Should we be ‘Leaf’-ing out vegetation when parameterising the aerodynamic properties of urban areas?


When modelling urban areas, vegetation is often ignored in attempt to simplify an already complex problem. However, vegetation is present in all urban environments and it is not going anywhere… For reasons ranging from sustainability to improvements in human well-being, green spaces are increasingly becoming part of urban planning agendas. Incorporating vegetation is therefore a key part of modelling urban climates. Vegetation provides numerous (dis)services in the urban environment, each of which requires individual attention (Salmond et al. 2016). However, one of my research interests is how vegetation influences the aerodynamic properties of urban areas.

Two aerodynamic parameters can be used to represent the aerodynamic properties of a surface: the zero-plane displacement (zd) and aerodynamic roughness length (z0). The zero-plane displacement is the vertical displacement of the wind-speed profile due to the presence of surface roughness elements. The aerodynamic roughness length is a length scale which describes the magnitude of surface roughness. Together they help define the shape and form of the wind-speed profile which is expected above a surface (Fig. 1).


Figure 1: Representation of the wind-speed profile above a group of roughness elements. The black dots represent an idealised logarithmic wind-speed profile which is determined using the zero-plane displacement (zd) and aerodynamic roughness length (z0) (lines) of the surface.

For an urban site, zd and z0 may be determined using three categories of methods: reference-based, morphometric and anemometric. Reference-based methods require a comparison of the site to previously published pictures or look up tables (e.g. Grimmond and Oke 1999); morphometric methods describe zd and z0 as a function of roughness-element geometry; and, anemometric methods use in-situ observations. The aerodynamic parameters of a site may vary considerably depending upon which of these methods are used, but efforts are being made to understand which parameters are most appropriate to use for accurate wind-speed estimations (Kent et al. 2017a).

Within the morphometric category (i.e. using roughness-element geometry) sophisticated methods have been developed for buildings or vegetation only. However, until recently no method existed to describe the effects of both buildings and vegetation in combination. A recent development overcomes this, whereby the heights of all roughness elements are considered alongside a porosity correction for vegetation (Kent et al. 2017b). Specifically, the porosity correction is applied to the space occupied and drag exerted by vegetation.

The development is assessed across several areas typical of a European city, ranging from a densely-built city centre to an urban park. The results demonstrate that where buildings are the dominant roughness elements (i.e. taller and occupying more space), vegetation does not obviously influence the calculated geometry of the surface, nor the aerodynamic parameters and the estimated wind speed. However, as vegetation begins to occupy a greater amount of space and becomes as tall as (or larger) than buildings, the influence of vegetation is obvious. Expectedly, the implications are greatest in an urban park, where overlooking vegetation means that wind speeds may be slowed by up to a factor of three.

Up to now, experiments such as those in the wind tunnel focus upon buildings or trees in isolation. Certainly, future experiments which consider both buildings and vegetation will be valuable to continue to understand the interaction within and between these roughness elements, in addition to assessing the parameterisation.


Grimmond CSB, Oke TR (1999) Aerodynamic properties of urban areas derived from analysis of surface form. J Appl Meteorol and Clim 38:1262-1292.

Kent CW, Grimmond CSB, Barlow J, Gatey D, Kotthaus S, Lindberg F, Halios CH (2017a) Evaluation of Urban Local-Scale Aerodynamic Parameters: Implications for the Vertical Profile of Wind Speed and for Source Areas. Boundary-Layer Meteorology 164: 183-213.

Kent CW, Grimmond CSB, Gatey D (2017b) Aerodynamic roughness parameters in cities: Inclusion of vegetation. Journal of Wind Engineering and Industrial Aerodynamics 169: 168-176.

Salmond JA, Tadaki M, Vardoulakis S, Arbuthnott K, Coutts A, Demuzere M, Dirks KN, Heaviside C, Lim S, Macintyre H (2016) Health and climate related ecosystem services provided by street trees in the urban environment. Environ Health 15:95.

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.

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.

Lunch with experts. Photo courtesy of Ariane Frassoni
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.

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.

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


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.


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!


Future of Cumulus Parametrization conference, Delft, July 10-14, 2017


For a small city, Delft punches above its weight. It is famous for many things, including its celebrated Delftware (Figure 1). It was also the birthplace of one of the Dutch masters, Johannes Vermeer, who coincidentally painted some fine cityscapes with cumulus clouds in them (Figure 2). There is a university of technology with some impressive architecture (Figure 3). It holds the dubious honour of being the location of the first assassination using a pistol (or so we were told by our tour guide), when William of Orange was shot in 1584. To this list, it can now add hosting a one-week conference on the future of cumulus parametrization, and hopefully bringing about more of these conferences in the future.


Figure 1: Delftware.


