Archive for the ‘Weather’ Category

Weather and climate evening at the Manchester Science Festival

October 25, 2011

It’s the Manchester Science Festival this week and as part of it there’s a Weather and Climate evening on Thursday. It starts with a live recording of the Barometer podcast, which I sometimes (well, not very often) contribute to. Anyway, sounds like they’ve got some fun stuff planned and they’ll be plenty of time for Q&A.

Why has this winter been so cold in Europe?

January 6, 2011

This post was chosen as an Editor's Selection for ResearchBlogging.orgI’ve written a couple of posts recently looking at the cold UK weather in context and how snow forms.

What I haven’t done, though, is looked at why it’s been so cold over Europe this winter (as well as last year’s winter).

It just so happens that a paper came out in the Journal of Geophysical Research recently (Petoukhov and Semenov, 2010) that might hold the answer.

They looked at whether changing the sea ice concentration in a particular area of the Arctic (the  Barents-Kara sea) in a long run of a climate model changed the conditions over Europe.

Their reason for doing this was that the very cold winter of 2005/06 was accompanied by very low sea ice in this region – they don’t mention 2009/10 and 2010/11 at all, though, as they would have been writing the paper before these cold European winters occurred.

To isolate the effect of the sea ice in this one region they use a mean state (a “climatology”) for most of the planet in the model but change the amount of ice on the  Barents-Kara sea. The results are quite surprising.

The winter wind patterns over Europe change dramatically when they changed the ice concentration from 80-100% to 40-80%. You can see this in the figure below, which is from the paper but I’ve highlighted the key areas on the wind plot and removed some of the panels.

Mean surface air temperature and 850 hPa wind anomalies for February from the model runs using 2 ice scenarios.

(When they set the sea ice close to 0%, Europe goes into a different state that is similar in temperature to the 80-100% case.)

So why does Europe get cold in a model world where the Barents-Kara sea has 40-80% sea ice concentration? In this model run, the result of this level of sea ice is to set up a big anti-cyclonic (high-pressure) anomaly over the pole. In the northern hemisphere air rotates clockwise around a high so this explains the switch in wind direction that drives the change over Europe. However, the hypothesis they present as to why the sea ice change leads to a high pressure anomaly over the pole is not straightforward and probably deserves a bit more study.

So, in essence, this all seems to be saying that it’s climate change that has led to our very cold winter! I can imagine some people finding that hard to swallow but here’s a quote from the paper that sums it up better than I just have:

Our results imply that several recent severe winters do not conflict the global warming picture but rather supplement it, being in qualitative agreement with the simulated large-scale atmospheric circulation realignment.

Anyway, all interesting stuff and I look forward to seeing some more analysis, especially a better climatology of winter temperatures in Europe and Arctic sea ice to see if that fits in with this hypothesis and a better physical model for the different changes linked with different sea ice concentrations.

UPDATE (10th Jan 2011): I just read someone claiming to have “debunked” this paper by showing that sea ice concentration and European temperature don’t correlate. However, this completely misses the non-linearity of the relationship. I think it’s fine to question the findings of the paper but I suspect that to “debunk”, or verify, the findings using the actual sea ice and temperature measurements you’d have to pick apart the contributions of other factors (e.g. polar jet changes, ENSO teleconnections) and then find some way of characterising the non-linear nature of the relationship with B-K sea ice.

ResearchBlogging.orgV. Petoukhov, & V. A. Semenov (2010). A link between reduced Barents-Kara sea ice and cold winter extremes over northern continents Journal of Geophysical Research, 115 : 10.1029/2009JD013568

Why does it snow?

December 22, 2010

The Barometer guys have just published a new episode on snow. It’s another good one. I’m starting to really miss being involved with the show!

In this episode they talk for a bit about how snow forms and it reminded me of this really nice video showing the melting layer in a precipitating cloud (click to play):

In the higher levels of the cloud (where it’s really cold) the precipitation starts out as ice and snow, which falls much slower than rain. But if you look closely at about 2 km you can see the point where the ice and snow melts and becomes rain – this is known as the melting layer. The rains falls much quicker than the snow above.

It snows at the ground when the lower levels of the atmosphere are particularly cold and so the snow doesn’t melt. This is what’s been happening recently.

Snow again.

November 26, 2010

I wrote a post last winter about how the snow doesn’t mean that climate change is over.

Well, it’s snowing again.

And the warming on a global scale still hasn’t stopped:

October 2010 temperature anomalies relative to the period 1951-1980 from the NASA GISS webpage.

Rainbowgate

November 18, 2010

I’m predicting that the next scandal to hit the atmospheric sciences will be rainbowgate: a plot to get people to book holidays on the promise of false atmospheric phenomena.

