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Torsten Linders and Øyvind Saetra

tornadoes over land, tropical cyclones over the low-latitude ocean, and polar lows over the high-latitude ocean ( Holton 2004 ; Rasmussen and Turner 2003 ). Today, CAPE and its negative counterpart of convective inhibition (CIN) are used operationally in the United States to asses the risk of extreme wind strengths. Charney and Eliassen (1964) introduced the theory of conditional instability of the second kind (CISK) for tropical cyclones. They proposed that the energy for such systems comes from the

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Birgitte Rugaard Furevik, Harald Schyberg, Gunnar Noer, Frank Tveter, and Johannes Röhrs

1. Introduction and state of art Since synoptic observations are sparse in the polar regions, forecasting and monitoring of polar lows (PL) are to a large degree based on satellite observations from passive radiometers, in particular the National Oceanic and Atmospheric Administration (NOAA) Advanced Very High Resolution Radiometer (AVHRR), and from scatterometer winds in addition to numerical models. Polar lows are relatively small (200–1000 km) short-lived (1–2 days) cyclones that form over

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Wataru Yanase, Hiroshi Niino, Shun-ichi I. Watanabe, Kevin Hodges, Matthias Zahn, Thomas Spengler, and Irina A. Gurvich

1. Introduction Polar lows (PLs) are intense maritime cyclones with horizontal scales between 200 and 1000 km that develop poleward of the main baroclinic zone ( Rasmussen and Turner 2003 ). Since satellite observation started to operate in the late twentieth century, there have been a number of studies on PL formation over high-latitude oceans including the Barents Sea ( Rasmussen 1985 ), the Norwegian Sea ( Wilhelmsen 1985 ), the Labrador Sea ( Mailhot et al. 1996 ), the Gulf of Alaska

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Wataru Yanase and Hiroshi Niino

1. Introduction Polar lows are mesoscale cyclones that develop over high-latitude oceans on the polar side of the polar front in cold seasons ( Harley 1960 ; Businger and Reed 1989 ; Heinemann and Claud 1997 ; Rasmussen and Turner 2003 ). Their horizontal scale ranges from 200 to 1000 km and the maximum wind speed exceeds 15 m s −1 . A number of meteorologists have been interested in the structure and dynamics of polar lows not only because they cause hazardous weather but also because they

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Burghard Brümmer, Gerd Müller, and Gunnar Noer

1. Introduction Polar lows are intense mesoscale vortices that form over the open ocean in the cold air mass north of the polar front in the wintertime season (for a definition see, e.g., Rasmussen and Turner 2003 ). Their generation mechanisms are manifold. Often weak disturbances in the low-level flow that, for example, are triggered orographically or by the ocean–sea ice temperature contrast grow if favorable forcing from the upper troposphere is present (e.g., Rasmussen et al. 2003

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Julia Smirnova and Pavel Golubkin

1. Introduction The most intense subclass of polar maritime mesoscale cyclones characterized by horizontal scales less than 1000 km and near-surface wind speeds near or above gale force are the polar lows (PLs), which are found on both hemispheres and form poleward of the main baroclinic zone ( Rasmussen and Turner 2003 ). These cold-season phenomena have a lifetime that is generally only 12–36 h ( Blechschmidt 2008 ; Smirnova et al. 2015 ), which in conjunction with their small size and a

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Annick Terpstra, Clio Michel, and Thomas Spengler

1. Introduction Polar lows are intense, mesoscale cyclones developing over ice-free oceans at high latitudes (e.g., Rasmussen and Turner 2003 ). Previous climatologies of polar lows focused mainly on the large-scale atmospheric conditions associated with polar low events and thereby neglected subsynoptic features (e.g., Businger 1985 ; Claud et al. 2007 ; Blechschmidt et al. 2009 ; Mallet et al. 2013 ). However, given the relatively small length scale of polar lows, with a diameter between

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Giuseppe Zappa, Len Shaffrey, and Kevin Hodges

1. Introduction Polar lows are intense maritime mesocyclones forming at high latitudes during cold air outbreaks ( Rasmussen and Turner 2003 ). They have a radius of the order of 100–500 km and surface wind speeds above 15 m s −1 . This makes them a major source of weather risk in high-latitude coastal areas and it raises interest in how they might be affected by climate change ( Kolstad and Bracegirdle 2008 ; Zahn and von Storch 2010 ). Moreover, polar lows are associated with large heat

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Longtao Wu and Grant W. Petty

1. Introduction A polar low is an intense mesoscale cyclone that often develops over high latitude oceans in the cold airstreams of the polar air mass. Rasmussen and Turner (2003) define a polar low as a “small, but fairly intense maritime cyclone that forms poleward of the main baroclinic zone (the polar front or other major baroclinic zone). The horizontal scale of the polar low is approximately between 200 and 1000 kilometers and surface winds near or above gale force

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Clio Michel, Annick Terpstra, and Thomas Spengler

1. Introduction Polar mesoscale cyclones (PMCs) are cyclonic systems with a horizontal extent less than 1000 km and a lifetime of about one day ( Rasmussen and Turner 2003 ). They occur poleward of the polar front, over maritime areas, in both hemispheres and mainly during the cold season. The most intense PMCs, referred to as polar lows, are associated with cold air outbreaks (CAOs) and feature strong winds (e.g., Businger 1985 ; Harold et al. 1999a ; Rasmussen and Turner 2003 ). The least

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