Final Report Summary - INTENSE (INTENSE: INTElligent use of climate models for adaptatioN to non-Stationary climate Extremes) INTENSE was a large funded European Research Council project “INTElligent use of climate models for adaptatioN to non-Stationary hydrological Extremes” led by Prof Hayley Fowler at Newcastle University, UK, with a number of international project partners. It provided the funded core of a community effort into the collection and analysis of sub-daily precipitation data and model outputs through the Global Energy and Water EXchanges (GEWEX) Hydroclimatology Panel cross-cut on sub-daily precipitation which Prof. Fowler leads. The project collected a global database of sub-daily precipitation data from a number of countries and from global datasets, comprising observations from >25,000 gauges. We developed methods by which to quality control the data and the codes are available as open-source. We also developed the first climatology of hourly rainfall extremes across the globe and have developed a set of indices for sub-daily precipitation globally which can be used by the community for further work, including climate model evaluation. These will become freely available to the public on a dedicated web-site. From trend analyses in the UK and US we find that trends in winter extremes emerge first in hourly precipitation for both magnitude and frequency statistics and that these can be in part linked to rising temperatures. Similar work over the Netherlands has shown that most hourly precipitation extremes are part of large-scale circulation systems, with considerable forcing from the larger scales. We have explored these large-scale drivers of hourly precipitation extremes further, by linking these to atmospheric circulation patterns over Europe, the US and Australia. We have also explored these relationships in high-resolution climate models over the UK with an analysis of large-scale precursors of small-scale storms. Results suggest that large-scale stability is skilful in predicting the occurrence of extremes in the 1.5km convection-permitting model. Missed events show some common features - they tend to have lower convective fraction, and may be related to orography or coastal convergence.The INTENSE team has also examined new high-resolution climate model simulations for the UK. A paper published in Nature Geoscience showed that high-resolution climate models show the same downturn in hourly rainfall scaling with high temperatures as seen in observations. The scaling relationships between temperature and precipitation for sub-hourly precipitation are found to be similar to hourly precipitation. Our paper in the Bulletin of the American Meteorological Society examines existing climate model projections made with convection-permitting runs for different regions and from different modelling groups and determines where their projections are different from coarse-resolution climate models, or where projections from coarse resolution models are robust. This allows us to say whether current regional climate scenarios are robust. Work on sub-hourly precipitation extremes from convection-permitting models has also explored whether the storm profile changes in a future warmer climate. The simulations find that storms become more intense and longer in duration in a warmer climate, similar to a study on radar observations also performed as part of INTENSE where storms become more intense and larger in size with warming. This corroborates work with convection-permitting models over the US but is different to profile changes of storm events identified in observations in Japan and Australia – which found intensification of the storm core but smaller size/duration of storms. Therefore, the evidence is unclear whether storm size will increase or decrease with warming; however, increases in rainfall intensity and the spatial footprint of the storm can compound to give significant increases in the total rainfall during an event.INTENSE results have gone a long way towards indicating the usefulness of temperature-scaling for projections of changes to precipitation extremes. Heavy rainfall extremes are intensifying with warming at a rate generally consistent with the increase in atmospheric moisture, for accumulation periods from hours to days. Studies have indicated the need for a moisture component in temperature-scaling and that sub-daily precipitation extremes are increasing at faster rates than would be expected with warming in parts of the world. In some regions, high-resolution modeling, observed trends and observed temperature dependencies indicate stronger increases in short-duration, sub-daily, extreme rainfall intensities, up to twice what would be expected from atmospheric moisture increases alone. We have also established at least some of the causes for this enhancement of intensity increases from local in-storm effects. This means that at a regional scale temperature-scaling of sub-daily extreme precipitation intensity increases is approximately consistent with Clausius-Clapeyron but that at local scales it is larger. It is still uncertain what this will mean for future projections of changes to precipitation intensities but evidence is emerging that sub-daily rainfall intensification is related to an intensification of flash flooding, at least locally. This will have serious implications for flash flooding on much of the planet and requires urgent climate-change adaptation measures.
