Deficiencies in climate model simulations of the seasonal rains in Africa

Email: c.m.dunning@pgr.reading.ac.uk

‘When is the wet season in Britain?’ a new student from Botswana asked me once. ‘Errrr, January-December?’ I replied flippantly. But in Botswana, and across much of Africa they experience one or two well-defined wet seasons per year, when the majority of the annual rainfall occurs. The timing and length of this wet season(s) is of significant societal importance; it replenishes water supplies used for drinking and other domestic purposes, affects the agricultural growing seasons and impacts the lifecycle of a number of vectors associated with the transmission of diseases such as malaria and dengue fever. Delays in the onset, or even failure of the wet season, can lead to reduced yields and potential food insecurity.

Future changes in climate will not be felt solely through changes in mean climate; projected shifts in atmospheric circulation patterns will also alter seasonality. Africa is acutely vulnerable to the effects of climate change so understanding future changes in the seasonal cycle of African precipitation is of utmost importance in establishing appropriate adaptation strategies. In order to produce reliable projections of seasonality, we require models to contain an accurate representation of current seasonality.

In our recent study we use a novel method to diagnose progression of the rainy seasons across continental Africa and identify important deficiencies in the climate simulations (a previous blog post and paper describes this method).

Firstly, when we use the method of Dunning et al. (2016) to identify the wet seasons in satellite-based precipitation estimates, atmosphere-only and coupled climate model simulations we find that the rainy seasons are differentiated more clearly from the dry seasons (shown by larger differences in the average rainfall per rainy day; Figure 1) than when fixed meteorological seasons (OND, MAM etc) are used, as this method accounts for interannual variability in seasonal timing and model timing biases.

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Figure 1. Average rainfall rate (mm day−1) during the wet/dry seasons over the Horn of Africa (a) and the Sahel (b) when defined using meteorological seasons (dashed bars) and dynamically varying seasons (Dunning et al. 2016, solid). See Figure 2 for a map of the regions.

Overall, climate model simulations capture the gross seasonal cycle of African precipitation on a continental scale, and seasonal timing exhibits good agreement with observations, however deficiencies manifest over key regions (Figure 2). The Horn of Africa (Somalia, southern Ethiopia, Kenya) experiences two wet seasons per year; the ‘long rains’ during March-May and the ‘short rains’ during October-November.  Whilst the simulations capture two wet seasons per year, they exhibit significant timing biases, with the long rains around 3 weeks late and the short rains nearly 4 weeks too long on average (Figure 2). Accounting for these biases may be crucial in interpreting the contrasting trend of observed declining rainfall during the ‘long rains’ in recent years and model projections of increasing ‘long rains’ rainfall in the future.

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Figure 2: Multi-model mean onset (open circles) and cessation (filled squares) for observations, atmosphere-only (AMIP) and coupled (CMIP) over selected regions (b). Shaded bars indicate the period of the wet season. For SWAC the mean annual regime onset/cessation in coupled simulations is plotted, along with mean onset/cessation for MIROC4h and BCC-CSM1-1-M (coupled simulations).

The most notable bias affects the southern coastline of West Africa, a region of complex meteorology with growing population and declining air quality. This region experiences the first wet season from April-June and the second wet season from mid-September-October, separated by the ‘Little Dry Season’ (LDS) in July-August. The LDS can be useful for weeding and spraying crops with pesticides between the two wet seasons, but can adversely affect crop yields if it is too long or pronounced. We find that simulations produce an unrealistic single summer wet season, with no mid-summer break in the rains and this is linked with biases in ocean temperature patterns. Given that climate simulations cannot capture the current seasonality, future projections for this region should be treated with caution.

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Figure 3: a) Location of the region that experiences the Little Dry Season (LDS; blue dots) and the SST region of interest (pink box). b) Mean annual cycle of rainfall in observations, atmosphere-only and coupled simulations over LDS region.

This work highlights important deficiencies in the representation of the seasonal cycle of rainfall by climate simulations with implications for the reliability of future climate projections and associated impact assessments, including water availability for hydropower generation, the length of the malaria transmission season and future crop yields.

