Dynamical constraints on the future of extratropical precipitation: atmospheric rivers and extratropical storms (DyARES)

Most precipitation in the middle latitudes comes from extratropical cyclones - low pressure systems with fronts which regularly bring precipitation to the UK and elsewhere. Many large-scale precipitation extremes, high winds and floods are also associated with these events such that they have a major impact on society. Where these cyclones have long, trailing plumes of moisture connecting them to the tropics ('atmospheric rivers') the precipitation can be particularly intense. However, understanding the behaviour and controls on atmospheric rivers remains an emergent field.

This proposal will support a collaboration between the University of Exeter, where there is considerable expertise in climate dynamics, including understanding the behaviour of extratropical cyclones, and Scripps Institution of Oceanography, where work at the forefront of understanding atmospheric rivers is being undertaken. This collaboration will provide access to proprietary atmospheric river datasets held at Scripps and an opportunity to work with researchers to understand and utilise those data. In particular, the influence of large-scale patterns of climate variability and their role in determining the behaviour of atmospheric rivers will be investigated. This is an avenue of research which is yet to be exploited but will be key to determining the response of atmospheric rivers (and their impacts) to a changing climate.

This work adds significant value to an on-going NERC funded large-grant led at Exeter ("Robust Spatial Projections of Real-World Climate Change"). That project specifically investigates the physical processes and phenomena associated with future climate change and the present proposal will enhance our understanding of a physical phenomena - atmospheric rivers - which impact extratropical precipitation extremes and how they interact with both the large-scale and with extratropical cyclones. The work envisaged in this proposal will also directly inform a larger grant to be written during the course of the project, providing a platform for long term collaboration between the two groups and ensuring considerable additional value is added to this initial funding.

The immediate beneficiaries, other than those within the research team itself, will be the wider academic community who are interested in the processes governing the behaviour of the climate in the extratropics, particularly precipitation and wind extremes. The direct beneficiaries extend to those dealing with government and industry, in particular insurers, since it is the extreme events which cause most of the significant societal impacts. Through combining the data and knowledge of the two institutions, a powerful repository of data that allows a deeper understanding of the physical processes governing the regional response to a changing climate will be obtained, and this data and knowledge will have wider utility in the UK academic community.

The Exeter collaborators already work with colleagues at the Met Office on a number of projects via the Met Office Academic Partnership, and PI Collins holds a joint chair between the two institutions, allowing the findings of this work to have direct influence on the next generation of climate models being developed in the UK. Improving our ability to represent key physical phenomena in numerical models enhances our ability to make accurate predictions of the future on both short and long timescales, leading to broad, indirect societal benefits.

During the project, the work will be exposed at the 2018 International Atmospheric Rivers Conference and a high impact paper will be produced with the initial findings from the collaboration. This work will feed into a larger grant proposal, assuring that the identified benefits discussed here are achieved through a longer collaborative relationship between the partners. With new reanalyses (e.g. ERA-20C) and the forthcoming CMIP6 data becoming available in the near future, the application of the techniques developed here on these new data will be invaluable for constraining confidence in the future weather and climate in the extratropics.

Given the major impact these events can have on precipitation, wind storms and flooding, their impacts on society are profound. This proposal and longer term collaboration will yield an enhanced understanding of the behaviour of atmospheric rivers and extratropical cyclones and the ability of state-of-the-art models to represent them and their impacts. This will give capacity to provide predictions of the future of large-scale extreme precipitation events in the extratropics which are physically based, increasing confidence in their accuracy when compared to the statistical approaches frequently used. Existing approaches often rely on the mean and spread of model simulations without reference to the ability of each model to represent the underlying phenomena and processes which control those metrics. With an enhanced understanding of these processes, an ability to discern the plausibility of projections from different models will be gained. In doing so, the information available to policymakers who shape adaptation planning will be enhanced, allowing better informed decision making when shaping the policies that will influence our ability to cope with changing extremes in a warming climate. This activity will specifically contribute to improving evidence based policy-making, managing environmental impacts and enhancing the research capacity and skills of private and public bodies through the availability of more useful and accurate data.