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Impact of Climate Change on Precipitation Extremes and Pluvial Flooding in Europe

Extreme precipitation and flood are among the major climate-related disasters which cause thousands of fatalities and billions of euros in damages each year. The risk of these events is expected to increase remarkably under changing climatic and socioeconomic conditions, increasing pressure on vulnerable societies and ecosystems. Yet, the impact of the changing conditions on sub-daily extreme precipitation and pluvial flooding and their related risk has not been well studied due primarily to the scarcity of sub-daily precipitation observations and model simulations, a limited availability of detailed and consistent data on future vulnerability components and the computationally expensive continental flood modeling. This PhD dissertation therefore explores the impact of future climatic and socioeconomic changes on sub-daily extreme precipitation, intensity-duration-frequency (IDF) curves and pluvial flooding over Europe.

First, the impact of future anthropogenic climate change on 3-hourly extreme precipitation with return periods ranging between 5 and 50 years over Europe is investigated using the simulations from regional climate models (RCMs) with 0.11° resolution from the Coordinate Downscaling Experiment over Europe (EURO-CORDEX) ensemble. The robustness of the signals is examined based on a regionalized signal-to-noise (S2N) technique by taking into account a spatial dependence incorporated spatial pooling (regionalization) to decrease the noise due to internal variability and to achieve improved statistics. The effectiveness of the spatial pooling  is evaluated by a sensitivity analysis to precipitation times scale, precipitation intensity, season, climate model resolution and greenhouse gas concentration. The results indicate an intensification of 3-hourly extreme precipitation over Europe at the end of the 21st century for all seasons except summer for which a bipolar pattern (increase in the north and decrease in the south) is discerned. For the non-mitigation scenario of Representative Concentration Pathway (RCP) 8.5, the regionalized winter 3-hourly extreme precipitation changes over 9×9 model grid cells are statistically significant in roughly 72%, 65%, 59% and 48% of the European area for 5-, 15-, 25- and 50-year return periods respectively, whereas 16-21% of the area will experience significant changes in summer. The S2N values for 3-hourly extreme precipitation changes rise after the spatial pooling by about a factor of 1.4-1.7 for all seasons except summer when they decline by about a factor of 0.78. The results of sensitivity analysis reveal that the regionalization influence is sensitive - in order of decreasing importance - to season, precipitation time scale, precipitation intensity, future greenhouse gas concentration scenario and climate model spatial resolution. Whenever and wherever short-duration convective precipitation is dominant, the precipitation time scale is found to be more important, which is the case seasonally for summer and regionally for south Europe.

Subsequently, the impact of future anthropogenic climate change on precipitation IDF relationships is examined. As a pilot study, future IDF curves are firstly developed for Belgium by using large ensembles of global climate models (GCMs) and RCMs. Building upon the findings from the case study in Belgium, the methodology is extended for Europe and the future IDF curves are developed considering duration ranging from 30 minutes to 24 hours and return periods ranging between 1 and 100 years. The future continental-scale IDF curves are developed in the framework of the quantile perturbation downscaling by applying climate change signals from the EURO-CORDEX RCMs on current IDF curves obtained from high spatial and temporal resolution remote sensing based precipitation products. The results reveal that under climate change, future IDF curves will be uplifted (16-27%, depending on duration and return period) and steepened (17-25%, depending on return period), with a larger degree under RCP8.5 compared to RCP4.5. The upward shift in IDF curves is caused by the intensification of rainstorms under climate change, while a larger increase for shorter duration rainstorms leads to a steepening of the curves. In addition to the intensity of rainstorms, their frequency is also projected to increase such that the frequency of 50-year and 100-year events will be doubled under RCP4.5 and tripled under RCP8.5.

Furthermore, this PhD dissertation analyzes the impact of future climatic and socioeconomic changes on pluvial flooding over Europe. To this end, pluvial flooding is quantified using a satellite-based data driven approach for 20-, 30-, 50- and 100-year return periods. To elucidate the impact of changes in both climatic and socioeconomic conditions on floods at the continental, regional and national levels, the Shared Socioeconomic Pathways (SSPs) are merged with RCPs, integrating hazard and several social, economic and agricultural exposure-vulnerability proxy indicators. The results reveal that future flood hazard increases for different return periods and scenarios by the end of this century. A ubiquitous drastic increase up to 87% is also projected for future flood risks of different return periods over Europe, with eastern and southern regions experiencing the highest risk increase. A fossil-fuel based development (SSP5 combined with RCP8.5) in the future would lead to 14-15% higher flood risk compared to a sustainable development (SSP1 combined with RCP4.5), which goes up to 23% in north Europe. The amplified future flood risk is predominantly driven by climate change, although with a large uncertainty, rather than socioeconomic drivers. In almost all the European countries, the changes in flood hazard are far larger than the exposure-vulnerability changes.

As the knowledge on the uncertainty of future projections is indispensable for future planning in the face of climatic and socioeconomic changes, special attention is paid to this matter throughout this dissertation. The uncertainty is quantified in the future projections of different variables including extreme precipitation, IDF curves and flood risk. The uncertainty analysis shows GCM uncertainty as the dominant source in extreme precipitation projections for Belgium for all return periods and durations compared to GCM initial conditions and RCPs. By taking an ensemble of the EURO-CORDEX RCMs, the uncertainty associated with the choice of RCMs is found to be larger than the other uncertainty components in the projection of more extreme precipitation. Similar to the Belgian case study, the uncertainty in the projections of extreme precipitation and IDF curves at the European scale is larger for longer return periods. The uncertainty is, with a lesser extent, dependent on duration, being larger for shorter durations. The uncertainty assessment in the projections of European pluvial flood risk indicates a spatially and temporally varying magnitude of total and fractional uncertainties over the continent, with climate model uncertainty as the dominant source.

Date:29 Apr 2016  →  9 Sep 2020
Keywords:Climate change, Precipitation extremes, Urban pluvial flooding
Disciplines:Structural engineering, Other civil and building engineering
Project type:PhD project