The full paper can be found here:

Dunning, C.M., Allan, R.P. and Black, E. (2017) Identification of deficiencies in seasonal rainfall simulated by CMIP5 climate models, Environmental Research Letters, 12(11), 114001, doi:10.1088/1748-9326/aa869e

 

Clear-Air Turbulence and Climate Change

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Figure 1: Percentage change of clear-air turbulence over Europe and the North Atlantic

Clear-Air Turbulence (CAT) is a major hazard to the aviation industry. If you have ever been on a plane you have probably heard the pilots warn that clear-air turbulence could occur at any time so always wear your seatbelt. Most people will have experienced it for themselves and wanted to grip their seat. However, severe turbulence capable of causing serious passenger injuries is rare. It is defined as the vertical motion of the aircraft being strong enough to force anyone not seat belted to leave the chair or floor if they are standing. In the United States alone, it costs over 200 million US dollars in compensation for injuries, with people being hospitalised with broken bones and head injuries. Besides passengers suffering serious injuries, the cabin crew are most vulnerable as they spend most of the time on their feet serving customers. This results in an additional cost if they are injured and unable to work.

Clear-air turbulence is defined as high altitude inflight bumpiness away from thunderstorm activity. It can appear out of nowhere at any time and is particularly dangerous because pilots can’t see or detect it using on-board instruments.  Usually the first time a pilot is aware of the turbulence is when they are already flying through it. Because it is a major hazard, we need to know how it might change in the future, so that the industry can prepare if necessary. This could be done by trying to improve forecasts so that pilots can avoid regions likely to contain severe turbulence or making sure the aircraft can withstand more frequent and severe turbulence.

Our new paper published in Geophysical Research Letters named ‘Global Response of Clear-Air Turbulence to Climate Change’ aims at understanding how clear-air turbulence will change in the future around the world and throughout the year. What our study found was that, the busiest flight routes around the world would see the largest increase in turbulence. For example, the North Atlantic, North America, North Pacific and Europe (see Figure 1) will see a significant increase in severe turbulence which could cause more problems in the future. These regions see the largest increase because of the Jet Stream. The Jet Stream is a fast flowing river of air that is found in the mid-latitudes. Clear-air turbulence is predominantly caused by the wind traveling at different speeds around the Jet Stream. Climate change is expected to increase the Jet Stream speed and therefore increase the vertical wind shear, causing more turbulence.

To put these findings in context, severe turbulence in the future will be as frequent as moderate turbulence historically. Anyone who is a frequent flyer will have likely experienced moderate turbulence at some point, but fewer people have experienced severe turbulence. Therefore, this study suggests this will change in the future with most frequent flyers experiencing severe turbulence on some flight routes as well as even more moderate turbulence. Our study also found moderate turbulence will become as frequent in the summer as it has done historically in winter. This is significant because although clear-air turbulence is more likely in winter, it will however now become much more of a year round phenomenon (see Figure 2).

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Figure 2: Percentage change of clear-air turbulence around the world in all four seasons. No Stipling (stipling) indicates it is (is not) significant at the 90% confidence level.

This increase in clear-air turbulence highlights the importance for improving turbulence forecasting. Current research has shown that using ensemble forecasts (many forecasts of the same event) and also using more turbulence diagnostics than the one we used in this study can improve the forecast skill. By improving the forecasts, we could consistently avoid the areas of severe turbulence or make sure passengers and crew are seat-belted before the turbulence event occurs. Unfortunately, as these improvements are not yet fully operational, you can still reduce your own risk of injury by making sure you wear your seat belt as much as possible so that, if the aircraft does hit unexpected turbulence, you would avoid serious injuries.

Storer, L. N., Williams, P. D., & Joshi, M. M. (2017). Global response of clear-air turbulence to climate change. Geophysical Research Letters, 44, 99769984. https://doi.org/10.1002/2017GL074618

This blog was originaly writen for EGU Blogs

Adventures in Modelling – NCAS Climate Modelling Summer School

At the beginning of September 3 PhD students from Reading, including myself, went to Cambridge to attend the NCAS Climate Modelling Summer School. This is an annual event aimed at PhD students and early career scientists who want to develop their understanding of climate models, with topics covering parameterisations to supercomputers.

