Lecture 22 Polar Lows Polar low seen in an infrared image from a polar orbiting satellite 1 What is a Polar Low? A small-scale (typically 200-300 mi in diameter) cyclone that forms in a cold air mass poleward of major jet streams or frontal zones. 2 Small Scale Polar low seen in an infrared image from a polar orbiting satellite NOAA-5 infrared satellite photograph of a polar low over the Bering Sea and Bristol bay; at 21:27 GMT on 19 January 1979. 3 Midlatitude Cyclone North-East Pacific Low 19 December 2002 4 Polar Low Animation Visible Satellite Loop, November 1, 2006 5 Polar Low Animation Infrared Satellite loop with surface pressure field overlay 6 Polar Low Animation Infrared Satellite loop with 500 mb height field overlay 7 Polar Low Characteristics • Non-polar frontal low-pressure systems • Cyclonic wind circulation (in Northern Hemisphere) • Anticyclonic outflow at upper levels • Warm Core • Winds decrease with height • Vertical structure, spiral or symmetric about clear "eye" • Sensible and latent heat from the ocean primary energy source • Formation over high latitude oceans adjacent to snow or ice covered surfaces 8 Forecast Challenge Polar Low over the Barents Sea area at 0240 UTC 13 December 1982. • Small scale - 200 - 300 mi in diameter • Data sparse habitat - over high latitude oceans • Rapid development - ~12 hour spin up 9 Weather Related Hazards: • High winds (up to 90 kt observed) • Large waves and swell • Low visibility, heavy snow, icing of ships • Thunderstorms and in some cases tornados 10 Form to the North and West of the Polar Front Polar lows do not form on the polar front Schematic Model 11 Comma Clouds 12 Comma Cloud This one produced tornados in CA! 13 a) Schematic depiction of a series of developing comma clouds forming within a 500 mb cold-core low (L). b) NOAA-7 infrared- satellite image at 1508 PST, 16 January 1982, showing high clouds associated with the incipient comma cloud Case 1 and a mature comma cloud that preceded it. 14 Schematic Model 15 Schematic Model 16 Schematic Model 17 Prerequisites for Formation • Cold temperatures in the upper troposphere • Air passing from snow or ice covered surface over the open ocean leading to enhanced surface sensible and latent heat fluxes • Curved flow (large vorticity or spin) at the surface and aloft 18 Prerequisites for Formation Habitat Sea-surface temperature and extent of the ice edge (shaded area) for 9-12 February 1984 19 Polar Low Climatology Paths of polar lows over the Norwegian Sea 1978-1982. Closed circles indicate point of origin, open circles indicate final position. 20 Polar low and cloud streets over the Barents Sea: NOAA-7 infrared satellite photograph for 0320 GMT, 22 November 1983 21 Polar Low over the Norwegian Sea area at 1340 UTC 27 February 1984. 22 Polar Low over the Norwegian Sea area at 1340 UTC 27 February 1984. 23 Sea-level pressure analysis of Polar Low over the Norwegian Sea area at 1340 UTC 27 February 1984. 24 Polar Low Habitat Mean sea-surface temperature (°C) and extent of the ice edge (heavy dashed line) for the Bering Sea and the North Pacific Ocean; 20 year mean (1957-1978) for January. 25 NOAA-5 infrared satellite photograph of a polar low and cloud streets over the Bering Sea at 22:24 GMT on 23 January 1979. 26 NOAA-4 infrared satellite photograph of polar low and cloud streets over the Gulf of Alaska at 20:21 GMT on 22 March 1975. 27 Polar Low Climatology Histogram of the number of days per month on which polar lows were observed in polar-orbiting satellite imagery over the Gulf of Alaska or the Bering Sea during the period 1975-1983. 28 Polar Lows over Mediterranean Sea Polar lows are sometimes called extra tropical hurricanes. 25 January 1982 26 January 1982 29 Polar Low in Southern Hemisphere NOAA-6 enhanced infrared-satellite photograph of a polar low near the ice edge just west of the Palmer Peninsula, Antarctica at 18:25 GMT on 15 march 1985. 30 Summary • Conditions favorable to Polar Low Development – Strong cold air advection over water. – Cold temperatures at 500 mb - unstable air • Forecast Challenge – Small scale - subsynoptic – Data sparse habitat - over high latitude oceans – Rapid development - ~12 hour spin up • Weather Related Hazards – High surface winds (up to 90 kt observed) – Large waves and swell – Low visibility, heavy snow, icing of ships – Thunderstorms and in some cases tornados 31 Questions? Why can’t I show you a nice satellite loop of this polar low? 32.