Relationships in errors between meteorological forecasts and air quality forecasts

Email: K.M.Milczewska@pgr.reading.ac.uk

Exposure to pollutants in the air we breathe may trigger respiratory problems. Pollutants such as ozone (O_{3}) and particulate matter (PM_{2.5}) – particles of about 1/20th of the width of a hair strand – can get into our lungs and cause inflammation, alter their function, or otherwise cause trouble for the cardiovascular system – especially in people with existing underlying respiratory conditions. Although high pollution episodes in the UK are infrequent, the public becomes aware of the associated problems during events such as red skies, in part caused by long-range transport of Saharan dust. Furthermore, the World Health Organisation (WHO) estimates that 85% of UK towns regularly exceed the safe annual PM_{2.5} limit. It is therefore important to forecast surface pollution concentrations accurately in order to enable the public to mitigate some of those adverse health risks.

Figure 1: Smog in London (December 1952). This 5-day event caused many deaths attributable to elevated concentrations of pollutants. The Clean Air Act of 1956 followed. Credit: TopFoto / The Image Works.

In general, air pollution can be difficult to forecast near the surface because of the multitude of factors which affect it. Incorrectly modelling chemical processes within the atmosphere, surface emissions or indeed the meteorology can lead to errors in predicting ground-level pollution concentrations. It is well accepted within the literature that weather forecasting is of decisive importance for air quality. Thus, my PhD project tries to link forecast errors in meteorological processes within the atmospheric boundary layer (BL) with forecast errors in pollutants such as O_{3} and NO_{2} (nitrogen dioxide) using the operational air quality forecasting model in the UK, the Air Quality in the Unified Model (AQUM). This model produces an hourly air quality forecast issued to the public by DEFRA in the form of a Daily Air Quality Index (DAQI) and is verified against surface-based observations from the Automatic Urban and Rural Network (AURN).

Figure 2: Automatic Urban and Rural Network (AURN) ground-based measuring sites for O_{3} and NO_{2}.

A three-month evaluation of hourly forecasts from AQUM shows a delay in the average increase of the morning O_{3} + NO_{2} (‘total oxidant’) concentrations when compared to AURN observations. We also know that BL depth is important for the mixing of pollutants – it acts as a sort of lid on top of the lower part of the troposphere. Since the noted lag in total oxidant increase in our model occurs exactly at the time of the morning BL development, we can form a testable hypothesis: that an inaccurate representation of BL processes – specifically, morning BL growth – leads to a delay in entrainment of O_{3}-rich air masses from the layer of air above it: the residual layer. It has been suggested in the literature that when the daytime convective mixed layer collapses upon sunset, the remaining pollutants are effectively trapped in the leftover (‘residual’) layer, and thus can act as a night-time reservoir of O_{3} above the stable or neutral night-time boundary layer (NBL).

Figure 3: Total oxidant (O_{3} + NO_{2}) average forecast (AQUM, red) and observations (AURN, black) diurnal cycle, averaged over JJA 2017 at 48 urban background sites. Shading is inter-quartile range.
Figure 4: Rate of change of the mean diurnal profile of the forecast (AQUM, red) and observations (AURN, black) of the total oxidant.

To test the hypothesis, semi-idealised experiments are conducted. We simulate a one-month long release of chemically inert tracers within the Numerical Atmospheric Dispersion Environment (NAME) using different sets of numerical weather prediction (NWP) outputs. This enables a process-based evaluation of how different meteorology affects tracers within the BL. Tracers are released within the lateral boundaries of the domain centred on the UK. The idea is to separate the effects of meteorology from chemistry on the tracer concentrations. In particular, we want to understand the role of entrainment of O_{3}-rich air masses from the residual layer down into the developing BL during the morning hours.

