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, 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.

Polar low hits the UK

This picture shows a nice polar low when its centre was north of Scotland in the early hours of 6 December 2011. (Image downloaded from the Dundee Satellite Receiving Station.) It later moved towards the south-east and led to snowfall in western Norway – snow still on the ground outside my window. The same cold air outbreak, which covered large parts of the Nordic Seas, also left some snow in the UK. You can see the cloud streets stretching all the way up to the East Greenland coast in the same image. It’s quite rare that polar lows move this far south, and this is a particularly nice specimen, with the beautiful spiral form near the low centre. Let’s hope that we get more of these this winter…

Here’s a StormGeo surface wind speed forecast for 0800 in the morning on the same day (click on the picture for a larger version):

A band of pretty strong winds, up to 24 m/s, hit Shetland according to the forecast. Although I don’t know how correct the forecast was, it’s very interesting to look at the structure of the winds. The area with strong winds is quite small – this is typical for polar lows. Eyewitness accounts report that they see a wall of clouds coming towards them from the north-east in otherwise calm weather. This is not what would happen in a regular storm coming in from the south-west. Then you can see the storm advancing hours ahead – typically you see high, wispy cirrus clouds first, followed by a denser, more stratified layer of clouds, and then you get the rain, snow and/or hail in the cumulus clouds or thunderstorms. Polar lows just sneak up on you, and that’s exactly why they’re so dangerous.

I’ve read lots of accounts of shipwrecks in northern Norway, and one pattern seems to stand out. The fishermen would stay ashore while large storms rolled by from the south, and then when the weather cleared, they were understandably anxious to go out in their boats. Thr trouble is that polar lows almost always form in the cold air outbreaks that follow behind these storms, and then they would get surprised by very strong, sudden winds. This can still happen; polar lows are notoriously difficult to forecast, but it is encouraging to see that we at least got the general structure of this one right.

Polar low in the wake of storm

A terrific storm moved north along the coast of northern Norway on 25 and 26 November 2011, causing massive damage to houses and boats. It was well forecast, so one can only hope that no one was out at sea during its passage. A cold air outbreak in the wake of the storm was also well forecast, and even the development of a polar low in those cold air masses.

This is the forecast for midnight on Saturday, where the storm, which was called “Berit” by the way, is clearly visible just off the coast of Lofoten in northern Norway:

If you look at the isobars (the black curves) circling around the centre of the storm, you can tell that there were northerly winds coming down from the Svalbard region and the Barents Sea. This is because the wind blows roughly parallel to the isobars, and the direction is always counterclockwise around the low centre.

Now take a look at the “analysis”, which was computed after the fact, taking all observations into account, for midnight on Sunday:

There seems to be a dual feature in pretty much the same location as the storm was in the previous picture. (Also note that there seems to be a polar low in development west of Iceland.) These are polar lows that have formed as the icy cold winds blew down from the north and interacted with the much warmer ocean surface. The process is the same as what happens when you blow on a bowl of hot soup. The air is a lot colder than the soup, and therefore gets heated from below and starts rising and producing steam. Over the ocean, the steam turns into clouds, and nasty ones at that if the temperature difference between the water and the air is large enough. The perfect recipe for polar lows.

The picture on the left is a satellite image taken at 3 in the morning on Sunday, three hours after the analysis above. Click on the image for a larger version. You’ll have no trouble picking out the polar low here, but I’ve marked it in the picture all the same. You can also see the original storm, which has moved up into the Barents Sea now. There’s also a new storm coming in from the south, which is producing heavy rain here in Bergen as I write this.

The current forecast predicts that the PL will move towards the south-east and hit the coast of Norway later today. It might also merge with the new storm coming in.

The consequences will probably be thunderstorms, heavy snowfall, and a drop in temperatures. These are common features of polar lows, and is why they have always represented a major hazard for the coastal communities in northern Norway, Iceland and Japan, which are the regions that are most prone to polar lows. The odd polar low also strikes the UK and the island communities further north (Shetland, the Faeroes), sometimes causing panic on the roads.

I’ll try to get a good capture from the MODIS satellite as well.

First cold air outbreak 2011-2012

This satellite image was taken on 10 October 2011, when we got the first snows in the mountains here in western Norway. The first brush with winter, but as you can see from the image, no proper polar lows formed over the Atlantic. But they’re coming soon.

First satellite image of a polar low

For some historical background, here’s what is probably the first satellite image of a polar low.  It was taken from the NASA Nimbus 3 satellite on 5 January 1970. The clouds associated with the polar low can be clearly seen to the north-west of Scotland.

 

Up to this point observational studies had been based on other sources of data such as weather balloons. This polar low was the subject of a case study in Lyall (1972), who used this image as part of the study. At this time there was uncertainty over whether polar lows were driven directly by heat transfer from the ocean (i.e. a mechanism similar to hurricanes) or mainly from rebalancing of atmospheric horizontal temperature gradients (i.e. a smaller version of the majority of mid-latitude cyclones). This picture suggested the latter. However, as more polar lows were studied and observed from satellite imagery it was found that some appear more similar in structure to hurricanes.