Just look here:

The photo was taken from a website advertising hotels in Hawaii. This looks fake to me. Amongst other things, the colours in the second rainbow aren’t reversed and it doesn’t even look like its raining where the rainbow is or that there’s a waterfall nearby. Besides which, triple rainbows are notoriously difficult to photograph (see Pledgley (1986), Weather 41, 401)

I’m suspicious.

How many people have been lured to that hotel expecting a triple rainbow? I hate to think. How deep does this go?

[UPDATE: Apologies if you came here looking for something serious about triple rainbows. If so, why not try the wikipedia page or this Science Daily article.]

Storms and climate change

March 25, 2010

I’ve been pretty distracted recently with the Institute of Physics issue. I’ll hopefully draw that chapter to a close in the next couple of weeks (it looks like the IoP are going to stick their head in the sand and wait for it to blow over) but right now I’m bringing my current project to a close so I thought I’d look at how that work has gone and where it’ll go in the future.

The work I’ve been doing at Manchester over the last few years is looking at how storms form – the kind of storms that lead to floods like the one seen in Boscastle in 2004.

These type of storms are pretty difficult to forecast because they are often smaller than the computer model grid box size that such models use to chunk up the atmosphere into a more manageable mathematical problem. I spoke about this in a recent edition of the NERC Planet Earth podcast.

I’ve been part of a couple of big projects that try and observe these storms as they form. We do this by getting big teams of scientists, around 50 or so, to go to a certain location for a few months and then use surface stations, aeroplanes, weather balloons, radar, lidar and satellite data to get a really good picture of what’s going on.

The video below shows the Chilbolton radar scanning convective clouds as they develop whilst someone launches weather balloons in the bottom left hand corner – this is pretty typical of what goes on at several locations during these campaigns, although this is the biggest dish we’ve used.

I’ve only been looking at a small part of the storm problem: how does air that descends from the stratosphere influence these storms?

I’ve shown that this upper-level air can sometimes make storms less likely by introducing thin layers into the atmosphere that can cap storms before they get going (Russell et al., 2008) and sometimes make storms more likely by changing the temperature structure of the atmosphere to make the convection more powerful (Russell et al., 2009).

That doesn’t sound much like progress, does it? Well, now that we know a lot more about these two types of effect, we can start to generalise them and use that knowledge to help forecasters improve their predictions and computer models. This is what we are working on now.

So how does climate change fit in to all this?

The next big step in this work is to try and define a “storm environment”, including what we now know about these upper-level features, and apply this to the kind of projections that climate models produce. These projections use even bigger grid boxes than weather models so this step will not be easy but if we can show that this method works using the data that is available now (and hope for better resolution soon) then we can start to think about the likely changes in this storm environment. These big storms can really affect people’s lives so this type of work is something that would influence how we start to prepare for the future.

References:

ResearchBlogging.orgRussell, A., Vaughan, G., Norton, E., Morcrette, C., Browning, K., & Blyth, A. (2008). Convective inhibition beneath an upper-level PV anomaly Quarterly Journal of the Royal Meteorological Society, 134 (631), 371-383 DOI: 10.1002/qj.214

ResearchBlogging.orgRUSSELL, A., VAUGHAN, G., NORTON, E., RICKETTS, H., MORCRETTE, C., HEWISON, T., BROWNING, K., & BLYTH, A. (2009). Convection forced by a descending dry layer and low-level moist convergence Tellus A, 61 (2), 250-263 DOI: 10.1111/j.1600-0870.2008.00382.x

Antarctic climate change – the exception that proves the rule?

March 1, 2010

Antarctica has been in the news recently because two large icebergs (one about 60 miles long and the other about 50) have broken off the continent. These “calving” events often occur naturally and these ones are probably not linked to climate change, although they might affect the global ocean circulation.

But I thought that this would be a good opportunity to have a look at the general climate situation in the South Pole region…

The clearest signal is rapid warming that has been seen on the Antarctic Peninsula (the bit that points up to South America) over the last 50 years.

The picture for the rest of the continent is not so clear, mainly because of the lack of data. For comparison, the USA has over 1000 climatological observing stations, some of which go back to the late 1800s; Antarctica currently has around 55 stations, very few of which go back to before 1957, (plus a similar number of automatic weather stations, which tend to not be maintained for long periods) and these data are used to represent a much bigger land area.

Antarctica compared to the USA[Image from NASA]

Nonetheless, there have been some high profile studies looking at Antarctic temperature trends, some finding cooling, some finding warming and nearly all being controversial.

So why is the warming on the Peninsula so clear?

The reason is that the warming is mostly driven by atmospheric circulation changes and not the increase in the greenhouse gas concentrations (although global climate change patterns forced by GHGs can include atmospheric circulation changes).