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Staff and students of the course pose outside the Chemistry department, which played host to morning lectures

The course ran over two weeks with lectures on the components of climate models in the morning, covering fundamental dynamics and thermodynamics, numerical methods and different parameterisations. This was followed by an afternoon of computer practicals and then more topical lectures in the evening, such as “User engagement in climate science” and “The Sun and Earth’s climate system”. The lectures were very fast paced but this was a great opportunity to cover so many topics in a short space of time and get a grounding in lots of different topics that I will definitely be looking over in future. A poster session on the second evening gave us the chance to learn about other people’s work and make connections with other people starting out their careers in climate science, including a few readers of the blog, that will hopefully last throughout our careers.

One of the highlights of the course was the chance to run some (rather interesting) experiments with an earth system model. This involved breaking into groups with each being given a different project. It was exciting to go  through the whole process of having an idea, developing a hypothesis, thinking of specific experiments to answer the hypothesis and then analysing the results in just a week – something that takes much longer when you’re doing a PhD! My group worked on the Flat Earth experiment, which looked at the effect of removing all of the earth’s orography not, to our dismay, turning the earth into a flat disk. I learned a lot about how to run models, something which I have never done even though I use the output. It also developed my understanding of different climate processes that I don’t work with such as the monsoons, and even dynamical vegetation.

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Flat earth experiment looking at the change in the monsoon winds

Throughout the course we stayed at St Catharine’s College. Right in the centre of Cambridge it quickly felt like a home from home, keeping us well fed to get through the intense science. Although the weekend was rainy, apparently breaking a run of excellent weather for the school, we still had plenty of time to explore beautiful Cambridge. A few people were even brave enough to go punting!

An interesting, hectic and inspiring two weeks later we may have been glad to head back to Reading for a good sleep but having thoroughly enjoyed the summer school.

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The beautiful St Catharine’s College, image from http://www.caths.cam.ac.uk/

 

1st Met Office Training and Research Summer School

Email: carlo.cafaro@pgr.reading.ac.uk

From the last week of June until the 1st September I took part in the Met Office Training and Research (MOTR), as part of the Mathematics of Planet Earth CDT.

Inspired by the highly popular and successful Geophysical Fluid Dynamics Summer School at the Woods Hole Oceanographic Insitution in the USA, it is a 10 week-programme, hosted by Met Office in Exeter. The PhD students have the opportunity to handle an applied research topic outside the area of their PhD, diversify their portfolio and experience the working and social life at the Met Office.

In the first two weeks we participated in a summer school. In particular in the first week there was a lecture course on ”Regional Climate Variability and Change”. In the morning the lectures were given by David Karoly from University of Melbourne on “patterns of climate change”, starting from the basic concepts of the climate systems and then expanding to the climate change attribution. In the afternoon we had specialist lectures by Met Office and University of Exeter scientists about El Nino, modelling paleoclimates and attribution of extreme weather events. 

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David Karoly giving one of his lecture on attribution of regional climate change patterns.

In addition, in the afternoon we had to do lab work working in pairs, using Climate Explorer, choosing a specific continent of the world and investigating past climate and future climate projections for that area. My colleague and I selected South America and we gave a presentation about that.

During the second week we participated in the workshop ”Future opportunities to inform UK regional projections”, with a lecture given by Ed Hawkins, amongst  others, talking about sources of uncertainty.

From the third week onward each student started a research project in different Met Office research groups. A different colleague and I worked within the Atmospheric Processes and Parametrization group (APP), supervised by Gabriel Rooney. My project was on numerical simulations and theoretical aspects of colliding density currents. Other colleagues were placed within the Climate Science, Dynamics groups and Informatics Lab, a partner of Met Office.

A typical day for us at Met Office started at 9am, meeting almost every day with Gabriel at 9.45 am, coffee break at 10.30 for half an hour or so (where I also met Annelize, previously in Reading), 1 hour lunch break and then “working” again until 5.30 pm or so.

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Not quite our typical day at Met Office (found on a desk at Met Office)

Also, once a week we had the meeting with the smaller convection group, where everyone was asked to give an update of their own work. We also attended journal club sessions on Fridays and a brainstorming meeting on 21st July. It was nice to take part in these events, even being only summer interns. During the project phase we had also weekly advanced seminars by Glenn Shutts and Mike Cullen, mainly about large-scale dynamics and hierarchies of operational models used in NWP.

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Glenn Shutts giving a lecture on Rossby waves breaking.

