Cyclogenesis in Polar Airstreams

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Cyclogenesis in Polar Airstreams

• Not all midlatitude cyclones develop along the
  main polar front/baroclinic zone.
• Range in size from comma clouds (big) to polar
  lows (small)

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Comma Clouds 500-1500 km
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Polar Lows 100-500 km
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Comma Clouds

• Convection and sometimes stratiform-type clouds
  organized into a comma-shaped patterns
• Smaller than normal synoptic systems and on the
  cold side of the midlatitude jet stream.
• Most apparent over the oceans during a period
  with high-amplitude, long-wave trough development

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Generally of a smaller scale (500 to 1000 km)
than classic midlatitude cyclones.
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• Frequently multiple with typical spacing of
  1000-1500 km

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Usually associated with the region of positive
vorticity advection (PVA) associated with a
short-wave trough aloft.
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Scale often grows in time, particularly as they
move through a long wave trough
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Three Stages of Comma Cloud Development

• Incipient Stage
• Two troughs large scale one and other
  associated with developing comma
• Appreciable baroclinicity with comma
• Intensifying Stage
• Convective elements grow in size and merge.
• Size of system increases
• Low center may appear
• Stronger advections and front-like
  characteristics
• Mature Stage
• Large size and movement to the forward side of
  the long-wave trough.
• Difficult to differentiate from a normal polar
  front cyclone

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• Associated with regions of appreciable
  baroclinicity (temperature gradient) on the cold
  side of a major baroclinic zone (polar front).

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Often develop in conditionally unstable
environments with lots of convection
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Comma Clouds

• Most apparent over oceans in winter, but can
  develop over land

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1445Z/05
GOES-12 Visible
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1
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1745Z/05
GOES-12 Visible
2
1
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2045Z/05
GOES-12 Visible
2
1
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15Z/05
2
2
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Some, But Not All, Associated with Lightning
Strikes
Jan 18-21 2010 Lightning in Yellow
Nov 15-17 2009
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• Sea-level low-pressure center is sometimes found
  under the comma head, with a trough of low
  pressure under the trailing edge of the comma
  tail.

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Sometimes the associated trough can develop
frontal characteristics
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There are a variety of ways for comma clouds, and
their associated vorticity maxima, to interact
with the main baroclinic zone/polar front
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Little Interaction
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Instant Occlusion
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Instant Occlusion

• The comma cloud/PVA maximum can excite the
  development of a wave on a preexisting front.
• The comma cloud combines with the developing wave
  to form what appears to be a more occlusion
  WITHOUT the usual occluded front evolution.

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Why such a small scale?

• Baroclinicity, latent heat release, and low
  stability appear to accompany most comma clouds.
• Several studies (e.g., Gall 76 and Staley and
  Gall 76) suggest that baroclinic instability in
  concert with low stability in the lower
  troposphere could contribute to such small
  scales.
• Needs more work.

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Polar Lows
Look somewhat like hurricanesspiral rain bands,
cloud free eye
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Polar Lows (also known as Arctic Hurricanes!)

• Small scale typically 300 to 800 km in size
• Usually develop near the ice margin where
  relatively warm, open water is adjacent to ice
  fields or cold continents.
• Thus, they develop in a region of very strong,
  low-level atmospheric baroclinicity.
• Low-stability environment as cold air moves over
  warm water. Usually convective clouds are
  present, frequently in linear, cloud streets.

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Polar Lows

• Form rapidly when short-wave troughs aloft
  approach such baroclinic, unstable regions.
• Favored locations Bering Sea, Greenland,
  Norwegian and Barents Seas, Gulf of Alaska.

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Polar Lows
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Mesoscale Structure
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Polar Low Mechanisms

• Because they look like hurricanes, some have
  suggested they grow by similar air-sea
  interaction mechanisms
• CISK (Conditional Instability of the Second Kind)
• WISHE (Wind Induced Surface Heat Exchange)
• Others have suggested that baroclinic instability
  in the presence of low stability is dominant.
• Probably both mechanisms are important.

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The END