Air Masses and Midlatitude Cyclones

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Air Masses and Midlatitude Cyclones

• ATS 351 Lab 11
• November 16, 2009

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Air Masses of North America

• Air mass extremely large body of air whose
  properties of temperature and humidity are fairly
  similar in any horizontal direction at any given
  latitude
• Named for their region of origin
• Continental Polar
• Continental Arctic
• Maritime Polar
• Maritime Tropical
• Continental Tropical

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Cold Front

• Cold, dry stable polar air is replacing warm,
  moist, conditionally unstable subtropical air
• Steep vertical boundary due to surface friction
  slowing down the surface front
• Has strong vertical ascent along the surface
  front
• Strong upper level westerlies push ice crystals
  far ahead of the front, creating cirrus advance
  of the front.

• Cold, dense air wedges under warm air, forcing
  the warm air upward, producing cumuliform
  clouds
• Can cause strong convection, severe weather, and
  squall lines.
• Air cools quickly behind the front

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Air Masses of North America

• Continental Polar (cP) Arctic (cA)?
• Cold, dry, stable air in winter
• In summer, cP air mass usually brings relief from
  oppressive heat in central and eastern US
• Maritime Polar (mP)?
• In winter, cold and dry continental air masses
  are carried over Pacific Ocean where moisture and
  warmth is added
• Air mass at Pacific Coast is cool, moist, and
  conditionally unstable
• East of Rockies - brings fair weather and cooler
  temperatures (moisture has been removed by
  mountains)?
• East coast mP air mass originates in N. Atlantic
  
• Storms may develop (heavy rain or snow, coastal
  flooding)?
• Late winter, early spring

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Air Masses of North America

• Maritime Tropical (mT)?
• Subtropical east Pacific Ocean very warm and
  moist by the time it reaches east coast
• Heavy precipitation
• Gulf of Mexico warm, humid subtropical air
• Formation of dew, fog and low clouds along Gulf
  coast
• Could lead to record heat waves
• Continental Tropical (cT)?
• Source is N Mexico and arid SW only exist in
  summer
• Hot, dry and conditionally unstable at low levels
• Clear skies and hot weather (severe drought
  possible)?

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Air Masses and Fronts

• A front is a transition zone between two air
  masses of different densities
• Fronts extend both horizontally and vertically

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Cold Front

• Rising motion causes decreased surface pressure
  ahead of the front
• On a surface pressure map, frontal location can
  be seen by kinks in the isobars, changes in
  wind direction from a southwesterly to a
  northwesterly wind, and decreases in temperature.
• Pressure is lowest at the surface front.
• On weather maps, cold fronts are indicated by
  blue lines with triangles pointing in the
  direction of frontal motion (towards warmer air)?

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Cold Front
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Warm Front

• Occurs at the leading edge of an advancing warm,
  moist, subtropical air mass from the Gulf
  replacing a retreating cold, maritime, polar air
  mass from the North Atlantic
• Slowly advances as cold air recedes moves at
  about half the speed of an average cold front
• Speed may increase due to daytime mixing
• Speed may decrease due to nighttime radiational
  cooling
• Smaller vertical slope than cold front

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Warm Front

• Warmer, less-dense air rides up and over the
  colder, more-dense surface air
• Overrunning
• Produces clouds and precipitation well in advance
  of the front

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Warm Front
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Stationary Front

• Essentially no movement
• Surface winds blow parallel to front, but in
  opposite directions on either side of it
• Separates two air masses
• Seen often along mountain ranges when cold air
  cannot make it over the mountain ridge

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Hourly surface observations at Gage, Oklahoma
showing the passage of a primary and secondary
cold front (left) and at Bowling Green, Kentucky
showing the passage of a warm front (right).
Source Wallace and Hobbs, 2006.
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Occluded Fronts

• Cold fronts generally move faster than warm
  fronts
• Occlusion occurs when cold front catches up to
  and overtakes a warm front
• Occlusions can be warm or cold

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Dry Lines

• Think of a dry line as a moisture boundary
• Separates warm, humid air from warm, dry air
• Drier air behind dry lines lifts the moist air
  ahead of it, triggering storms along and ahead of
  it
• Induces lifting along front
• Often produces severe thunderstorms in OK TX
• Unique to southern great plains of US because of
  the Rocky mountains and the Gulf of Mexico

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• A guide to the symbols for weather fronts that
  may be found on a weather map
• 1 cold front
• 2 warm front
• 3 stationary front
• 4 occluded front
• 5 surface trough
• 6 squall/shear line
• 7 dry line
• 8 tropical wave

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Midlatitude (Extratropical) Cyclone

• A cyclone (area of low pressure) in the middle
  latitudes (35-70)?
• Important for global heat transport
• Help to redistribute energy between the tropics
  (equator) and the poles
• Often associated with significant weather events
• Described by the Polar Front Theory
• Form on boundaries between warm and cold air
• Cold Polar air meeting warm tropical air

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Features of a Midlatitude Cyclone

• Deep low pressure area with attached cold and
  warm fronts
• Often an occlusion forms, the triple point
  lending to the formation of severe weather
• Precipitation associated with the cold and warm
  fronts organizes in typical comma cloud
  structure

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Stages in Wave Cyclone Development
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Polar Front Theory

• Initially, there is a stationary front that acts
  as the boundary separating cold, continental
  polar air from warm, maritime tropical air
• Winds blow parallel to this front on either side
• Polar Fronts are discontinuous

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Cyclogenesis
Central Pressure

• A wave forms on the front due to a shortwave
  disturbance
• Frontal Wave
• The front develops a "kink" where the wave is
  developing
• Precipitation will begin to develop along the
  front
• Overrunning and lifting

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Strengthening

• The cyclonic circulation around the low becomes
  more defined
• The central pressure intensifies
• The cold front and warm front have more organized
  motion
• Cyclone usually pushed east or northeast by the
  winds aloft

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Mature Cyclone

• The cold front catches up with the warm front and
  an occlusion forms
• The cyclone is at its strongest at this point
• Severe weather often develops near the triple
  point
• - Intersection of cold, warm, and occluded fronts

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Dissipation

• The occlusion grows with time
• Eventually, the occlusion is so great that the
  supply of warm, moist air into the low is cut off
• Cold air on both sides
• When this happens, the system starts to dissipate

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Interaction with Upper Levels

• Previous model for cyclone development only
  includes surface characteristics but what
  happens higher up can determine what happens
  below
• Divergence aloft can help to remove mass from a
  column, hence lowering the surface pressure even
  more

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Interaction with Upper Levels

• Downstream of an upper level trough, the air
  tends to diverge
• If a surface low is located slightly downstream
  of an upper level trough, the divergence will be
  located above the low and help to intensify it

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Mid-latitude Cyclones The Upper Level Life Cycle

• A 500 mb trough develops westward of a surface
  stationary front
• As the 500 mb trough deepens, the associated
  upper level divergence strengthens, helping to
  intensify the surface low
• Stronger winds aloft force the upper level trough
  to move eastward faster, and eventually it
  becomes located above the surface low
• When the surface and upper level low are
  stacked, convergence at both levels starts to
  fill the low pressure area, weakening the
  cyclone

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Mid-latitude Cyclones The Upper Levels
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Closed Low

• A closed low is an upper level area of low
  pressure that is completely encircled by at least
  one isobar
• This may be partially or completely detached from
  the main flow

• Often results from occlusion of surface
  cyclone
• These can persist for several days without
  moving and produce several days worth of
  precipitation over the area where they park
• Called cut-off low when completely
  separated from main flow at upper levels