 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

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