Download - Cyclogenesis in Polar Airstreams
Cyclogenesis in Polar Cyclogenesis in Polar AirstreamsAirstreams
• 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 kmPolar Lows: 100-500 km
Comma Clouds• Convection and sometimes stratiform-type clouds Convection and sometimes stratiform-type clouds
organized into a comma-shaped patternsorganized into a comma-shaped patterns• Smaller than normal synoptic systems and on the cold Smaller than normal synoptic systems and on the cold
side of the midlatitude jet stream.side of the midlatitude jet stream.• Most apparent over the oceans during a period with Most apparent over the oceans during a period with
high-amplitude, long-wave trough developmenthigh-amplitude, long-wave trough development
Generally of a smaller scale (500 to 1000 Generally of a smaller scale (500 to 1000 km) than classic midlatitude cyclones.km) than classic midlatitude cyclones.
Frequently multiple with typical spacing of 1000-1500 kmFrequently multiple with typical spacing of 1000-1500 km
Usually associated with the region of positive Usually associated with the region of positive vorticity advection (PVA) associated with a vorticity advection (PVA) associated with a short-wave trough aloft.short-wave trough aloft.
Scale often grows in time, Scale often grows in time, particularly as they move particularly as they move
through a long wave troughthrough a long wave trough
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
Associated with regions of appreciable Associated with regions of appreciable baroclinicity (temperature gradient) on the baroclinicity (temperature gradient) on the cold side of a major baroclinic zone (“polar cold side of a major baroclinic zone (“polar front”).front”).
Often develop in conditionally unstable environments with lots of Often develop in conditionally unstable environments with lots of convectionconvection
Comma CloudsComma Clouds
Most apparent over oceans in Most apparent over oceans in winter, but can develop over landwinter, but can develop over land
1445Z/05 GOES-12 Visible
12
1745Z/05 GOES-12 Visible
12
2045Z/05 GOES-12 Visible
12
15Z/05
2
2
Some, But Not All, Associated Some, But Not All, Associated with Lightning Strikeswith Lightning Strikes
Jan 18-21 2010Lightning 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 Sometimes the the
associated associated trough can trough can
develop develop frontal frontal
characteristicharacteristicscs
There are a variety of ways for comma clouds, and their
associated vorticity maxima, to interact with
the main baroclinic zone/polar front
Little InteractionLittle Interaction
““Instant” OcclusionInstant” Occlusion
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 mature occlusion WITHOUT the usual occluded front evolution.
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.
Polar LowsPolar LowsLook somewhat like small hurricanes—spiral rain bands, cloud free eye
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.
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.
Polar Lows
Mesoscale Structure
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.
The END