FUTURE-STORMS: Quantifying uncertainties and identifying drivers of future changes in weather extremes from convection-permitting model ensembles

Climate change is arguably the biggest challenge facing people this century, and changes to the intensity and frequency of climatic and hydrologic extremes will have large impacts on our communities. We use climate models to tell us about what weather in the future will be like and these computer models are based on fundamental physical laws and complicated mathematical equations which necessarily simplify real processes. One of the simplifications that really seems to matter is that of deep convection (imagine the type of processes that cause a thunderstorm). However, computers are so powerful now that we are able to produce models that work on smaller and smaller scales, and recently we have developed models which we call "convection-permitting" where we stop using these simplifications of deep convection. These "convection-permitting" models are not necessarily better at simulating mean rainfall or rainfall occurrence but they are much better at simulating heavy rainfall over short time periods (less than one day) which cause flooding, in particular flash-flood events. They are also better at simulating the increase in heavy rainfall with temperature rise that we can observe; therefore we are more confident in their projections of changes in heavy rainfall for the future.

A few "convection-permitting" modelling experiments have now been run for different parts of the world but all of these have been over small regions, only the same size as the UK, or smaller. All of the experiments so far have concentrated on rainfall and none have examined how "convection-permitting" models might improve the simulation of other types of extreme weather such as hail, lightning or windstorms. In fact we know very little about how these types of extremes might change in the future. We also have no idea of the uncertainty in our experiments in terms of our predictions of future changes as we have only run one model simulation in each region - this is not useful for planning climate adaptation strategies where we really need to understand the uncertainties in our future predictions so we can plan for them.

In FUTURE-STORMS we are running these "convection-permitting" models over a very large area (the whole of Europe) and we are comparing models from two different climate modelling teams at the UK Met Office and ETH Zurich in Switzerland. In addition to this we are now able to run a number of different climate models over the same region, which allows us to assess some of the uncertainties in future changes to heavy rainfall and other storm-related extreme weather. This will let us explore how heavy rainfall might change across Europe and what might be causing this. It will also allow us to look at whether these new models are able to simulate other types of extreme weather like hail, lightning and windstorms which have a huge impact on Europe, and how these might change in the future.

Ultimately, we need better information on how extreme weather events might change in the future on which to make adaptation decisions and FUTURE-STORMS intends to provide this important advance, alongside translating this information into useful tools and metrics for use in climate change adaptation.

FUTURE-STORMS will use an ensemble of convection-permitting climate models (CPMs) to quantify, for the first time, the role of large-scale dynamics, thermodynamical moist processes, and local-scale (storm) dynamics in contributing to uncertainty in precipitation extremes and other storm-related extreme weather in Europe; thus providing more robust projections and tools with which practitioners can use CPM outputs. This will include the first examination of the simulation of hail, lightning and wind-storms by CPMs at km-scales and on climate timescales and will provide the first projections of these variables. This will lead to both improved projections of extreme weather associated with future storms and the development of new tools for European climate adaptation. FUTURE-STORMS will produce a report on predictions of change in storm-related weather extremes (extreme precipitation, hail, lightning and windstorms) for selected European cities which can be used to update national climate scenarios and give better predictions of the probability of infrastructure failure - needed for design of adaptation strategies.

The understanding of model deficiencies in the simulation of high-impact weather, particularly for local-scale convective type events will provide a key resource for the international climate modelling community. The MOHC anticipate benefits from high resolution in terms of potentially providing more accurate projections of changes in other high impact events which will inform the next generation of climate models. Knowledge transfer visits are proposed with international partners to facilitate the flow of information and ideas between researchers in order to maximise the benefits of the links developed. Through collaboration with the UK Climate Impacts Programme (UKCIP) we aim to bring together climate modelling and numerical weather forecasting communities and the end users of climate information in two joint workshops. Suggested improvements and specific outputs will be sought from end users in conjunction with the climate modelling community, focusing on those that are both viable and best meet user needs. Engaging these end users in a workshop in the first year of the project will maximise the potential for this community to contribute to the research agenda and continued communication through a website and newsletters will seek to develop an active community. The delivery of outputs over the proposed website will ensure users can derive benefit from the project outputs.

The work will benefit a wide range of bodies engaged in building adaptive capacity to the risk of increased flash-floods, such as the Environment Agency who believe that the proposed research is particularly important for sewer design and managing urban flooding, and those with responsibility for water resource planning and with obligations under the EU Floods Directive which aims to prevent and limit floods and their damaging effects on human health, the environment, infrastructure and property. The research will provide more robust projections for government departments and agencies engaged in determining flood policy and for identifying and managing flood risk across Europe.

The re(insurance) industry have a particular interest in changes in hail and windstorms. There are growing insurance claims from extreme weather events, and hail and windstorms provide the largest insurance hazard in Europe (even greater than flooding). Improved, CPM projections of future high-impact weather will enable each of these users to better plan for future changes. The re(insurance industry, through our project partner WillisTowersWatson, would use outputs from this project to validate their catastrophe model and better advise their clients on potential impacts of climate change on future extreme weather.

Ultimately, this research will enable future policies and practice to better reflect the risks associated with changes to extreme weather.