IAP-26-080

The Heatwave That Never Was: Assessing Scotland’s Vulnerability to Plausible Extreme Heat

1. Background and scientific rationale

Extreme heat is an increasingly important climate hazard across the United Kingdom, with implications for ecosystems, human health and critical infrastructure. Although Scotland experiences lower average temperatures and fewer extreme heat events than southern England, its exposure to physically plausible but historically unobserved extremes remains poorly understood. Conventional assessments of extreme heat often rely on observed temperature records or projections of future climate change. However, the relatively short observational record captures only a small subset of the atmospheric conditions that are physically possible within the current climate. Consequently, extreme events capable of producing substantial impacts may remain absent from historical observations. The increasing availability of large ensembles of numerical weather prediction simulations, seasonal hindcasts and high-resolution climate model archives offers new opportunities to explore these unrealised extremes. These datasets contain alternative realisations of atmospheric evolution, including rare combinations of circulation patterns and local meteorological conditions that may produce exceptionally high temperatures.

A key scientific challenge is understanding which atmospheric circulation patterns generate the most extreme heat events in Scotland, what large-scale processes drive these patterns, and how their characteristics and likelihood may change under global warming. Scottish heat extremes may be influenced by several interacting mechanisms, including persistent atmospheric blocking, anomalous pressure distributions, warm-air advection, subsidence, land–atmosphere feedbacks and variations in the North Atlantic jet stream. The relative importance of these processes, particularly for the most extreme simulated events, requires further investigation. Importantly, extreme heat impacts are not determined by air temperature alone. Solar radiation, cloud cover, diurnal temperature range, antecedent conditions and the persistence of particular weather regimes can influence the severity of thermal stresses experienced at the surface. Understanding the relationship between large-scale atmospheric circulation and these local-scale hazards is therefore essential for characterising the full range of plausible extreme heat conditions.

Scotland’s railway network provides a valuable case study through which to investigate the consequences of these meteorological extremes. Railway infrastructure is sensitive to thermal stresses, yet historical exposure to severe heat has been relatively limited. This raises questions about whether existing infrastructure and operational procedures are adequately prepared for extreme events that are meteorologically plausible but have not yet been experienced. By combining large ensemble analysis, atmospheric dynamics and infrastructure-relevant hazard assessment, this PhD will advance understanding of the physical drivers of extreme heat in Scotland and demonstrate how this knowledge can inform climate risk assessment.

2. Overall aim

To understand the atmospheric circulation patterns and physical processes responsible for plausible extreme heat events in Scotland, investigate how these may change under global warming, and assess their implications for climate-related hazards affecting railway infrastructure.

3. Research questions

1. How extreme could heat events plausibly become in Scotland under the present-day climate? What can large ensembles of weather and climate simulations reveal about the magnitude, duration, spatial extent and seasonality of extreme heat events beyond the observational record?

2. What are the dominant atmospheric circulation patterns and large-scale drivers of plausible extreme heat in Scotland? How do atmospheric blocking, jet-stream configurations, air-mass transport, subsidence and land–atmosphere interactions contribute to the development, persistence and intensity of the most extreme events?

3. What meteorological characteristics influence the potential severity of extreme heat impacts on railway infrastructure? In particular, what roles do solar radiation, diurnal temperature range, event duration and seasonal timing play? How spatially variable is Scotland’s railway exposure and vulnerability to plausible extreme heat events? How do large-scale atmospheric patterns translate into local-scale heat hazards relevant to railway infrastructure?

4. How might the characteristics and likelihood of plausible extreme heat events change under continued global warming? How can information about plausible extreme heat events support railway adaptation, stress-testing and operational preparedness?

1. Characterising plausible extreme heat in Scotland: Analyse large ensembles of historical weather forecasts and hindcasts, potentially including ECMWF medium-range reforecasts and seasonal prediction archives. Identify extreme temperature events exceeding observed historical records and assess their spatial extent, duration, intensity and meteorological drivers. This work would establish what extreme heat could plausibly look like in Scotland, while evaluating model biases, ensemble dependence and sampling limitations.

2. Atmospheric circulation patterns and physical drivers of extreme heat: The project will identify the dominant circulation patterns associated with extreme heat in Scotland and investigate the physical processes responsible for their development and persistence.

Potential approaches include:
– Atmospheric circulation classification using clustering, empirical orthogonal functions or self-organising maps.
– Composite analysis of sea-level pressure, geopotential height, jet-stream position and atmospheric temperature anomalies.
– Investigation of blocking frequency, persistence and geographical location.
– Analysis of air-mass trajectories, temperature advection, subsidence and diabatic heating.
– Comparison of the circulation patterns associated with observed and unrealised extreme events.
A particularly interesting question is whether the most severe Scottish heat extremes arise from unusually intense versions of familiar summer circulation regimes, or from rare combinations of atmospheric processes that have not yet occurred in the observational record.

