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A polar low is a small-scale, short-lived atmospheric low pressure system
Low pressure area

A low pressure area, or "low", is a region where the atmospheric pressure is lower in relation to the surrounding area. Low pressure systems form under areas of upper level divergence on the east side of upper troughs, or due to localized heating caused by greater insolation or active thunderstorm activity....
 (depression) that is found over the ocean areas poleward of the main polar front
Polar front

In meteorology, the polar front is the boundary between the polar cell and the Ferrel cell in each hemisphere. At this boundary a sharp gradient in temperature occurs between these two air masses, each at very different temperatures....
 in both the Northern and Southern Hemispheres. The systems usually have a horizontal length scale of less than and exist for no more than a couple of days.






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Polar Low
A polar low is a small-scale, short-lived atmospheric low pressure system
Low pressure area

A low pressure area, or "low", is a region where the atmospheric pressure is lower in relation to the surrounding area. Low pressure systems form under areas of upper level divergence on the east side of upper troughs, or due to localized heating caused by greater insolation or active thunderstorm activity....
 (depression) that is found over the ocean areas poleward of the main polar front
Polar front

In meteorology, the polar front is the boundary between the polar cell and the Ferrel cell in each hemisphere. At this boundary a sharp gradient in temperature occurs between these two air masses, each at very different temperatures....
 in both the Northern and Southern Hemispheres. The systems usually have a horizontal length scale of less than and exist for no more than a couple of days. They are part of the larger class of mesoscale
Mesoscale meteorology

Mesoscale Meteorology is the study of weather systems smaller than synoptic scale systems but larger than Microscale meteorology and storm-scale cumulus systems....
 weather systems. Polar lows can be difficult to detect using conventional weather reports and are a hazard to high-latitude operations, such as shipping and gas and oil platforms. Polar lows have been referred to by many other terms, such as polar mesoscale vortex, Arctic hurricane, Arctic low, and cold air depression. Today the term is usually reserved for the more vigorous systems that have near-surface winds of at least 17 m/s.

History

Polar lows were first identified on the meteorological satellite imagery that became available in the 1960s, which revealed many small-scale cloud vortices at high latitudes. The most active polar lows are found over certain ice-free maritime areas in or near the Arctic
Arctic

The Arctic is the region around the Earth's North Pole, opposite the Antarctica region around the South Pole. The Arctic includes the Arctic Ocean and parts of Canada, Greenland , Russia, the United States , Iceland, Norway, Sweden and Finland....
 during the winter, such as the Norwegian Sea
Norwegian Sea

The Norwegian Sea is part of the North Atlantic Ocean northwest of Norway, located between the North Sea and the Greenland Sea.It adjoins the Iceland Sea to the west and the Barents Sea to the northeast....
, Barents Sea
Barents Sea

The Barents Sea is a part of the Arctic Ocean located north of Norway and Russia. It is a rather deep Continental shelf sea , bordered by the shelf edge towards the Norwegian Sea in the west, the island of Svalbard in the northwest, and the islands of Franz Josef Land and Novaya Zemlya in the northeast and east....
, Sea of Japan
Sea of Japan

The Sea of Japan is a marginal sea of the western Pacific Ocean, bordered by Japan, South Korea, North Korea and Russia. It is referred to in North Korea as the Korea East Sea and in South Korea as the East Sea....
, and Gulf of Alaska
Gulf of Alaska

The Gulf of Alaska is an arm of the Pacific Ocean defined by the curve of the southern coast of Alaska, stretching from the Alaska Peninsula and Kodiak Island in the west to the Alexander Archipelago in the east, where Glacier Bay and the Inside Passage are found....
. Polar lows dissipate rapidly when they make landfall. Antarctic systems tend to be weaker than their northern counterparts since the air-sea temperature differences around the continent are generally smaller. However, vigorous polar lows can be found over the Southern Ocean
Southern Ocean

The Southern Ocean, also known as the Great Southern Ocean, the Antarctic Ocean and the South Polar Ocean, comprises the southernmost waters of the World Ocean south of 60th parallel south latitude....
.

Structure

Polar lows can have a wide range of cloud signatures in satellite imagery, but two broad categories of cloud forms have been identified. The first is the "spiraliform" signature consisting of a number of cloud bands wrapped around the centre of the low. Some polar lows have the appearance in satellite imagery of tropical cyclones, with deep thunderstorm clouds surrounding a cloud-free ‘eye
Eye (cyclone)

The eye is a region of mostly calm weather found at the center of strong tropical cyclones. The eye of a storm is a roughly circular area and typically 30?65 km in diameter....
’, which has given rise to the use of the term "Arctic hurricane" to describe some of the more active lows. These systems are more common deep within the polar air. The second is a "comma-shaped" signature that is found more frequently with systems closer to the polar front.

Formation

Polar lows form for a number of different reasons, and a spectrum of systems is observed on satellite imagery. A number of lows develop on horizontal temperature gradients through baroclinic instability, and these can have the appearance of small frontal depressions. At the other extreme are the polar lows with extensive cumulonimbus clouds, which are often associated with cold pools in the mid- to upper-troposphere
Troposphere

The troposphere is the lowest portion of Earth's atmosphere. It contains approximately 75% of the atmosphere's mass and almost all of its water vapor and particulate....
.

Forecasting

Polar lows are very difficult to forecast and a nowcasting
Weather forecasting

Bold text'Weather forecasting is the application of science and technology to predict the state of the Earth's atmosphere for a future time and a given location....
 approach is often used, with the systems being advected with the mid-tropospheric flow. Numerical weather prediction models are only just getting the horizontal and vertical resolution to represent these systems.

See also

  • Polar cyclone
    Polar cyclone

    Polar cyclones are vast areas of low pressure which strengthen in the winter and weaken in the summer.. A polar cyclone is a low pressure weather system usually spanning 1,000–2,000 kilometers in which the air is circulating in a counter-clockwise fashion ....


External links

  • by MetEd