Lyall, IT (1972). The polar low over Britain. Weather, 27, 378-390.

 

Barents Sea polar low 9 Feb 2011

There was a classic Barents Sea polar low yesterday. It’s apparent at the top right of this NOAA 16 image from 1740 on 9 Feb, downloaded from the NERC Dundee Satellite Receiving Station.

Polar low and von Karman vortices

Check out the incredible detail in this image, downloaded from NASA’s MODIS Rapid Response System. The island at the top is Spitsbergen, the biggest island in the Svalbard achipelago. South of it there’s a polar low. It’s also cool to see the wake to the west of Svalbard. There’s probably very weak winds in that area, but there’s a strong tip jet around Spitsbergen’s south cape. I sailed through this jet when I was onboard a coast guard icebreaker in 2008. Standing on the bridge, I could see that the wind speed increased rapidly from 22 knots to 44 knots inside the narrow jet, and then it dropped down to 22 again as we sailed out of it.

Near the bottom of the image, the small island of Jan Mayen is seen, and downstream of it, some amazing von Kármán vortices are very evident. I zoomed in on these vortices here:

Cloud streets

Although there are no polar lows in this picture, I thought it was too nice not to share. It was captured by NASA’s Terra satellite and shows cloud streets off the coast of the eastern US [click here for their article]. These are cloud features that form during cold air outbreaks, when you get really cold air streaming off a cold continent (or sea ice) onto a warm ocean surface. Much in the same way that you get steam when you blow on hot soup, this leads to evaporation from the sea, and cloud formation. The most common cloud type during cold air outbreaks is Stratocumulus or Cumulus, but sometimes you get Cumulonimbus thunderstorms as well.

The clouds in the cloud streets are often called roll clouds. Here’s what Liu et al. (2004, Geophysical Research Letters) [read their paper here] had to say about them:

Convective roll clouds are one of the most common forms of shallow boundary layer convection. They typically develop during cold-air outbreaks that are characterized by large air-sea temperature and humidity differences and high surface wind speeds. These characteristics provide favorable conditions for the development of convection that is organized into long quasi two-dimensional rolls. This organization results in cloud streets or roll clouds oriented along the direction of the mean low-level wind. Typically, the roll clouds are observed to evolve into cellular convection as one moves downstream from the coastline or ice edge.

In NASA’s story you’ll find a high-resolution version of the same image. I’ve cropped the image to show some details:

Click the image for even more detail. What interests me here is that there is clearly a narrow band near the coast with no clouds. Liu et al. managed to capture that with their numerical model:

This means that “we” (not I) have a certain understanding of the physics behind roll clouds and cloud streets. And that’s important, because polar lows always form in cold air outbreaks.

Polar lows in the future

In September 2010, Nature published an interesting article on the future of polar lows. [Read and download it here]

A good friend of ours, Matthias Zahn, who like us did his Ph.D. on polar lows, performed an interesting experiment with Hans von Storch. Recognizing that climate models are unable to simulate actual polar lows (because their resolution is too low), they had to find another way to tell if the frequency and severity of polar lows will change in the future. They therefore took the results from one of the models and fed them into a high-resolution regional climate model (pretty much the same as a weather forecasting model), to “translate” the climate model output to more realistic weather.

In Zahn and von Storch’s simulations, the number of polar lows decreased quite a lot for the future scenarios that they used. The reason for this was that in a warmer future, the air heats up faster than the ocean. This makes cold air outbreaks occur less often (because the air isn’t so cold anymore), and as a consequence, you get fewer polar lows. Tom and I did a similar analysis using the same climate models a few years ago [download paper here], and came to the same conclusion: in the typical polar low regions the frequency of polar lows is going to decline in the future. But the other half of our conclusion was:

In the regions that are covered by sea ice in today’s climate, the number of polar lows will increase dramatically if the sea ice retreats.

I expect this to be true for many regions near today’s sea ice edge, such as the north-eastern Barents Sea, north of Russia (the Kara and Laptev and East Siberian Seas), north of the Bering Strait (the Chukchi Sea) and north of Alaska (the Beaufort Sea). The reason is that polar lows need open ocean and cold air, and when the sea ice disappears, they get just that.

This story was picked up by Reuters in 2009 [read the article here], and was published on many web sites (Scientific American, USA Today etc.). Zahn and von Storch’s paper got a lot of attention; see for instance this article in the Guardian.

Polar low south of Svalbard

A fine polar low under development off the Norwegian coast on 20 November 2008. Spitsbergen is seen at 12 o'clock, the Norwegian coast in the lower right quadrant and the Greenland coast and sea ice in the top left corner

This is an interesting case because I think Spitsbergen has an influence on the development. The elongated cloud stretching down from Spitsbergen’s north-west corner separates the cold Greenland air on the left from the warmer air on the right. The southern edge of the front starts twisting as it gets far enough south, and this is when the polar low starts developing. There is another development just to the east of the incipient polar low. The whiter shades of this second cloud indicates deep convection and high clouds. I’m going to try to find out just what happened in this case. I’ve seen quite a few of this kind of polar low, and it would be fun to see just how much Spitsbergen influences their developments.

Erik