Personally, it was a wonderful experience for several reasons. The Met Office is a very pleasant place to work, with very friendly and flexible people. Since I think my project was quite academic I did not find many differences with working at university itself. Nevertheless, interacting with new people in a new environment has provided me with new inspirations and insights. I had the chance to talk with several scientists and also a chief Meteorologist, since I was curious about the activities carried out in the Operational room and how much communication there is with the research side. There were some social and sports events organised by MOSSA (Met Office Social and Sports Association) which I really enjoyed (picnic and sports day), getting the chance to meet and to talk with people of other research divisions.

Finally, to top it off, I visited Exeter a lot and the area around, mainly during the weekends and the 5 days of holidays agreed at the beginning, even going to Cornwall for 2 days. 

In the end I would like to thank all of the organisers, my supervisor and all the people I talked with for giving me and my colleagues this very valuable opportunity which I will keep always in mind for my future career.

Thunderstorms and showers: an insight into UK operational radar rainfall estimation.

If there’s one thing you can count on in Britain it’s that at any given time someone, somewhere, is talking about rain.  Either it’s raining, or we want it to rain, or it absolutely mustn’t be raining today.  It’s just one of those things we love to complain about – and we do!

My work isn’t in forecasting rain, but observing it. It’s a little known fact that the rainfall map you see on the front page of the Met Office website doesn’t just come straight out of one giant weather radar, neatly packaged.  There’s a lot of work that has to happen before we can turn the “reflectivities” from many different radar echoes into a sensible estimate of how heavily it’s raining on your street.

The Met Office owns and manages a network of 15 weather radars across the UK, and receives data from three more, in Ireland and the Channel Islands.  We’ve now almost completed a major upgrade of the network, replacing key components of the old radars – some of which had been running operationally for over 30 years! – with new technology.  The dual polarisation and Doppler information we obtain from the upgraded radars improves our ability to distinguish between “rain” and “non-rain” echoes, and to measure how fast the rain is moving, feeding improvements in short range “nowcasts” and flood forecasting models. It can also help us estimate the quantity of rainfall and other types of precipitation in real time.

For my PhD, I’m looking at how to improve Met Office estimates of surface rain rates from radar measurements at long range.  When a radar measures weather, it does so with a beam of energy that spreads out with distance.  20 km from the radar, the echo received represents a volume of space around 600 m by 400 m by 400 m.  At 50 km, the volume is 600 m by 1 km squared.  By 100 km, the beam has spread out to be 2 km wide.  This effect is called “beam broadening”, and limits the spatial detail with which we can measure rainfall.

The other effect of range is the increasing height of the radar beam above the ground.  This means the radar isn’t always measuring liquid rain drops, but may be measuring frozen ice crystals or snow, high up in colder parts of the atmosphere, which will melt before they reach the surface.  Snow, melting snow and rain all have different reflectivities, so we have to correct for this “vertical profile” to calculate how much rain is falling at ground level.

The Met Office corrects for the vertical reflectivity profile (VPR) using an iterative scheme (Kitchen et al. 1994, Kitchen 1997).  We know roughly the VPR shape, and the amount of beam broadening at a given range, so we can scale this shape to the actual radar reflectivity measurement.  This allows us to correct for the impacts of melting snow, which causes a huge enhancement of the radar measurement and would – if uncorrected – lead to extreme overestimates of rainfall.  In the early days of weather radar, this caused rings of very high rain rates to appear in the image: an effect called “bright banding”.  VPR correction also compensates for the underestimation of rain rates at very long distances from the radar.

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Vertical slice “range height indicator” scan of rain from October 2014.  There is a “bright band” at 2 km height due to melting snowflakes in the reflectivity and linear depolarisation ratio (LDR).

In my work, I’m using measurements from the upgraded dual polarisation radar network to choose between different VPR shapes in making this correction.  Specifically, I’m investigating the depolarising properties of different melting drops, to identify the rare situations where we don’t need to correct for “bright banding”.  The linear depolarisation ratio (LDR), which I’m using to identify the large melting snowflakes that cause radar bright bands, has to be measured using different scans from the ones used to collect reflectivities for rainfall rates, and the Met Office is one of very few meteorological services capable of measuring LDR operationally.  Using LDR in this way can improve rainfall estimates significantly in cases where there is no bright band (Smyth and Illingworth, 1998; Sandford et al., in press).