Figure 2: Delft with canopy of cumulus clouds. By Johannes Vermeer, 1661.


Figure 3: AULA conference centre at Delft University of Technology – where we were based for the duration of the conference.

So what is a cumulus parametrization scheme? The key idea is as follows. Numerical weather and climate models work by splitting the atmosphere into a grid, with a corresponding grid length representing the length of each of the grid cells. By solving equations that govern how the wind, pressure and heating interact, models can then be used to predict what the weather will be like days in advance in the case of weather modelling. Or a model can predict how the climate will react to any forcings over longer timescales. However, any phenomena that are substantially smaller than this grid scale will not be “seen” by the models. For example, a large cumulonimbus cloud may have a horizontal extent of around 2km, whereas individual grid cells could be 50km in the case of a climate model. A cumulonimbus cloud will therefore not be explicitly modelled, but it will still have an effect on the grid cell in which it is located – in terms of how much heating and moistening it produces at different levels. To capture this effect, the clouds are parametrized, that is, the vertical profile of the heating and moistening due to the clouds are calculated based on the conditions in the grid cell, and this then affects the grid-scale values of these variables. A similar idea applies for shallow cumulus clouds, such as the cumulus humilis in Vermeer’s painting (Figure 2), or present-day Delft (Figure 3).

These cumulus parametrization schemes are a large source of uncertainty in current weather and climate models. The conference was aimed at bringing together the community of modellers working on these schemes, and working out which might be the best directions to go in to improve these schemes, and consequently weather and climate models.

Each day was a mixture of listening to presentations, looking at posters and breakout discussion groups in the afternoon, as well as plenty of time for coffee and meeting new people. The presentations covered a lot of ground: from presenting work on state-of-the-art parametrization schemes, to looking at how the schemes perform in operational models, to focusing on one small aspect of a scheme and modelling how that behaves in a high resolution model (50m resolution) that can explicitly model individual clouds. The posters were a great chance to see the in-depth work that had been done, and to talk to and exchange ideas with other scientists.

Certain ideas for improving the parametrization schemes resurfaced repeatedly. The need for scale-awareness, where the response of the parametrization scheme takes into account the model resolution, was discussed. One idea for doing this was the use of stochastic schemes to represent the uncertainty of the number of clouds in a given grid cell. The concept of memory also cropped up – where the scheme remembers if it had been active at a given grid cell in a previous point in time. This also ties into the idea of transitions between cloud regimes, e.g. when a stratocumulus layer splits up into individual cumulus clouds. Many other, sometimes esoteric, concepts were discussed, such as the role of cold pools, how much tuning of climate models is desirable and acceptable, how we should test our schemes, and what the process of developing the schemes should look like.

In the breakout groups, everyone was encouraged to contribute, which made for an inclusive atmosphere in which all points of view were taken on board. Some of the key points of agreement from these were that it was a good idea to have these conferences, and we should do it more often! Hopefully, in two years’ time, another PhD student will write a post on how the next meeting has gone. We also agreed that it would be beneficial to be able to share data from our different high resolution runs, as well as to be able to compare code for the different schemes.

The conference provided a picture of what the current thinking on cumulus parametrization is, as well as which directions people think are promising for the future. It also provided a means for the community to come together and discuss ideas for how to improve these schemes, and how to collaborate more closely with future projects such as ParaCon and HD(CP)2.

Experiences of the NERC Atmospheric Pollution and Human Health Project.


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!


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:

RMetS Impact of Science Conference 2017.

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“We aim to help people make better decisions than they would if we weren’t here”

Rob Varley CEO of Met Office

This week PhD students from the University of Reading attended the Royal Meteorological Society Impact of Science Conference for Students and Early Career Scientists. Approximately eighty scientists from across the UK and beyond gathered at the UK Met Office to learn new science, share their own work, and develop new communication skills.


Across the two days students presented their work in either a poster or oral format. Jonathan Beverley, Lewis Blunn and I presented posters on our work, whilst Kaja Milczewska, Adam Bateson, Bethan Harris, Armenia Franco-Diaz and Sally Woodhouse gave oral presentations. Honourable mentions for their presentations were given to Bethan Harris and Sally Woodhouse who presented work on the energetics of atmospheric water vapour diffusion and the representation of mass transport over the Arctic in climate models (respectively). Both were invited to write an article for RMetS Weather Magazine (watch this space). Congratulations also to Jonathan Beverley for winning the conference’s photo competition!

Jonathan Beverley’s Winning Photo.