We located around 50 AURN sites in urban locations and compared hourly BL depths from June 2017 in the two sets of NWP output used for the tracer simulations: the UKV and UM Global (UMG) configurations of the Met Office Unified Model. It was found that although the average diurnal profiles of BL depth were quite similar, there was a lag in the morning increase of BL depth within the UMG configuration. This may be because the representation of surface sensible heat flux (SSHF) differs in the two NWP models: the UMG uses a single tile scheme to represent urban areas, whereas the UKV uses a more realistic, two-tile scheme (‘MORUSES’) which distinguishes between roof surfaces and street canyons. SSHF is a measure of energy exchange at the ground, where positive fluxes represent a loss of heat from the surface to the atmosphere. Therefore, a more realistic representation of SSHF results in the UKV being better at capturing and storing urban heat. This leads to a faster development of the BL depth in the UKV compared to the UMG, which in turn could mean that there is more turbulent motion and mixing within the atmosphere.

Assuming that the vertical gradient in pollutant concentrations is positive between the morning BL and the free troposphere, mixing air from above should enhance pollutant concentrations nearer to the surface. Our tracer results show that during days when synoptic conditions are dominated by high pressure, the diurnal cycle in forecast and observed surface pollutant concentrations can be adequately replicated by our simplified set-up. Differences between the diurnal cycle between tracer simulations with the two different meteorological set-ups show that the UKV is not only entraining more tracer from above the boundary layer than the simulation using UMG, but also the concentrations increase on average 1 – 2 hours earlier in the morning. These results suggest that indeed the model meteorology – in particular, representation of BL processes – is important to entrainment of polluted air masses into the BL, which in turn has a significant influence on the surface pollutant concentrations.

Within the past two decades, it has been recognised by the weather and air quality modelling communities that neither type of model can truly exist without the other. This post has discussed just one aspect of how meteorology influences the air quality forecast – there are, of course, many other parameters (e.g. wind speed, precipitation, relative humidity) which affect the forecast pollutant concentrations. We therefore also evaluated night-time errors in the wind speed and found that these errors are positively correlated with the total oxidant forecast errors. This means that when the wind speed forecast is overestimated, it is likely to affect the night-time and morning forecast of both O_{3} and NO_{2} in a significant way.

References

Ambient Air Pollution: A global assessment of exposure and burden of disease. WHO, 2016.

Bohnenstengel S., Evans S., Clark P., Belcher S.: Simulations of the London urban heat island, Quarterly Journal of the Royal Meteorological Society, 2011 vol: 137 (659) pp: 1625-1640

Cocks A., 1993: The Chemistry and Deposition of Nitrogen Species in the Troposphere, The Royal Society of Chemistry, Cambridge 1993

Savage N., Agnew P., Davis L., Ordonez C., Thorpe R., Johnson C., O’Connor F., Dalvi M.: Air quality modelling using the Met Office Unified Model (AQUM OS24-26): model description and initial evaluation, Geoscientific Model Development, 2013 vol: 6 pp: 353-372

Sun J., Mahrt L., Banta R., Pichugina Y.: Turbulence Regimes and Turbulence Intermittency in the Stable Boundary Layer during CASES-99, Journal of the Atmospheric Sciences, 2012 vol: 69 (1) pp: 338-351

Zhang, 2008: Online-coupled meteorology and chemistry models: History, current status, and outlook. Atmos. Chem. Phys, 2008 vol: 8 (11) pp: 2895-2932

On relocating to the Met Office for five weeks of my PhD

Some PhD projects are co-organised by an industrial CASE partner which provides supervisory support and additional funding. As part of my CASE partnership with the UK Met Office, in January I had the opportunity to spend 5 weeks at the Exeter HQ, which proved to be a fruitful experience. As three out of my four supervisors are based there, it was certainly a convenient set-up to seek their expertise on certain aspects of my PhD project!

One part of my project aims to understand how certain neighbourhood-based verification methods can affect the level of surface air quality forecast accuracy. Routine verification of a forecast model against observations is necessary to provide the most accurate forecast possible. Ensuring that this happens is crucial, as a good forecast may help keep the public aware of potential adverse health risks resulting from elevated pollutant concentrations.

