Category Archives: PhD

The Stratosphere – why do we care?

I study the stratosphere, the layer of atmosphere that extends above the troposphere from about 10-50 km. Friends and colleagues of mine often joke (I hope…) that “nobody cares about the stratosphere” *, primarily because it contains no real ‘weather’ – such as what happens in the troposphere. With little to no water vapour, it can’t be seen on visible satellite imagery – unlike the huge and beautiful weather systems in the troposphere. To visualise the stratosphere, we rely primarily on computer-generated graphics – and it’s not like you can walk outside and experience it, either. So, why do we care? What follows is a relatively simple (I hope!) explanation.

Weather forecasts, particularly on TV, often explain that our weather is “all down to the position of the jet stream” (the band of fast flowing air high in the troposphere that forms on the boundary between warmer and cooler airmasses). Now, that’s almost always true in the UK, but it’s particularly potent in winter – when the temperature contrasts either side of the jet become enhanced thanks to the Polar Night. One of the main driving factors behind the speed and position of the jet stream (particularly the Atlantic jet stream) in winter is… the stratosphere!

Rather like the jet streams we know and love/loathe in the troposphere that guide the development and evolution of weather systems, in the stratosphere there exists another jet stream – the Polar Night Jet (Figure 1). This encircles the Stratospheric Polar Vortex (SPV). Both of these form as the pole tilts away from the Sun in winter, leading to intense cooling. The strong temperature gradient then forms a jet stream and cyclonic vortex, which isolates the air within the vortex, and it cools further…etc. The Polar Vortex is a normal phenomenon which forms each winter – nothing sensational like some headlines suggest.

ZMZW_00Z_04022018

Figure 1: GFS zonal wind analysis from February 4th 2018. Reds indicate westerly winds. A strong Polar Night Jet exists in the stratosphere, associated with a strong tropospheric jet.

Through a process known as stratosphere-troposphere coupling, the stratosphere and the troposphere beneath can ‘talk’ via the influence of planetary/Rossby waves. These very large waves in the mid-latitude westerly flow can propagate vertically from the troposphere into the stratosphere and influence the circulation there – a process known as wave-mean flow interaction. Sometimes, this is strong enough to strongly disrupt the SPV, and when that happens, the isolated reservoir of cold air is broken down and the temperature sky-rockets… by as much as 50C in only a few days. This is known as a Sudden Stratospheric Warming (SSW). Very strong SSWs – called major SSWs – occur in approximately 6 winters per decade, and result in a reversal of the Polar Night Jet to easterlies. The Polar Vortex is either displaced, split up, or destroyed (2018’s SSW is shown in Figure 2).

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Figure 2: The February 2018 Major SSW, as told through daily analyses from the GFS of 10 hPa wind (filled) and geopotential height (contours). This is classified as a ‘split’ SSW, for obvious reasons.

This has implications for our weather, because anomalies in the strength and position of the SPV and the Polar Night Jet can propagate downwards and influence the tropospheric jet stream. A stronger than normal SPV is associated with a strengthened tropospheric jet stream – and for us in the UK, that means Atlantic westerlies and generally mild winter weather. In contrast, following a major SSW, the easterlies propagate downwards (Figures 3 and 4) – resulting in a reduction in strength of the Atlantic westerlies. Sometimes, there can be a complete reversal of circulation – this happened in March 2018 with the infamous ‘Beast from the East’, bringing cold and snowy weather.

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Figure 3: As in Figure 1, but for February 17th, following the major SSW. Note the weaker tropospheric jet and surface easterlies as the ‘Beast from the East’ developed in response.

Thus, being able to predict the state of the Stratospheric Polar Vortex is a source of skill for wintertime forecasts. Moreover, because there tends to be some lag between the events in the stratosphere and their maximum impact at the surface (~2 weeks), stratospheric predictability can provide increased predictability on the sub-seasonal timeframe (~15-30 days). Additionally, anomalies associated with a major SSW tend to persist in the lower stratosphere for even longer – which again, is a source of skill.

