STORMY-WEATHER: Plausible storm hazards in a future climate

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. The 2017 Climate Change Risk Assessment identified floods and windstorms as likely to have a strong impact on key infrastructure sectors in the UK with climate change. Extreme rainfall is becoming more intense with warming, and short-duration bursts within storms appears to be increasing at a higher rate. However, we still don't understand how changes in large scale atmospheric patterns, the storm track, the release of energy from evaporation and other factors will influence the profile of the storm in time and as well as their frequencies and how long they last for. This is partly due to the fact that most scientific studies have concentrated on 'peak intensity' changes over fixed durations, e.g. daily, multi-day, hourly, etc. Alongside this, most studies look at the likely range of change even though the most important risks rarely lie within this range. Instead, the most important risks are often associated with the 'plausible worst case' scenario.

In STORMY-WEATHER we are producing a new methodology based on different 'storm' types to understand the drivers behind the changes and to produce a set of physically-plausible high-impact storm hazard storylines and metrics that people can use to plan for the future. These will use the latest climate projections. 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 intense 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.

We will use these new models as well as global climate models more commonly used to assess the uncertainty in our projections of the future. We will consider changing temperatures as the potential driver of change to storm hazards, including precipitation and wind as joint hazards. Our storm-type approach will help clarify hazard from different rainfall mechanisms and their scaling rates with temperature, alongside combined wind and rain hazard from storms, as well as their changing nature with warming; characteristics that are vital for planning for impacts (e.g. flooding, infrastructure failure, transport and energy systems, etc.) The focus on storm properties is balanced against the need to understand the impact of potential changes to large-scale circulation patterns on storm hazards, e.g. frequency/persistence changes, and, in particular, the possibility of circulation-driven changes to the dominant event type across regions.

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