Polar Lows 461 Definition

Polar Lows are intense maritime mesocyclones of typically 100-500 km in diameter. They may intensify rapidly and surface wind speeds can sometimes reach hurricane force (Businger and Reed, 1989). They tend to be short lived, generally lasting only 3-36 hours. Polar Lows are the most intense category of the family of mesoscale cyclonic vortices found poleward of the main polar front, which are known generically as polar mesoscale cyclones. Polar Lows, which can present significant hazards to shipping, are also known as Arctic Instability Lows, comma clouds and, in the Southern Hemisphere, Antarctic coastal vortices. In the north polar regions, Polar Lows are particularly common in the Nordic Seas, the Labrador Sea, the Bering Sea, the Gulf of Alaska and the Sea of Japan. Hundreds of mesoscale lows can develop in these regions on an annual basis (Renfrew, 2003). Satellite images of two Polar Lows appear in Figures 4.15 and 4.16. The characteristics of Polar Lows have been examined in a number of studies. Papers by Rasmussen (1979), Shapiro et al. (1987a), Businger and Reed (1989), Emanuel and Rotunno (1989), Gr0nas and Kvamst0 (1995) and Carleton (1996) are worth reading. Rasmussen and Turner (2002) provide a comprehensive assessment of Polar Lows while Renfrew (2003) provides a useful shorter review, which we draw from here.

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Figure 4.15 NOAA-9 image for February 27, 1987 of a Polar Low off the coast of northern Norway. Note the spiral cloud signature and central eye in this example (by permission of the NERC Satellite Receiving Station, University of Dundee, http//www.sat. dundee.ac.uk/).

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