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Vertical slice “range height indicator” scan of a convective shower from July 2015.  There is no bright band in reflectivity, and very little melting layer enhancement in LDR.

As a logical extension to this work, I’m also looking into new VPR shapes for “non-bright band” rain, using vertical slice “range height indicator” scans from our research radar at Wardon Hill.  Correction for bright band is well established in the radar literature, as this is the most common type of rain at high latitudes (where the freezing level is low enough to affect radar measurements), but other types of VPR (e.g. Fabry and Zawadzki, 1995) are rarely discussed.  With the improvements in classification achieved by the new LDR algorithm, a suitable VPR shape is needed to correct for underestimation far from the radar in cases without bright band.  I’ve recently developed a test profile shape for non-bright band VPRs, and demonstrated improvements to rain rates in localised convective case studies. The method is currently being trialled for use in the Met Office’s operational radar processing software.

In the future it’s hoped that the work I’m doing will further improve the accuracy of Met Office rainfall estimates, particularly in thunderstorms and convective showers. And when the weather is doing things like this, that’s good to know!

References

Fabry, F. and I. Zawadzki, 1995: Long-term radar observations of the melting layer of precipitation and their interpretation. Journal of the Atmospheric Sciences, 52, 838-851.

Kitchen, M., 1997: Towards improved radar estimates of surface precipitation rate at long range. Quarterly Journal of the Royal Meteorological Society, 123, 145-163.

Kitchen, M., R. Brown. and A. Davies, 1994: Real-time correction of weather radar data for the effects of bright band, range and orographic growth in widespread precipitation. Quarterly Journal of the Royal Meteorological Society, 120, 1231-1254.

Sandford, C., A. Illingworth, and R. Thompson, in press: The potential use of the linear depolarisation ratio to distinguish between convective and stratiform rainfall to improve radar rain rate estimates. Journal of Applied Meteorology and Climatology.

Smyth, T. and A. Illingworth, 1998: Radar estimates of rainfall rates at the ground in bright band and non-bright band events. Quarterly Journal of the Royal Meteorological Society, 124, 2417-2434.

5th WGNE workshop on systematic errors in weather and climate models

The 5th Working Group on Numerical Experimentation (WGNE) workshop on systematic errors in weather and climate models was held in Montréal, Canada from 19 to 23 June 2017. The principal goal of the workshop is to increase understanding of the nature and cause of errors in models used for weather and climate prediction, including intra-seasonal to inter-annual scales.

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Centre Mont-Royal, venue for the workshop

The workshop is held every four years. The 5th WGNE workshop focused on processes that models currently fail to represent accurately, based around six themes: atmosphere-land-ocean-cryosphere interactions, clouds and precipitation, resolution issues, teleconnections, metrics and diagnostics, and model errors in ensembles. For each of the themes, the workshop started off with talks from invited keynote speakers, followed by contributed oral presentations, a conclusion session and a poster session.

My PhD project studies mean-state precipitation biases over the Maritime Continent in CMIP5 atmosphere-only experiments, which aligns well with the “model errors in ensembles” workshop theme. I received a lot of constructive feedback and suggestions during the discussions in the poster session.

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Lunch with experts. Photo courtesy of Ariane Frassoni
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Pub night. Photo courtesy of Ariane Frassoni

A mixture of scientific and social activities were organized in this workshop dedicated to Early Career Scientists (ECS). We had the opportunity to be a session rapporteur and participate in a best poster competition. Then we were given the chance to get to know more established scientists during the more social ‘lunch-with-experts’ and ‘pub night’ activities. The lunch-with-experts was truly entertaining – with conversations about PhD life and challenges, future career path advice, variations between countries in PhD education systems and much more! ECS were also given the opportunity to become co-reviewer of a poster competition session where we work in pairs with an expert scientist to review posters in a session we are not competing in. By becoming the co-reviewer, we get to experience the review process and get in contact with expert scientists.

