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What is a 'Polar low'?

When arctic-origin air in winter flows southward (northward in the southern hemisphere) across (relatively) warmer seas, strong surface heating acts both to enhance the degree of instability, and trigger vigorous moist convective towers. This is sufficient alone to give rise to heavy, wintry showers/cumulonimbus clusters etc., but often marked troughing, or even a closed circulation in the isobaric flow is found; the resultant low-level convergence/positive vorticity enhancement, plus the localised concentration of the latent heat energy released, enhances development within the system, and an intense (but synoptically small) area of rain, hail, sleet or snow & squally winds can result - a polar low (or polar depression or polar meso-cyclone in some texts).

The dynamics of such systems are not fully understood, and it is only with the (recent) arrival of very high-resolution satellite imagery & sensors in a wide variety of spectral bands that the detail within such systems can be studied. Even so, for operational meteorologists, careful monitoring of all available data is required; Geostationary satellite imagery has a rather course resolution at high latitudes, and the visible channels are of little use in the winter season. Polar orbiter passes (which give much higher resolution imagery) may not be frequent enough to maintain a continuous watch on developments.

Numerical models also have difficulty with such events; they are born in data-sparse regions, and most schemes 'paramaterize' convection i.e. models don't explicitly forecast each individual convective event, but rather indicate the degree of instability expected, its areal extent etc., and thus have problems going one step further and turning an area of disorganised (model) convection into an organised self-sustaining polar low/trough, where upper troughs are not the primary forcing mechanism. The one remaining Norwegian weather ship, and a handful of research and fishing vessels may be the only clues to developments taking place in, for example, the Norwegian Sea.

Polar Lows can develop, and move (in the prevailing flow) with surprising speed, and lead to considerable dislocation of normal life in regions directly affected. Preferred locations for genesis are to the west of large, slow-moving occluded depressions - i.e. those with a pre- existing rear-flank arctic flow. It may be that the geography of the regions in question play a significant part in genesis of polar lows - Dave Wheeler, who I am grateful to for checking much of the above, suggests that vortices shed by high-arctic island groups (e.g. Svalbaard) are enhanced by the land mass of Scandinavia (Norway) to the east and Greenland and its ice shelf to the west.