Clouds marching past

The cold air outbreak that I wrote about a few days ago will hit the UK tomorrow (Sunday), with subzero temperatures as far south as London. Here’s a picture of the cold air on its way southwards, taken from Bergen today:


The tall clouds are “mild” Cumulonimbus clouds forming as the moist, warm air over the sea surface rises in the cold air from the Arctic. I remember seeing such clouds, albeit more vigorous ones, during the IPY-THORPEX polar low field campaign in Andøya in northern Norway in March 2008.

Polar low brewing outside northern Norway

I just got an alert from the excellent BarentsWatch site about a polar low off the coast of northern Norway. Here’s the latest satellite image (downloaded from


You can clearly see the dual system between Iceland and Norway, and what’s nice about this is that we also see the edge of the cold air outbreak stretching west from the polar lows. Here are some satellite-based wind speeds from about 1600 GMT yesterday, taken from this site:


This shows nicely how strong the winds are along the edge of the outbreak.

Update: I just found winds from this morning, too, and here wind speeds of more than 50 knots (Force 10 on the Beaufort wind force scale) were estimated by the satellite:


That’s pretty hefty. We can trace the development back to the disturbance seen in this picture from Wednesday, a few hours before the wind snapshot above:


It’s moving slowly, but seems to have built some strength since yesterday.

First polar low of 2015/16 season

Our good friend at the Norwegian Met Office in Tromsø, Gunnar Noer, just told me that they observed the first polar low (with an impact on Norway) this season a few days ago. Here’s a satellite image taken at 18:21 on 26 October 2015:


The polar low is clearly visible just north of northern Norway. At 20:00 UTC the polar low is more developed:


It’s also clear that the polar low formed in a cold air outbreak well north of the well polar front (the region that separates cold air in the north from warm air in the south), which is one of the main characteristics of polar lows.

I also found an image of satellite-derived surface wind speeds from the ASCAR radar (click image for higher resolution):


This shows that the polar low produced fairly strong winds. There’s plenty of other interesting features as well (as in pretty much every satellite image):

  • A jet along the sea ice edge just north of Iceland, probably due to the high mountains of Greenland. It’s more common to get jets in northerly flow in this region.
  • The left-side jet off the northern tip of Novaya Zemlya, also due to topography. Left-side jets are normally stronger than right-side jets because flow towards a barrier creates a small-scale high pressure anomaly in front of the barrier, and this strengthens the jet.
  • The strong southerly flow along the coast north of southern Norway, also a left-side jet.
  • The strong winds along the polar front, which isn’t unusual in any way.

Polar lows for Christmas

At the end of last year, a cold air outbreak over the Nordic Seas spawned a couple of nice polar lows. This picture was taken on Christmas Day:


The cold air outbreak reached down to Iceland, but there was a boundary-layer cold front, or what we would call an Arctic front, just north of 70 degrees north. These fronts can be nasty in themselves, with strong surface wind speeds and lots of snow, and they also breed polar lows. In the picture above there were two such lows. The only one that developed further, though, was the one on the right.

Here’s a picture taken about 36 hours later:


By this time the eastern part of the Arctic front has moved south, while the polar low stayed put. The white clouds indicate deep convection, meaning that it had developed tall clouds and probably lots of precipitation. The official meteorological station in Tromsø got 35 mm of snow during 25 and 26 December. Arctic fronts are much more shallow, as testified by the clouds along the one in the picture, but they can easily produce thunderstorms and heavy snow.

Labrador Sea Polar Low in Extreme Detail

We received the following e-mail from Prof. Kent Moore at the University of Toronto:

NASA today published a striking image of cloud streets over the Lab Sea. […] The low that was causing the trouble in Iceland can be seen up along the SE coast of Greenland. The flow distortion traced out by the clouds is striking as is the nascent polar low. You can see a slight ‘kink’ in the sea-leve pressure field from the ECMWF analysis.

