Formation of the Extratropical Cyclone Cyclogenesis

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Formation of the Extratropical Cyclone
(Cyclogenesis)
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Cyclogenesis

• Extratropical cyclones often form when an upper
  level trough (e.g. a shortwave) approaches a
  front that is stationary or moving relatively
  slowly.
• In order for a low pressure system to form at the
  surface, there must be net divergence in the
  column of air and the air must start to rotate
  counterclockwise.

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Cyclogenesis (Cont.)

• During the development of the extratropical
  cyclone the system typically tilts to the west
  with height.
• This means that the upper level trough is usually
  found to the west of the surface trough or low.

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Cyclogenesis (Cont.)
N
E
Upper Level Shortwave at 500 mb
Upper Level Divergence
Positive Vorticity
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Cyclogenesis (Cont.)
Colder air mass
N
H
E
Upper Level Shortwave at 500 mb
H
Warmer air mass
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Cyclogenesis (Cont.)

• Once the stationary front begins to rotate
  cyclonically the eastern section becomes a warm
  front as warmer air begins to move north and to
  push out colder air.
• The western section of the front begins to move
  south and becomes a cold front as the colder air
  begins to push out warmer air.

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Cyclogenesis (Cont.)

• Colder air is denser than warmer air and the
  colder air is more affected by the gravitational
  force.
• The colder air tends to stay near the surface,
  while the warmer air tends to rise over denser
  colder air.

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Cyclogenesis (Cont.)

• The different densities mean that the warmer air
  often has a hard time pushing out the denser
  colder air.
• By contrast the denser colder air has much less
  trouble pushing out the less dense warmer air.

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Cyclogenesis (Cont.)

• The net result is that cold front moves faster
  than the warm front and a bend or kink develops
  along the front.
• The bend in the front is called a wave on the
  front (or frontal wave).

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Wave on the Front
N
H
Retreating cold air mass
E
500 mb flow
H
Warm air mass
H
Developing new cold air mass
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Extratropical Cyclone
N
H
E
H
L
H
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Changes at 500 mb

• As the colder air moves southward behind the cold
  front, it causes the 500 mb heights to decrease
• As the warmer air moves northward with the warm
  front, it causes the 500 mb heights to increase
  ahead of the developing surface low.

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Changes at 500 mb (Cont.)

• The effects of the advection of the warmer air is
  to build the downstream ridge.
• The effects of the advection of the colder air is
  to deepen the trough.
• Thus, the thermal advection increases the
  amplitude of the shortwave.

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Changes at 500 mb (Cont.)
N
H
500 mb flow
E
Warm air advection builds the downstream ridge.
H
L
Cold air advection deepens the trough at 500 mb.
H
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Clouds and Precipitation

• The movement of the fronts tends to produce
  favored regions of rising and sinking air that
  may produce clouds and precipitation.

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Extratropical Cyclone
2
.. ..
N
H
.. .. ..
500 mb flow
E
..
H
L
3
H
1
4
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Cross-Section of a Warm Front
Clouds
Warmer air rises over denser colder air
Warm front aloft
Warmer air mass
Colder air mass
1
2
Precipitation
Warm front at surface
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Warm Front (Cont.)

• Along the warm front the warmer air is forced to
  rise over the retreating colder air in a process
  called overrunning.
• If there is sufficient water vapor in the air,
  then the process will produce clouds and may
  produce precipitation.

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Warm Front (Cont.)

• Far ahead of the warm front the first clouds are
  the high clouds made up of ice crystals called
  cirrus clouds.
• The clouds gradually become lower and thicker and
  the more gradual slope of the warm front tends to
  produce the layered stratus clouds.

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Warm Front (Cont.)

• Precipitation is typically found ahead (i.e. to
  the north) of the warm front.
• During the colder seasons the precipitation may
  be rain, sleet, freezing rain or snow.
• During the warmer seasons the precipitation is
  typically rain.

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Warm Front (Cont.)

• If enough precipitation evaporates back into the
  air, then fog or mist may occur.

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Warm Front (Cont.)

• North of a warm front
• Temperatures are colder
• The winds are from the east
• It is often cloudy
• The dew point temperature is lower
• Precipitation may be falling.

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Warm Front (Cont.)

• South of a warm front
• Temperatures are warmer
• The winds are from the south or southeast
• The sky may be partly cloudy or clear
• The dew point temperature is higher
• The precipitation may end.

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Cold Front
Cold front aloft
Colder air mass
Warmer air mass
4
3
Cold front at surface
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Cold Front (Cont.)

• The slope of the cold front is typically much
  steeper than the slope of the warm front.
• The steeper slope usually produces more vertical
  clouds like the cumulus and cumulonimbus clouds.

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Cold Front (Cont.)

• If the air ahead of the cold front is
  sufficiently warm and moist, then the lifting by
  the cold front may produce thunderstorms and
  heavy rainfall.

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Cold Front (Cont.)

• East of a cold front
• The temperatures are warmer
• The winds are from the south or southwest
• It may be cloudy or partly cloudy
• Thunderstorms and heavy precipitation may be
  occurring
• The dew point temperature is higher.

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Cold Front (Cont.)

• West of a cold front
• The temperatures are colder
• The winds are from the north or northwest
• It may be cloudy or partly cloudy
• Showers may occur if the air is unstable
• The dew point temperature is lower.

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Warm Sector

• The region between the advancing cold front and
  the warm front is called the warm sector.
• The weather in the warm sector can be highly
  variable depending on the source region of the
  air mass and its temperature, humidity and
  stability.

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The Occluded Front
..
N
L
..
E
Cut off Low at 500 mb
500 mb flow
..
H
H
..
6
5
..
H
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warm air aloft
Occluded front aloft
Remainder of warm front aloft
Advancing colder air mass
Retreating colder air mass
5
6
Occluded front at surface
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Occluded Front (Cont.)

• By the time an occluded front forms the upper
  level trough is located almost directly above the
  surface low and the system is said to be
  vertically stacked.
• If the pool of cold air aloft becomes separated
  from the circumpolar vortex, then a cut off low
  may form.

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Occluded Front (Cont.)

• The development of an occluded front usually
  indicates that the extratropical cyclone has
  peaked in intensity.
• Sometimes a new surface low forms at the triple
  point where the occluded front, the warm front,
  and the cold front meet.

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