The project deals with two sides of one coin: evaluating forecasts of regional surface pollutant concentrations; and evaluating those of meteorological fields such as wind speed, precipitation, relative humidity or temperature. All of the above have unique characteristics: they vary in resolution, spatial scale, homogeneity, randomness… The behaviour of the weather and pollutant variables is also tricky to compare against one another because the locations of their numerous measurement sites nearly never coincide, whereas the forecast encompasses the entirety of the domain space. This is kind of the crux of this part of my PhD: how can we use these irregularly located measurements to our advantage in verifying the skill of the forecast in the most useful way? And – zooming out still – can we determine the extent to which the surface air pollution forecast is dependent on some of those aforementioned weather variables? And can this knowledge (once acquired!) be used to further improve the pollution forecast?

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Side view of the UK Met Office on a cold day in February.

While at the Met Office, I began my research specifically into methods which analyse the forecast skill when a model “neighbourhood” of a particular size around a particular point-observation is evaluated. These methods are being developed as part of a toolkit for evaluation of high resolution forecasts, which can be (and usually are) more accurate than a lower resolution equivalent, but traditional metrics (e.g. root mean square error (RMSE) or mean error (ME)) often fail to demonstrate the improvement (Mittermaier, 2014). They can also fall victim to various verification errors such as the double-penalty problem. This is when an ‘event’ might have been missed at a particular time in the forecast at one gridpoint because it was actually forecast in the neighbouring grid-point one time-step out, so the RMSE counts this error both in the spatial and temporal axes. Not fair, if you ask me. So as NWP continues to increase in resolution, there is a need for robust verification methods which relax the spatial (or temporal) restriction on precise forecast-to-observation matching somewhat (Ebert, 2008).

One way to proceed forward is via a ‘neighbourhood’ approach which treats a deterministic forecast almost as an ensemble by considering all the grid-points around an observation as an individual forecast and formulating a probabilistic score. Neighbourhoods are made of varying number of model grid-points, i.e. a 3×3 or a 5×5 or even bigger. A skill score such as the ranked probability score (RPS) or Brier Score is calculated using the cumulative probability distribution across the neighbourhood of the exceedance of a sensible pollutant concentration threshold. So, for example, we can ask what proportion of a 5×5 neighbourhood around an observation has correctly forecasted an observed exceedance (i.e. ‘hit’)? What if an exceedance forecast has been made, but the observed quantity didn’t reach that magnitude (i.e. ‘false alarm’)? And how do these scores change when larger (or smaller) neighbourhoods are considered? And, if these spatial verification methods prove informative, how could they be implemented in operational air quality forecast verification? All these questions will hopefully have some answers in the near future and form a part of my PhD thesis!

Although these kind of methods have been used for meteorological variables, they haven’t yet been widely researched in the context of regional air quality forecasts. The verification framework for this is called HiRA – High Resolution Assessment, which is part of the wider verification network Model Evaluation Tools (which, considering it is being developed as a means of uniformly assessing high-resolution meteorological forecasts, has the most unhelpful acronym: MET). It is quite an exciting opportunity to be involved in the testing and evaluation of this new set of verification tools for a surface pollution forecast at a regional scale, and I am very grateful to be involved in this. Also, having the opportunity to work at the Met Office and “pretend” to be a real research scientist for a while is awesome!

Email: k.m.milczewska@pgr.reading.ac.uk

Macmillan Coffee Morning / Bake-Off 2017

(Written by Hannah Gough & Kaja Milczewska)

Following on from last year, the Macmillan Coffee Morning 2017 proved to be another storming success in the Met department. Four tables full of cake and other goodies were sold across the morning, raising well over £300 for Macmillan Cancer. We combined this with a ‘Bake Off’, where two tables of bakes were entered into four categories: ‘traditional’, ‘pumpkin’, ‘WCD’ (weather and climate discussion) and the technical challenge: scones. Competition was fierce, with no bake being disliked by our judges: Steve Woolnough, Claire Morris, Rob Thompson and Michael Jonhston.

The goods which did not make it into any category (but definitely into our bellies!) made up two other tables, featuring the likes of rum cake, banana bread, Swiss roll, cookies and cupcakes of various flavours and a very shiny-chocolate topped salted caramel slice.

Those with allergies were well catered for with gluten free chocolate orange iced cupcakes and chocolate fudge cake. Vegan entrants included chocolate muffins and a lemon and chia seed loaf with rain cloud decoration!

