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