Alongside student presentations, two keynote speaker sessions took place, with the latter of these sessions titled Science Communication: Lessons from the past, learning for future impact. Speakers in this session included Prof. Ellie Highwood (Professor of Climate Physics and Dean for Diversity and Inclusion at University of Reading), Chris Huhne (Co-chair of ET-index and former Secretary of State for Energy and Climate Change), Leo Hickman (editor for Carbon Brief) and Dr Amanda Maycock (NERC Independent Research Fellow and Associate Professor in Climate Dynamics, University of Leeds). Having a diverse range of speakers encouraged thought-provoking discussion and raised issues in science communication from many angles.

Prof. Ellie Highwood opened the session challenging us all to step beyond the typical methods of scientific communication. Try presenting your science without plots. Try presenting your work with no slides at all! You could step beyond the boundaries even more by creating interesting props (for example, the notorious climate change blanket). Next up Chris Huhne and Leo Hickman gave an overview of the political and media interactions with climate change science (respectively). The Brexit referendum, Trump’s withdrawal from the Paris Accord and the rise of the phrase “fake news” are some of the issues in a society “where trust in the experts is falling”. Finally, Dr Amanda Maycock presented a broad overview of influential science communicators from the past few centuries. Is science relying too heavily on celebrities for successful communication? Should the research community put more effort into scientific outreach?

Communication and collaboration became the two overarching themes of the conference, and conferences such as this one are a valuable way to develop these skills. Thank you to the Royal Meteorology Society and UK Met Office for hosting the conference and good luck to all the young scientists that we met over the two days.


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Also thank you to NCAS for funding my conference registration and to all those who provided photos for this post.

4th ICOS Summer School


The 4th ICOS Summer School on challenges in greenhouse gases measurements and modelling was held at Hyytiälä field station in Finland from 24th May to 2nd June, 2017. It was an amazing week of ecosystem fluxes and measurements, atmospheric composition with in situ and remote sensing measurements, global climate modelling and carbon cycle, atmospheric transport and chemistry, and data management and cloud (‘big data’) methods. We also spent some time in the extremely hot Finnish sauna followed by jumps into a very cold lake, and many highly enjoyable evenings by the fire with sunsets that seemed to never come.

sunset_Martijn Pallandt
Figure 1. Sunset in Hyytiälä, Finland at 22:49 local time. Credits: Martijn Pallandt

Our journey started in Helsinki, where a group of about 35 PhD students, with a number of postdocs and master students took a 3 hours coach trip to Hyytiälä.  The group was very diverse and international with people from different backgrounds; from plant physiologists to meteorologists. The school started with Prof. Dr. Martin Heimann  introducing us to the climate system and the global carbon cycle, and Dr. Alex Vermeulen highlighted the importance of good metadata practices and showed us more about ICOS research infrastructure. Dr. Christoph Gerbig joined us via Skype from Germany and talked about how atmospheric measurements methods with aircrafts (including how private air companies) can help scientists.

Figure 2. Hyytiälä flux tower site, Finland. Credits: Truls Andersen

On Saturday we visited the Hyytiälä flux tower site, as well as a peatland field station nearby, where we learned more about all the flux data they collect and the importance of peatlands globally. Peatlands store significant amounts of carbon that have been accumulating for millennia and they might have a strong response to climate change in the future. On Sunday, we were divided in two groups to collect data on temperature gradients from the lake to the Hyytiälä main flux tower, as well as on carbon fluxes with dark (respiration only) and transparent (photosynthesis + respiration) CO2 chambers.

Figure 3: Dark chamber for CO2 measurements being used by a group of students in the Boreal forest. Credits: Renato Braghiere

On the following day it was time to play with some atmospheric modelling with Dr. Maarten Krol and Dr. Wouter Peters. We prepared presentations with our observation and modelling results and shared our findings and experiences with the new data sets.

The last two days have focused on learning how to measure ecosystem fluxes with Prof. Dr. Timo Vesala, and insights on COS measurements and applications with Dr. Kadmiel Maseyk. Timo also shared with us his passion for cinema with a brilliant talk entitled “From Vertigo to Blue Velvet: Connotations between Movies and Climate change” and we watched a really nice Finnish movie “The Happiest Day in the Life of Olli Mäki“.

Figure 4: 4th ICOS Summer School on Challenges in greenhouse gases measurements and modelling group photo. Credits: Wouter Peters

Lastly, it was a fantastic week where we were introduced to several topics and methods related to the global carbon budget and how it might impact the future climate. No doubt all information gained in this Summer School will be highly valuable for our careers and how we do science. A massive ‘cheers’ to Olli Peltola, Alex Vermeulen, Martin Heimann, Christoph Gerbig, Greet Maenhout, Wouter Peters, Maarten Krol, Anders Lindroth , Kadmiel Maseyk, Timo Vesala, and all the staff at the Hyytiälä field station.

This post only scratches the surface of all of the incredible material we were able to cover in the 4th ICOS Summer School, not to mention the amazing group of scientists that we met in Finland, who I really look forward to keeping in touch over the course of the years!