Josh Talib won the scone category with butternut squash, thyme and goat’s cheese scones, Rebecca Frew won the traditional cake category with a tasty Bara Brith (speckled bread), a Welsh speciality. The pumpkin category was won by Wendy Neale with some scrumptious pumpkin and ginger cupcakes, whilst the lightning bolt biscuits by Dan Hodson zapping the competition in the WCD category.

A big thanks to all who donated cakes, time and cash to this event. Macmillan Coffee mornings are held frequently all over the country, raising money towards cancer care. We hope this continues in our department in the future! For more information on the work Macmillan do, visit https://www.macmillan.org.uk/

 

Experiences of the NERC Atmospheric Pollution and Human Health Project.

Email: k.m.milczewska@pgr.reading.ac.uk

One of the most exciting opportunities of my PhD experience to date has been a research trip to Beijing in June, as part of the NERC Atmospheric Pollution and Human Health (APHH) project. This is a worldwide research collaboration with a focus on the way air pollution in developing megacities affects human health, and the meeting in Beijing served as the 3rd project update.

Industrialisation of these cities in the last couple of decades has caused air pollution to rise rapidly and regularly exceed levels deemed safe by the World Health Organisation (WHO).  China sees over 1,000,000 deaths annually due to particulate matter (PM), with 76 deaths per 100,000 capita. In comparison, the UK has just over 16,000 total deaths and 26 per capita. But not only do these two countries have very different climates and emissions; they are also at very different stages of industrial development. So in order to better understand the many various sources of pollution in developing megacities – be they from local transport, coal burning or advected from further afield – there is an increased need for developing robust air quality (AQ) monitoring measures.

The APHH programme exists as a means to try and overcome these challenges. My part in the meeting was to expand the cohort of NCAS / NERC students researching AQ in both the UK and China, attending a series of presentations in a conference-style environment and visiting two sites with AQ monitoring instruments. One is situated in the Beijing city centre while the other in the rural village of Pinggu, just NW of Beijing. Over 100 local villagers take part in a health study by carrying a personal monitor with them over a period of two weeks. Their general health is monitored at the Pinggu site, alongside analysis of the data collected about their personal exposure to pollutants each day, i.e. heatmaps of different pollutant species are created according to GPS tracking. Having all the instruments being explained to us by local researchers was incredibly useful, because since I work with models, I haven’t had a great deal of first hand exposure to pollutant data collection. It was beneficial to get an appreciation of the kind of work this involves!

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In between all our academic activities we also had the chance to take some cultural breaks – Beijing has a lot to offer! For example, our afternoon visit to the Pinggu rural site followed the morning climb up the Chinese Great Wall. Although the landscape was somewhat obscured by the pollution haze, this proved to be a positive thing as we didn’t have to suffer in the direct beam of the sun!


I would like to greatly thank NERC, NCAS and University of Leeds for the funding and organisation of this trip. It has been an incredible experience, and I am looking forward to observing the progess of these projects, hopefully using what I have learnt in some of my own work.

For more information, please visit the APHH student blog in which all the participants documented their experiences: https://www.ncas.ac.uk/en/introduction-to-atmospheric-science-home/18-news/2742-ncas-phd-students-visit-four-year-air-quality-fieldwork-project-in-beijing

Meteorology Ball 2017

Email: K.M.Milczewska@pgr.reading.ac.uk

On Friday 17th February, the annual Meteorology Ball provided a great excuse for members of the department and their guests to dress up for the evening. But for all the excitement of this year’s masquerade theme, the Ball is mainly a charity event. Through the sale of raffle tickets and an auction of promises, the event aims to raise money for the David Grimes Trust, administered by the Reading San Francisco Libre Association (RSFLA), in honour of the well-remembered academic from our department who devoted a great deal of his time to the charity.

RSFLA supports environmental and educational projects in the rural Nicaraguan town of San Francisco Libre, which was ‘twinned’ with Reading in 1994 in order to encourage the exchange of culture and knowledge. Over the past few years, the Meteorology department has supported this link through regular cake sales, running the Reading Half Marathon and, of course, the annual ball.

