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