This is a thumbnail of the NASA image (click for more detail) and click here for the NASA page with even higher resolution:


This is the ECMWF analysis, where you can see the trough off the coast of Labrador:




Polar low in massive cold air outbreak

This picture was taken just before noon on Friday 6 December 2013 (courtesy of the Dundee Satellite Receiving Station):


There is little reason to doubt that that’s a polar low up at 70 degrees north. It moves towards the south inside a cold air outbreak that covers much of the North Atlantic north of the British Isles.

There have been strong winds here in Bergen (at 60N on the west coast of Norway) ever since yesterday, when a strong low moved in from the west. The cold air outbreak is due to the northerly winds in the wake of that low. Check out the supply ships in the harbour of Bergen earlier today:


They have nowhere to go right now. The winds are fierce, there is a storm surge, and last night we had about 10 inches of snow. Pretty chaotic, in other words.

As for the polar low heading south, it’s a big one. This is an ECMWF wind speed forecast for later today (2100 GMT):


The wind speed is shown with colours in knots. On the Beaufort  scale, the lightest one of the two red colours is strong gale and the darkest one is storm. The tiny black speck is violent storm, just one step below hurricane force. Now wonder those ships are staying put in Bergen.

For the weather nerds, note that the air pressure in the polar low is quite high compared to other storms. The strong winds are due to the gradients between the polar low and the ridge over Greenland.  In addition, the  low is moving southwards, so that adds to the strong southerly winds near the core of the low.

Also note the extreme wind speed gradients. It is all but calm to the north-east of the low. This is another reason that polar lows are dangerous, the insane local variations, from violent storm to nothing in just a few tens of kilometers.

As for the path of the polar low, it looks like it’s going to die before it hits land. By early Saturday morning it will be over.

Polar lows influence ocean circulation

The strong winds and large ocean-atmosphere temperature differences associated with polar lows result in significant local impacts on the surface of the ocean. Indeed in some cases polar lows can generate heat fluxes of a similar magnitude to those seen in category one hurricanes. Althougth the sensitivity of the intensification of polar lows to sea surface conditions has been studied extensively, up to now the extent to which polar lows themselves influence large-scale ocean circulation has not been clearly demonstrated (“large-scale” here refers to the spatial scale of seas such as the Norwegian Sea). This is a difficult problem to study because ocean circulation varies on longer time scales and responds slowly to the cumulative effect of many polar lows. However, a study published in Nature Geosciences this month by Alan Condron and Ian Renfrew (C&R) is a major advance in understanding the effect of polar lows on ocean circulation. In particular their study demonstrates that polar lows probably have a significant influence on the strength of ocean currents in the northern North Atlantic. Their approach was to run a state-of-the-art ocean model both with and without the effects of small-scale (less than approximately 500 km) atmospheric cyclones (see Figure below). C&R note that they don’t exclusively assess polar lows in their study, but the combined effect of all detected small-scale cyclones. It is likely however that polar lows are the most important subset of these small-scale cyclones in generating the ocean response, although it would be interesting to confirm this in a future study since the wind drag of the other small-scale cyclones may be important.

Example case (b) with and (a) without the surface wind associated with a polar low. In more detail: (a) The near-surface wind speed without the polar low resolved, (b) as in (a) but with the addition of an estimated wind field from the polar low and (c) best estimates of the actual wind field. For details of the methodology see supplementary material of Condron and Renfrew, from which Figure S1 is reproduced here by permission from Macmillan Publishers Ltd: Nature Geoscience, copyright (2013).

C&R demonstrate that small-scale cyclones have a significant effect on simulated northeast Atlantic Ocean circulation. Most significantly, the current that flows westward around Greenland and then southward down the Labrador Sea (the North Atlantic subpolar gyre) increases in strength by 5.5% when small-scale cyclones are taken into account. As a result the northward transport of heat to northern Europe and North America is increased. This effect is missing in most global climate models, which do not have a sufficiently high spatial resolution to capture small-scale cyclones. This motivates the use of higher-resolution climate models to capture this and other important processes in seasonal and climate forecasting.

Full reference

Condron, A. and I. A. Renfrew (2013). The impact of polar mesoscale storms on northeast Atlantic Ocean circulation. Nature Geoscience, 6, 34-37, doi:10.1038/ngeo1661.