3. Translating meteorological extremes into railway-relevant hazards, assessing spatial vulnerability and plausible disruption scenarios: Investigate how air temperature, solar radiation, diurnal temperature range and seasonal timing combine to produce conditions potentially hazardous to railway infrastructure. Develop railway-relevant heat hazard indicators, potentially including estimates of rail temperature or other component thermal stresses, subject to the availability of engineering information. Compare conventional air-temperature-based indicators with more physically informed measures of heat exposure. Combine extreme heat scenarios with available information on Scotland’s railway network, infrastructure characteristics and operational thresholds. Identify locations or assets potentially exposed to unusually severe heat stresses. Where Network Rail data are accessible, investigate how infrastructure condition, regional preparedness and historical disruption records influence vulnerability. Develop a small number of physically plausible extreme heat scenarios to illustrate potential consequences for railway operations.

4. Climate adaptation and future resilience: The research will explore whether future changes are primarily associated with a warmer background climate or whether changes in circulation patterns, blocking persistence, air-mass transport or land–atmosphere feedbacks also contribute. Where suitable data are available, the analysis could compare extreme events across different warming levels or climate periods, using circulation-conditioned approaches to separate changes in background temperatures from changes in atmospheric dynamics. Examine how extreme heat exposure may change as the climate warms and how these changes interact with existing railway vulnerabilities. Explore implications for infrastructure stress-testing, seasonal preparedness and longer-term adaptation priorities.
Consider the trade-offs involved in maintaining resilience to both cold and hot extremes, particularly as their relative frequencies change.

5. Scientific novelty

The focus on unrealised present-day extremes rather than relying solely on future warming scenarios, means the student would examine severe events that could plausibly occur within the current climate. This has immediate relevance to infrastructure risk management.

Second, the use of high-resolution weather ensemble archives for infrastructure stress-testing. The project could develop an approach for extracting physically plausible, high-impact weather scenarios from existing model archives and translating them into infrastructure-relevant hazards.

Third, the consideration of compound meteorological drivers. A key contribution could be demonstrating that maximum air temperature alone is insufficient to characterise railway heat risk, particularly in Scotland, where clear skies, strong solar radiation and large diurnal temperature ranges may create challenging conditions outside conventional summer heatwaves.

Year 1

Year 1: Literature review, dataset evaluation, identification and validation of plausible heat extremes

– Review literature on extreme heat, atmospheric circulation and large ensemble approaches to studying rare events.
– Identify, acquire and evaluate suitable datasets (e.g., ECMWF hindcasts, ERA5 and climate model ensembles).
– Develop methods to identify plausible extreme heat events beyond the observational record
– Characterise the intensity, duration, spatial extent and seasonality of simulated extremes.
– Undertake preliminary analysis of associated large-scale atmospheric circulation patterns.

Year 2

Year 2: Understanding atmospheric drivers and physical mechanisms

– Identify dominant circulation patterns associated with Scottish extreme heat using clustering, composites and circulation diagnostics.
– Investigate the roles of atmospheric blocking, jet-stream variability, warm-air advection and subsidence.
– Examine how large-scale circulation interacts with local processes, including solar radiation, cloud cover and land–atmosphere feedbacks.
– Determine whether the most extreme events arise from familiar circulation regimes or rare combinations of atmospheric conditions.
– Produce storylines of plausible heatwave events for testing in infrastructure systems.

Year 3

Year 3: Climate change and implications for railway infrastructure

-Investigate how the frequency, intensity and characteristics of plausible extreme heat events may change under global warming.
– Assess the relative contributions of thermodynamic warming and changes in atmospheric circulation
– Examine how atmospheric conditions influence railway-relevant heat hazards, including solar radiation, diurnal temperature range and event persistence.
– Develop physically plausible extreme heat scenarios and assess their spatial distribution across Scotland’s railway network.
– Explore implications for railway resilience and adaptation, potentially in collaboration with Network Rail.

Year 3.5

– Integrate findings on plausible extremes, atmospheric drivers, climate change and infrastructure impacts.
– Refine extreme heat scenarios and evaluate key uncertainties.
– Complete research papers and disseminate findings to academic and infrastructure stakeholders.
– Write and submit the PhD thesis.

The PhD will provide interdisciplinary training in atmospheric science, climate modelling, computational data analysis and climate risk assessment, developing skills relevant to both academic research and applied environmental science.

– Atmospheric dynamics and climate science: Develop expertise in the physical processes driving extreme heat, including atmospheric blocking, jet-stream variability, large-scale circulation and land–atmosphere interactions.
– Climate and weather model analysis: Gain experience working with large, high-resolution weather and climate model ensembles, reanalysis datasets and seasonal hindcasts.
– Computational and statistical methods: Develop advanced programming skills (particularly Python), high-performance computing, large dataset processing, statistical analysis and techniques for identifying rare extreme events.
– Climate change and uncertainty assessment: Gain experience evaluating climate projections, distinguishing dynamical and thermodynamic contributions to climate extremes, and quantifying model and sampling uncertainties.
– Spatial analysis and climate risk assessment: Develop GIS and spatial mapping skills, translating meteorological information into infrastructure-relevant hazard indicators and plausible extreme-event scenarios.
– Interdisciplinary and stakeholder engagement: Gain experience collaborating with infrastructure stakeholders, potentially including Network Rail, and communicating climate science findings to non-academic audiences.
– Transferable research skills: Develop scientific writing, publication, presentation, project management and independent research skills through conference participation, peer-reviewed publications and thesis preparation.

Where opportunities arise, the student could also undertake a short placement or secondment with Network Rail or another climate resilience organisation, gaining practical experience in the application of climate science to infrastructure adaptation and decision-making.