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Session rapporteur presentation. Photo courtesy of Ariane Frassoni

On the last day, the session rapporteur presented a summary on the main issues discussed in each session, followed by a panel discussion and an overall conclusion to the workshop. I am very happy that my poster on ‘Maritime Continent seasonal climate biases in AMIP experiments of the CMIP5 multimodel ensemble’ was given the Best Poster Award, alongside with Falko Judt for his poster on ‘Effect of model error on the predictability of hurricane intensity’ and Danahé Paquin-Ricard for her poster on ‘The role and impact of a deep convective parameterization on Km-scale atmospheric forecasts’ during the closing session.

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ECS group photo. Photo courtesy of Ariane Frassoni

Lastly, I also got to do some sightseeing while I was in Montréal after the workshop. From the amazing Notre-Dame Basilica, great views of the city from Mont Royal and the underground city to escape the weather, Montréal has so much to offer!

 

I am thankful to the World Meteorological Organization (WMO) for providing me the travel funding to attend the workshop and present my poster. Also many thanks go to Ariane Frassoni for organising the pub nights and facilitating the ECS activities, as well as for providing the photos for this post.

Two Weeks in Paris Learning about Fluid Dynamics and Sampling French Pastries

Email: r.frew@pgr.reading.ac.uk

The Fluid Dynamics of Sustainability and the Environment (FDSE) residential summer school runs every summer for two weeks, alternating between Cambridge University and Ecole polytechnique, which run the summer school in partnership. I attended this years hosted by Ecole polytechnique, situated to the South of Paris. 40 PhD students attended from institutes around the world, all working on a range of topics who want to learn more about environmental fluid dynamics.

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The lectures covered topics on fundamentals of fluid dynamics, flow instabilities, environmental fluid dynamics, cryosphere, atmosphere, physical oceanography and renewable energy. The lectures went at a very fast pace (approximately triple speed!), aiming to familiarise us with as many concepts as possible in the two weeks, resulting in everyone taking home a large overflowing folder full of lecture notes to refer back to in the future.

We were kept very busy throughout the two weeks. Each day started with breakfast (coffee and croissants) between 7.30-8.20 am, followed by two back to back lectures 8.30-10.30 am. There was then half an hour for everyone to fuel their brain with coffee and (warm!) mini pastries before another hour lecture before lunch break. Lunch was roughly 12-1.30 pm, although typically there were so many interesting questions after each lecture that we ran progressively later relative to the schedule meaning that I think we only actually started lunch on time on the first day. There were also a number of guest speakers speaking on topics such as public engagement, climate policy, meteorology on mars and air quality.

After lunch we had the final lecture of the day, followed by a short break before numerical sessions and lab experiments, which ran until roughly 6 pm. These sessions gave us the chance to really learn about a particular topic in more detail and to have a more hands on experience with some of the material being lectured. My labs were on tidal energy where we explored the energy output and efficiency of tidal turbines, and Art and Science, which encouraged us to engage with Science in new and more playful ways and also to challenge us to look at it differently.

However the day didn’t end after the labs, the evenings were also jam packed! The first evening was a poster session, giving us all the opportunity to learn more about what all of the other students work on and to mingle. Other evenings consisted of learning to row sessions, visits to the observatory, movie nights and discussions about the ‘science’ in The Day After Tomorrow movie and barbeques enjoying the warm light evenings (definitely missing those now I’m back in Reading).

During the weekend sandwiched in the middle of the two weeks, we were all transferred to a hostel in the centre of Paris, setting us all up perfectly for some weekend sightseeing in Paris. On the Friday evening there was a boat party reception on the Siene, supplying us all with lots of wine, many difference French cheeses to sample and a lively dance floor.

The school ended on Friday July 14th, Bastille Day. After a morning presenting a few slides on the labs we had completed in groups to share what we had learnt, we travelled into the centre of Paris ready for an evening enjoying the spectacular Bastille Day fireworks around the Eiffel tower, ending the summer school with a bang.

Personally the main take away from the summer school was not to learn the entirety of the lecture content, but to become familiar with a wide range of topics gain more hands on experience of laboratory experiments and to have a (rather large) folder full of lecture notes to refer back to whenever I stumble across a particular concept again in the future. And of course, it was great having the opportunity to meet lots of other PhD students from around the world working on related topics and to be able to discuss, engage and get to know each other over the two weeks. I would like to thank all of the organisers and lecturers of the summer school for a really interesting and enjoyable two weeks!