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)
Comma Clouds: 500-1500 km
Polar Lows: 100-500 km
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
Generally of a smaller scale (500 to 1000 km) than
classic midlatitude cyclones.
Frequently multiple with typical spacing of 1000-1500 km
Usually associated with the region of positive vorticity
advection (PVA) associated with a short-wave trough
aloft.
Scale often grows in time,
particularly as they move
through a long wave trough
Three Stages of Comma Cloud
Development

Incipient Stage
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Intensifying Stage
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Two troughs: large scale one and other associated with
developing comma
Appreciable baroclinicity with comma
Convective elements grow in size and merge.
Size of system increases
Low center may appear
Stronger advections and front-like characteristics
Mature Stage
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
Large size and movement to the forward side of the long-wave
trough.
Difficult to differentiate from a normal polar front cyclone
Associated with regions of appreciable
baroclinicity (temperature gradient) on the cold
side of a major baroclinic zone (“polar front”).
Often develop in conditionally unstable environments with lots of
convection
Comma Clouds

Most apparent over oceans in winter, but
can develop over land
1445Z/05
GOES-12 Visible
2
1
1745Z/05
GOES-12 Visible
2
1
2045Z/05
GOES-12 Visible
2
1
15Z/05
2
2
Some, But Not All, Associated with
Lightning Strikes
Jan 18-21 2010
Lightning in Yellow
Nov 15-17 2009
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.
Sometimes the
associated
trough can
develop frontal
characteristics
There are a variety of ways for
comma clouds, and their
associated vorticity maxima, to
interact with the main baroclinic
zone/polar front
Little Interaction
“Instant” Occlusion
Instant Occlusion
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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 mature occlusion WITHOUT the
usual occluded front evolution.
Why such a small scale?
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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.
Polar Lows
Look somewhat like small hurricanes—spiral rain bands, cloud
free eye
Polar Lows (also known as
Arctic Hurricanes!)
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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.
Polar Lows
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Form rapidly when short-wave troughs
aloft approach such baroclinic, unstable
regions.
Favored locations: Bering Sea,
Greenland, Norwegian and Barents Seas,
Gulf of Alaska.
Polar Lows
Mesoscale Structure
Polar Low Mechanisms
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Because they look like hurricanes, some have
suggested they grow by similar air-sea
interaction mechanisms:
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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.
The END