David Grimes was a respected, integral member of the department and there are many among us who reminisce about his goodwill, interactive lectures and Panto appearances. There are also those among us who, despite never having had the chance to meet David, can easily imagine the positive impact he had both in and outside of our department, through our continued support of the charity under his name. The money  raised is mainly spent on educational support in the San Francisco Libre district: helping to fund a scholarship programme, build a library and toilet facilities among various other projects – and the people who benefit directly have a special message for us all!
https://youtu.be/vWsf9TWwWp4

The generosity of over 80 people attending made the event a great success, raising over £1500 through bidding on bizzarre auction items and lessons, as well as purchasing raffle tickets. To add to this, Santander will be chipping in with an extra £1500 to match, bringing the total raised to over £3000 for the charity! Such success would never have happened, had it not been for all the help we received from Santander, local businesses offering prizes for the raffle, and most importantly: all of those who bought a ticket to come! On behalf of all the organisers, I would like to finish this post with a massive bout of thanks for making the evening worth all the effort and continuing the important tradition of fundraising for the David Grimes Trust.

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Met Festivities.

The Christmas period is a busy time for many and PhD students are no exception. Below are quick highlights from the department written by three of our PhD students. Read below to learn more about our recent Royal Meteorological Society South-East local centre meeting, the adventures of the Met Choir, and the much-anticipated departmental pantomime.

“Will we have a white Christmas in Reading this year? What does the term “white Christmas” even mean? Both of these questions were addressed at the beginning of the Royal Meteorological Society’s local south-east centre meeting on the 7th of December by the department of meteorology’s recently retired Ross Reynolds.

The evening began with inevitable mince pies and a poster showcase by eight PhD students from a variety of research areas, which initiated lively discussions. The Met choir singers added to the festive spirit with a repertoire of carols before the oral presentations began.

First up was Jake Gristey, whose research project investigates satellite constellations to measure energy flux in and out of the Earth’s atmosphere. Updating the satellite constellation will allow satellites to measure outgoing energy flux to a higher accuracy than any instrument has done previously, allowing for an accurate calculation of Earth’s radiation budget. Eunice Lo spoke about a geoengineering method, Sulphate Aerosol Injection (SAI) which involves releasing sulphate particles into the atmosphere with the aim of increasing the Earth’s albedo. The idea is based on historical volcanic aerosol release which led to a decrease in global temperatures. Eunice is basing her studies of the effects of SAI on a future world following a particular economic scenario. Our last speaker of the evening was James Shaw, who researches the modelling of atmospheric transport over terrain. He is currently developing a new mesh for numerical transport schemes over mountains, with a focus on the accurate representation of near-surface cells.

The meeting exhibited the huge variety of research happening in the department and was an overall success. This was the last local-cente meeting of the year, with the next one taking place on 11th January 2017.”

Kaja Milczewska, K.M.Milczewska@pgr.reading.ac.uk

“An important part of the festive season for PhD students is the infamous Met Pantomime. Twice a week we all get together over our lunchtimes to practice and perfect all the jokes accrued by the members of the department this year. Although planning begins in September, it’s only come December when it all comes together. That crazy wig arrives from Amazon and we’ve created oversized comic props from all the cardboard Hobbycraft can spare. The jokes and jibes get funnier every time we practice them and staff just keep providing more and more material (oh no they don’t!). There’s definitely an undercurrent of excitement – and a little apprehension – as the big evening draws near. This years’ comic spectacle: Snow White and the Research Dwarves, complete with lights, sound, and a fantastic buffet.”

Sarah Bentley, S.Bentley@pgr.reading.ac.uk

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“This year the meteorology choir have been busy rehearsing for performances both within the department and externally. A recently formed tradition and definite highlight has been singing for the residents of the Lakeside care home as well as for the annual department Christmas celebration. We also have been lucky enough this year to perform at a local and national meeting of the Royal Meteorological Society. The choir is open to all, regardless of musical ability and we have members ranging from students all the way up to head of department.”

Samantha Buzzard, s.c.buzzard@pgr.reading.ac.uk

Merry Christmas and Happy New Year from the Social Metwork.