Fronts and Jets

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Fronts and Jets

• How we analysis them and why

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But first AIR MASSES

• Air masses are typically classified by their
  source region.
• A arctic m - maritime
• P polar c - continental
• T tropical
• (E equitorial, never in Canada)

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Types of Air Masses

• mT maritime tropical moist warm
• cT continental tropical dry warm
• mP maritime polar moist cool
• cP (also mA) continental polar dry cool
• (arctic air moving over north Pacific gaining
  some moisture)
• cA continental arctic very cold very dry
• E equitorial never comes to Canada, under the
  subtropical Highs near the equatorial zone

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Fronts

• Fronts separate air masses
• Name of the front comes from the colder air mass
• Polar Front separates mT mP air masses
• Arctic Front separates mP cP (or mA) air
• A2 Front (only in Canada) separates cold arctic
  air from the very cold dry continental arctic
  air (-30 and colder temperatures)

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Frontal Concepts

• front is a boundary between two air masses
• zone of contrast (usually sharp)
• Tight gradient (isotherms, isodrothotherms, etc.)
• frontal surface fronts extend upward into the
  atmosphere, sloping toward the cold air
• frontal inversion frontal surface slopes back
  over cold air, so warm air aloft (think tephi)
• Temperature steady or increases with height in
  the frontal zone

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Frontal Concepts

• frontal movement moves at speed equal to
  component of the wind perpendicular to the front
• frontal slope the degree of slope of the
  frontal surface over the cold air.
• miles of rise to miles of run (e.g. 150 1 mile
  of rise per 50 miles of run)
• cold front 150
• warm front 1100

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Frontal Concepts

• Frontogenesis new or intensification of an
  existing front by low-level convergence or by
  diabatic processes (surface heating/surface
  cooling)
• Frontolysis dissipation or decrease in
  intensity of front.
• Decrease temperature gradient through divergent
  flow or diabatic processes

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Cold Front

• Cold air mass is replacing warm air mass
• Cold air is advancing
• Pressure tendency show a rise
• Anafront active cold front warm air mass is
  being forced to ascend along frontal surface,
  ususally steep slope.
• Clouds precipitation behind cold front
• Katafront inactive cold front warm air mass
  is being forced to descend the frontal surface,
  usually shallow slope.
• Skies generally clear no precipitation

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Warm Front

• Warm air mass is replacing cold air mass at
  surface.
• Pressure tendency checks, was falling and is now
  rising
• Warm fronts tend to move more slowly than cold
  fronts (cold air is more dense than warm air)
• Warm fronts have shallower slope than cold
  fronts, usually.

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Other fronts

• Quasistationary front no motion
• Occluded front air masses mix
• Cold occlusion cold air captures the warm air
  (usual or common)
• Warm occlusion warm air captures the cold air
  mass (uncommon or rare)
• Dryline moisture discontinuity, shallow
  usually below 850mb
• EML elevated mixed layer

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Sea Breeze (or land breeze) front

• Local circulation caused by differential heating
  (cooling) of land and sea surfaces
• In daytime wind blows from cold water to warm
  land
• In nighttime wind blows from cold land to warm
  water
• Wind always blows from area of high pressure to
  area of low pressure

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Jets

• Any narrow band of strong winds
• Usually separates two air masses
• Due to temperature gradient tighter the
  gradient the stronger the jet

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4 cell jet max concept

• 4 quadrants around jet core
• Left exit (LFQ)
• Right exit
• Left entrance
• Right entrance (RRQ)
• Left exit and right entrance are areas of
  enhanced vertical velocity (lift)
• cyclonic curved jets stronger lift in left exit
• Anticyclonic curved jet stronger lift in right
  entrance

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Low level jet

• Narrow band of strong winds below 850 mb
• Important source of convergence (lift) moisture
  transport

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Why do we care?

• Air mass characteristics
• maximum/minimum temperature
• Moisture/humidity
• Stability
• Areas of lift
• Clouds and precipitation
• Wind (speed and direction)
• Simple answer to help us forecast!