CR2025_22 The role of the North Atlantic jet stream and cut-off low cyclones in the changing nature of summertime European flood risk
Lead Supervisor: Ben Harvey, National Centre for Atmospheric Science and Department of Meteorology, University of Reading
Email: b.j.harvey@reading.ac.uk
Co-supervisors: Kevin Hodges, National Centre for Atmospheric Science and Department of Meteorology, University of Reading; Alison Kay, UK Centre for Ecology and Hydrology; Christine McKenna, JBA Consulting.
Summertime precipitation extremes are increasing across much of Europe due to climate change. Two recent examples include the exceptional rainfall associated with storm Boris in September 2024, which broke some local records in Austria by over 200%, and an event in July 2021 which resulted in over 200 fatalities across Germany and Belgium. Both were caused by “cut-off low” weather systems bringing heavy, convective rainfall. Both systems, crucially, remained nearly stationary for several days allowing large rainfall totals to accumulate and widespread flooding to result.
Such cut-off low systems form when large-amplitude meanders of the North Atlantic jet stream break away from the core of the jet. Warming temperatures provide a direct thermodynamic influence on these systems (greater moisture availability enhances latent heat release, supercharging the strength of embedded convective systems), but understanding the extent to which climate change also modifies the large-scale evolution of the North Atlantic jet stream remains an important, unsolved research topic with significant consequences for assessments of future flood risk.
This project will investigate future changes in the summertime North Atlantic jet stream, cut-off low weather systems, and their associated flood impacts. The North Atlantic jet stream is the dominant driver of weather variability over the UK and NW Europe. On daily timescales, individual weather systems are steered by the jet, and on longer timescales, variations in the position and strength of the jet are highly correlated with seasonal climate anomalies. One robust feature of future climate projections is a northward shift in the mean position of the summertime North Atlantic jet, taking it away from the UK and NW Europe. Consistently, summers are projected to become dryer, on average, in these regions, as fewer weather systems pass over them. But this poses several questions related to cut-off lows and precipitation extremes: how will the projected shift of the jet modify the location and frequency of slow-moving cut-off low pressure systems? Will the weaker jet stream over the UK and NW Europe result in slower-moving cut-off lows, and therefore larger local rainfall extremes? How will this influence the occurrence and intensity of flooding events?
This project will address these questions by utilizing three major new modelling activities being undertaken by the project supervisors as part of the multi-disciplinary CANARI program (www.canari.ac.uk). The first is a 40-member single model large-ensemble experiment (LE) performed with the UK’s state-of-the-art climate model, aimed at characterizing climate risks associated with forced and internal variability over the coming decades. The second is a 300-member case study set of extreme storms, downscaled from the LE at very high convection-permitting resolution, and the third is detailed hydrological simulations driven by the outputs from 1 and 2. Together, these provide an unprecedented opportunity to examine the changing nature of the convective dynamics associated with cut-off lows, the large-scale conditions in which they form, and the resulting flood impacts.
Initial work will characterize future changes in the summertime North Atlantic jet stream in the LE (e.g. how robust is the northward shift among different members? do some members show particularly large and/or rapid shifts?) before employing feature tracking software to characterize the statistics of cut-off lows over Europe (e.g. what characteristics of cut-off lows are associated with the highest precipitation impacts? how are the geographic location, propagation speeds and associated precipitation of cut-off lows projected to change in future?). Combined, these analyses will allow a novel assessment of the links between future changes in the large-scale atmospheric circulation and changes in precipitation extremes. Subsequent work will follow the interests of the student but could explore flood risks in more detail, for example by quantifying the changing flood risks associated with cut-off lows using the hydrological model outputs, or by creating event-based storylines of how the changing characteristics of cut-off lows will impact flood risk regionally. Alternatively, the convective-scale dynamics of cut-off low weather systems could be examined, utilizing novel dynamics diagnostics in the high-resolution simulations to understand the changing interaction between latent release and the large-scale environment.
Training opportunities:
The project is CASE supported by JBA Trust (www.jbatrust.org), and will include guidance from and a placement within JBA teams working on flood risk and impacts analysis, to explore applications of the research, translating hydrological expertise learnt into flood management and climate resilience implications.
In addition, the Flood Estimation Handbook (FEH) offers guidance on rainfall and river flood frequency estimation across the UK, required for performing flood risk assessments and flood defence planning. During the project the student will, under the advisement of the FEH Team at UKCEH (Dr Adam Griffin), undertake a study of these industry-standard methods.
Student profile:
The project would be suited to a student with a degree in a physical science. Prior knowledge of atmospheric and environmental science is desirable, but not essential. Some experience with computer programming and data analysis would be beneficial.
Co-Sponsorship details:
The project will receive a CASE award from JBA Trust.