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Apolar low is amesoscale, short-lived atmosphericlow pressure system (depression) that is found over the ocean areas poleward of the mainpolar front in both the Northern and Southern Hemispheres, as well as theSea of Japan.[1] The systems usually have a horizontal length scale of less than 1,000 kilometres (620 mi) and exist for no more than a couple of days. They are part of the larger class ofmesoscale 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 andoil platforms. Polar lows have been referred to by many other terms, such aspolar mesoscale vortex,Arctic hurricane,Arctic low, andcold air depression. Today the term is usually reserved for the more vigorous systems that have near-surface winds of at least 17 m/s (38 mph).[2]
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 theArctic during the winter, such as theNorwegian Sea,Barents Sea,Labrador Sea andGulf of Alaska; however, polar lows also have been found in theSea of Japan and theSea of Okhotsk. 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. Still, vigorous polar lows can be found over theSouthern Ocean.
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’, 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.
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 throughbaroclinic instability, and these can have the appearance of small frontal depressions. At the other extreme are the polar lows with extensivecumulonimbus clouds, which are often associated with cold pools in the mid- to upper-troposphere. During winter, when cold-core lows with temperatures in the mid-levels of the troposphere reach −45 °C (−49 °F) move over open waters, deep convection forms which allows polar low development to become possible (polar lows usually occur with cold air outbreaks[3]).[4]
Although cyclonic activity is most prevalent in the Eurasian Arctic with approximately 15 lows per winter, polar lows also occur inGreenland and theCanadian Arctic. Polar lows occur in the extended winter season, with seldom occurrences during the summer. They are not well studied and seldom destructive as they typically take place in sparsely populated areas. The only infrastructure damage that occurs as a direct result of a polar low is to oil and gas rigs present throughout the Antarctic Ocean (sometimes known as theSouthern Ocean). Some cargo and shipping vessels are also affected, although there are minimal or no reports of losses in recent years as the result of a polar low.
Despite the more equatorward location, numerous polar lows can form over theSea of Japan every year, due to theJapan-Sea Polar-Airmass Convergence Zone (JPCZ) contributed by thecold-core low aloft and the warmTsushima Current. They would bring severe impacts toJapan owing to the proximity to populous regions, with strong winds and heavy snowfall. On December 28, 1986, seven passenger cars of a railway train were blown from theAmarube Viaduct due to a polar low, causing six deaths.[5]
Polar lows are very difficult to forecast and anowcasting 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.[when?]
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