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Figure 4: Anomalies in geopotential height for January-March 2018. Note how anomalies associated with the major SSW (red blob in the centre) propagate downwards like ‘dripping paint’.

And that is why we care about the stratosphere!

Further reading:

Kidston et al., 2015: Stratospheric influence on tropospheric jet streams, storm tracks and surface weather. Nature Geoscience, 8, 433-440.

*A tongue-in-cheek quote from Reading Meteorology’s weekly ‘Weather and Climate Discussion’ a few years ago that stuck with me was “the stratosphere – nothing of interest lies therein”. I plan to use that in my thesis…

So…why the stratosphere?

On February 23rd, I accepted an offer of a SCENARIO-funded PhD studentship in the Department of Meteorology at the University of Reading for the project “How can the stratosphere help us predict the weather several weeks ahead?“. The project is supervised by Andrew Charlton-Perez and Steve Woolnough at Reading and Jason Furtado at the University of Oklahoma – giving the exciting opportunity to spend some time working in the School of Meteorology at the National Weather Center and attend conferences in the USA.

It’s a big thing taking on a PhD and I felt it made sense I elaborate somewhat on why I’m interested in the stratosphere. I’ve been talking about the stratosphere a lot recently thanks to the first major Sudden Stratospheric Warming (SSW) in 5 years. I found myself presenting about the SSW in the Department’s weekly ‘Weather and Climate Discussion‘ in the presence of several Year 10 work experience students – hopefully attempting to inspire future stratosphere researchers as I myself embark on that journey! The timing of my PhD offer couldn’t have been better – what’s better than having an interview for a stratosphere-related PhD whilst there’s a big fat ridge sat over the Pole at 10 hPa?

Boundary Layer meteorologists have it easy explaining their interest: “it’s where we live!”. Tornado researchers simply find awe in the atmosphere’s most destructive phenomenon that you can go out and watch. But the stratosphere? It’s not even the ‘sphere’ where weather in the traditional sense occurs! So how did I get ‘into’ it?

January 2009 saw one of the strongest SSWs on record – a wave-2 vortex split event that reversed the 10 hPa 60°N zonal mean zonal wind from record-strong westerly to record-strong easterly. The Met Office announced the SSW and explained how this meant we were going to see cold conditions with a risk of snow. As a 13 year-old I hadn’t seen much in the way of snow (yet…) so this was very exciting! I was fascinated by this phenomenon and it immediately made me realise the atmosphere was far more complex and beautiful than I’d first thought (and I was already in love with it).

The Met Office news release was accompanied with this diagram:

met office ssw

I remember being captivated by this and had a thirst for a better understanding. I began asking questions: How does this happen? How do the stratosphere and troposphere interact? How does this lead to cold outbreaks in the UK? Why does this give increased predictability?

As time went on, I continued to find the stratosphere fascinating – the almost intangible nature of its dynamics – vertically propagating planetary waves, the Polar Vortex, the Polar Night Jet Stream, the Brewer-Dobson circulation…perhaps it was something to do with this being a less well-understood subject, and certainly one so complex it wasn’t often in the public eye. I had always been fascinated by the large-scale motions of the atmosphere – and it seemed in the stratosphere, I’d found the largest scale of all.

In the first year of my undergraduate Meteorology degree I attended an RMetS meeting entitled “Stratosphere-troposphere coupling in the Earth System: where next?“. Much of it went way over my head, but it didn’t put me off and made me want to know more! In second year, I gave a presentation on the unique 2015-16 Polar Vortex, whilst in third year I took a ‘Weather Briefings’ class with Steven Cavallo and always found myself talking about the Polar Vortex! At any opportunity, I’d talk about the stratosphere.

And with all of the meteorology sub-disciplines, most of which interest me on some level, the stratosphere was the one that stuck. I couldn’t be more happy to be researching it further, and helping to improve subseasonal forecasting. It’s a dream come true – especially since I’ll be able to return to Oklahoma, too!

I summed up my interest to the work experience students as:

“Isn’t it incredible that something so entirely abstract that happens 50 km above your head can directly influence the temperature of the wind blowing in your face?”