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.

Cold air outbreak in amazing detail

The current cold air outbreak, the first proper one this winter (see previous post), reveals incredible small-scale structure in the satellite image below. The land in the upper middle part of the picture is the Svalbard archipelago in the Northeast Atlantic.Image

The resolution of the original image (which you can see by clicking on the image above or go here) is 500 metres, which means that each pixel in the image is 500 metres wide. For maximum detail, check out this image at a resolution of 250 metres. There are so many things to see in the image, but here are some of the highlights.

In the upper left part of the picture, the white tendril-like features are sea ice. We also see some large floes. It’s all being blown southwards and at the same time broken up by the strong surface winds.

The north-south-oriented stripes that are made up of individual white dots are what we call cloud streets. They only occur when there’s a lot of convection (rising air) going on, and the dots themselves are cumulus clouds. Further south, away from the sea ice edge, the convection becomes less intense, and the cumulus clouds merge together into large patches of stratocumulus clouds. Cloud streets are a tell-tale sign of strong surface winds.

Downstream from Svalbard and in the lower part of the picture, the clouds get organized into a vortex. This is because there’s a small low-pressure system there, and the air starts to move in spiral-like, anti-clockwise patterns around the low (because of the earth’s rotation and the Coriolis force). The beautiful pattern we see in the image is, in other words, a polar low. Not a big one, but a polar low nonetheless.

Another interesting thing in the image are the wave clouds over and downstream from Svalbard. They form because the air moves over the mountains, much in the same way that waves form when water in a river hits rocks.

If you want to see more images from this region, take a look at NASA’s Hornsund subset.

First polar low this season

The satellite image on the left shows what I think must be the first polar low in the Northeast Atlantic this winter. At least it’s the first time snow is forecast for large parts of Norway, a sure sign that there’s a cold air outbreak in its way. The cold air outbreak itself is also clearly visible in the image, with its patchy or dotty structure. These are clouds that are organized into what we call “convective cells”.

A satellite image of the Northeast Atlantic, taken at 0940 on 23 October 2012. Downloaded from the Dundee Satellite Receiving Station in Scotland.

Convection is the meteorological term for rising air, and rising air is what creates all kinds of clouds. The convective cells in the picture are cumulus clouds which are formed because the cold air is heated from below by the (relatively) warm ocean surface, much the same as what happens when you blow on a bowl of hot soup. When air is heated, it gets lighter, and therefore it rises, letting colder air flow in from all sides to replace it.

So, the cold air outbreak in the picture sets up a lot of rising motion in the air masses. At the same time, there’s evaporation going on so that the air gets humid. And when humid air rises fast to create cumulus clouds, you get precipitation, in this case in the form of massive snow amounts forecast for Norway. Along the coast, people are going to witness tall cumulus and cumulonimbus clouds marching past them on their way south. Cold air outbreaks don’t always come as far south as Bergen, but I sometimes see these majestic clouds trotting along. It’s a very nice spectacle.

Arctic hurricane?

Do hurricanes form in the Arctic? Sometimes polar lows closely resemble hurricanes and a detailed study of one such case has just been published. The polar low (PL) in question occurred on 18-21 December 2002 over the Barents Sea, north of Norway, and is clearly visible at the top right of the image below. This shows spiral bands of cloud surrounding a clear eye at the centre of the storm, which are common features of hurricanes. The study, led by Ivan Fore, used a state-of-the-art weather computer model to answer the intriguing question of what caused this “hurricane-like”PL to intensify. Using this weather computer model they were able assess the details of the polar low dynamics that are not apparent from satellite imagery.

Infra-red satellite image of the polar low over the Barents Sea taken at 0204UTC, 20 December 2002. Source: NERC Dundee Satellite Receiving Station, http://www.sat.dundee.ac.uk

Very strong fluxes of thermal energy from the ocean to the atmosphere were found around the central eye, which are similar in magnitude to those observed in hurricanes. These large fluxes come mainly from the very large temperature difference between the winter atmosphere and open ocean, which can often be around 20 deg C. However, the maximum simulated surface winds of about 90 km/hour were not as strong as the threshold for category one hurricanes of 118 km/hour.

The results showed that the period of rapid intensification did not resemble a hurricane. The dynamics were closer to a standard mid-latitude cyclone (known as baroclinic). However a maintenance period followed the intensification, during which hurricane-like dynamics dominated. The polar low then decayed after moving over land, which cuts off the supply of energy from the ocean.

Would hurricane-like maintenance, or even intensification, have continued if the PL had not moved over land? Theory suggests that in winter the temperature difference between the atmosphere and ocean (and therefore potential for large energy fluxes) is large enough to support a genuine Arctic hurricane. One possible reason that PLs never develop into mature hurricanes is that there is simply not a large enough expanse of relatively warm ocean over the Arctic in winter. Hurricanes take of the order of five days to drift across the tropical Atlantic and slowly intensify, whereas most polar lows move over land or ice in one day.

Citation: Fore, I., J.E. Kristjansson, E.W. Kolstad, T.J. Bracegirdle, O. Saetra and B. Rosting (2012). A ‘hurricane-like’ polar low fuelled by sensible heat flux: High-resolution numerical simulations. Quarterly Journal of the Royal Meteorological Society, published online, doi:10.1002/qj.1876.