Future changes in European windstorm severities and impacts Jennifer L Catto Alex Little* Matthew Priestley University of Exeter *Now at JBA Risk. EGU 2021 Session AS1.6 - Fri, 30 Apr, 13:30–15:00 (CEST) Introduction Motivation Research Questions • Future climate changes will be felt through changes in the weather systems. • How will the severity of European windstorms change in the future? • In Europe one of the most important weather systems is extratropical cyclones. • How will the impacts of European • There are currently a lot of uncertainties windstorms change in the future? around how the frequency and intensity of extratropical cyclones will change over • What will be the role of adaptation to Europe, associated with competing storm severity for decreasing the impacts? dynamical effects, and global climate model uncertainties. • How will future population changes • Another aspect of uncertainty comes from influence the impacts of European the different ways in which intensity is windstorms? defined. • Using models from the latest suite of CMIP, and applying a storm severity index, we investigate future changes in characteristics of windstorms over Europe. December 2020 Alex Little II. POPULATION DENSITY PROJECTIONS Methods and Data December 2020 Alex Little H SSP5 Lagrangian Feature Tracking - Using TRACK (HodgesII. PopulationPOPULATION data DENSITY1980 – 2010 PROJECTIONS 1994,1995) applied to 6-hourly 850hPa relative SSP2 Population data are vorticity (truncated to T42 resolution) from ERA5 taken from the (1980-2010) and 8 CMIP6 models. Socioeconomic data Models and applications ACCESS-CM2 MIROC6 center (SEDAC) at https://sedac.ciesin.co BCC-CSM2-MR MPI-ESM1.2-HR H lumbia.edu/data/set/pSSP5 SSP2 1980 – 2010 EC-Earth3 MPI-ESM1.2-LR opdynamics-1-8th- pop-base-year- KIOST-ESM MRI-ESM2-0 F1 F1 2040 – 2070 2040 – 2070projection-ssp-2000- Present day: Historical simulations for 1980-2010. 2100-rev01 Future: SSP2-4.5 and SSP5-8.5 simulations for 2070- 2100. Adaptation: To consider the impact of adaptation, we considered two cases (1) where the wind F1 F1 thresholds from the historical period are applied2040 – 2070 to 2040 – 2070 the future simulations (no adaptation), and (2) where the wind thresholds from the future F2 F2 simulations are used (adaptation). 2070 – 2100 2070 – 2100 F2 F2 2070 – 2100 2070 – 2100 Methods – storm footprints • Along the identified track, maximum 10-m • Two storm severity indices are calculated: cyclonic wind speeds are calculated and • Met-SSI, which considers only the attributed to the cyclone if they are within 5 severity of the storm, degrees of the cyclone centre, within 12 • Soc-SSI, which includes information hours of the present time and above the 98th about population density. percentile threshold. • The maximum associated winds are retained for each point along the track to give a maximum cyclonic wind speed footprint (Figure 1a). • The Storm severity indices and regions of interest • To evaluate the impact on different regions, we have considered 3 areas of Europe – NW, NE, and MED, shown below. NW Europe NE Europe S Europe Model evaluation – winds and storm tracks (Top) This shows the maximum wind speed associated with the retained cyclones. Models overestimate the maximum wind speeds over large parts of the continent, particularly in the south and the north of the domain. (Bottom) This shows the track density in cyclones per month per 5 degree spherical cap for ERA5 and the difference between CMIP6 and ERA5. The zonal bias of the storm tracks that is widely known (e.g. Priestley et al 2020) shows up in this 8-member ensemble as an overestimation of the track density over central and eastern Europe, and a slight underestimation to the north and south. Model evaluation - SSI (Top) This shows the Met-SSI for ERA5 and the multi-model mean bias. Largest Met-SSI values are seen over the north-west of the continent, where track density and maximum wind speeds are high. The model overestimates the Met-SSI over the south and over Scandinavia, and underestimates it over the UK and central Europe. This broadly corresponds to biases in the storm tracks. (Bottom) This shows the Soc-SSI. The highest values occur over densely populated regions with high Met-SSI (e.g. England, north of France, Belgium, the Netherlands and Germany. Overall the models do well, but underestimate the values in these regions by around 50%. Future changes in frequency of winter cyclones • In SSP2-4.5 the track density increases over the UK and over the continent around 50N up to 0.8 cyclones per season. There are decreases to the north and south. • In SSP5-8.5 there are larger increases over the UK and over Scandinavia, and larger decreases over the Mediterranean. • Overall there are decreases over the whole region (see bar chart below). Track density changes (tracks per month per 5 degree spherical cap) for SSP2-4.5 and SSP5-8.5. Changes to Met-SSI (Top) In the no-adaptation case (using the wind speed threshold from the historical period), there is an increase in the Met-SSI over northwest Europe in both SSP2 and SSP5, which is stronger in SSP5. In SSP5 the increase is also large over parts of Scandinavia and further east. (Bottom) When windspeed thresholds from the future are used, this acts to reduce the increase in Met-SSI. Changes to Soc-SSI (Top) There are increases in Soc- SSI over the NW Europe region, mainly associated with the large population centres. Although Met- SSI decreases over S Europe, there are barely any decreases in Soc- SSI. (Bottom) Considering adaptation reduces the increase. Summary of changes over different regions • Wind speeds in the future are expected to increase over large parts of Europe, associated with high track density. • When historical wind speed thresholds are used to determine future storm severity index, there are increases in Met-SSI and Soc-SSI over all regions apart from S Europe. • When we consider “adaptation”, i.e. assuming infrastructure is designed to withstand the future wind speeds, there is a decrease in Met-SSI across Europe. However, Soc-SSI still increases (except in S Europe) due to increased population density. Take home message Over large parts of Europe the frequency of storms is likely to increase in the future, leading to higher wind speeds and higher Met-SSI. Increasing population means the Soc-SSI also increases. By adapting to the increasing wind speeds, the impact of the storms can be reduced, but there will still be increased storm impacts (losses) in the future. 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