Ozone "hole"The key factor is that the ozone hole above the South Pole has changed the wind patterns – when ozone is removed from the stratosphere, less solar UV radiation is absorbed so the polar stratosphere cools. This increases the temperature change as you move away from the pole and, in turn, has changed the westerly (clockwise) winds that circle the pole – they are now further south and faster.

This wind pattern spreads down through the atmosphere towards the planet’s surface and has, therefore, brought more warm air from over the Southern Ocean to the Peninsula. This circulation change has less effect on the Antarctic interior and possibly even isolates it from the rest of the Earth system.

This climate change pattern is really interesting to study and we can even use ice core data from the Antarctic to look at how these winds have changed in the past – I’ve recently reviewed the literature on this subject (Russell and McGregor 2010).

Korhonen et al. (2010) have even found another mechanism of how these wind changes have affected the climate. As the wind speed over the ocean increases, it throws up more spray and this means that more clouds can form over the Southern Ocean and Antarctica (I’ll write a post later about how clouds form). If there are more, bright clouds around then these reflect away more incoming sunlight, which will cool the region beneath these clouds.

So, to bring all this together, if the Antarctic continent has been cooling (which isn’t clear) then this could be because the normal rules don’t apply to Antarctica. Does this mean that we can say that Antarctic climate change is the exception that proves the rule of GHG forced climate change?

Probably not, but it does highlight just how complicated the climate system is and how much more there is find out about it!

References:

ResearchBlogging.orgKorhonen, H., Carslaw, K., Forster, P., Mikkonen, S., Gordon, N., & Kokkola, H. (2010). Aerosol climate feedback due to decadal increases in Southern Hemisphere wind speeds Geophysical Research Letters, 37 (2) DOI: 10.1029/2009GL041320

ResearchBlogging.orgRussell, A., & McGregor, G. (2009). Southern hemisphere atmospheric circulation: impacts on Antarctic climate and reconstructions from Antarctic ice core data Climatic Change DOI: 10.1007/s10584-009-9673-4

Britain’s snow and climate change

January 8, 2010

NOTE: This post is from January 2010. I put a temperature anomaly plot from October 2010 here and I’ll do one for November 2010 as soon as the data is available.

I’m sure most of the Brits out there have seen this amazing NASA image of Britain covered in snow.  I love satellite images and use them a lot in my research – they really help me get a grasp of the big picture.

But what does this cold weather tell us about climate change?  Well, if we examine the whole northern hemisphere and look at how the temperatures for December compared to those from the last 30 years, then we get an interesting picture:

So, northern Europe and North America were colder than usual.  But southern Europe, Greenland, the Arctic and north Africa were all warmer than usual.  The situation for January will probably be quite similar.  So, looking at the bigger picture, the recent cold conditions in the UK don’t really tell us much about climate change – we need to look on big scales in both time and area.

Proposed industry standards for the terms “TREACHERY” and “CHAOS”

January 6, 2010

As much as I love reporters using the same terms over and over again, I feel that without any clear definition some of the power of these words may be lost upon the audience.  Therefore, I propose official industrial standards for these two terms.

Treachery

Clearly, in cases where this term is used, the conditions must not be merely and/or obviously dangerous.  The situation should initially appear tranquil – perhaps a sunny day with happy rabbits frolicking on the hard shoulder.  However, over time, the road-based conspiracy to undermine and betray the driver will become clear.  A quiet malevolent laughter will be audible underfoot.

Possible alternatives: dangerous, unsafe, difficult.

Chaos

A long line of stationary cars is clearly not chaotic; this situation is, in fact, relatively ordered.  If the term chaos is used, the following should be expected:

  • At least one overturned, burning car;
  • Screaming women pulling their hair out;
  • Helicopters crashing on the horizon;
  • Complete confusion (as opposed to the clear realisation that you should have stayed at home)

[Apologies to anyone that got trapped or had an accident in the snow.]

Snow in Manchester

January 5, 2010

I’ve not gone to work today.  There’s quite a bit of snow out there.  But why?

Well, the main reason why it’s cold here is because it is winter.  This sounds obvious but it’s worth remembering why it gets cold in winter.  Earth rotates with a tilt so, throughout the year, different parts of the planet get more sunlight.  At the moment, the UK is getting less sunlight so it’s colder.

However, the reason why it is just so cold and snowy right now is a bit more complicated.  If you look at the pressure chart below then you can see that the isobars are almost parallel  from the Arctic all the way to the north of the UK.  This means that very cold air is flowing right to our doorstep.  Brrr!  Watch out further south as the front (the region where the cold air meets slightly warmer air, which produces the precipitation) moves southwards and takes the snow with it.


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