Spatiotemporal Characteristics And Future Projections Of Extreme Precipitation Events And Population Exposure In The Yangtze River Delta Under Climate Change

Global warming has increased the occurrence of extreme precipitation events(EPEs,hereafter),which pose a significant threat to ecosystems,socio-economic security,and human health.At present,systematic studies on daily,daytime,and nighttime EPEs at the regional scale are still insufficient.The new generation of Coupled Model Intercomparison Project Phase 6(CMIP6)global climate models provide richer climatic information than CMIP5,however,the historical simulation and future prediction capability of extreme precipitation at the local scale need to be further evaluated and explored.In addition,the long-term dynamic changes of population exposure to EPEs are still unclear.The Yangtze River Delta(YRD,hereafter)is one of the most densely populated and economically developed regions in China,which is vulnerable to the adverse effects of EPEs.Based on observations,reanalysis,and downscaling model simulation datasets,this study revealed the spatiotemporal characteristics of daily,daytime,and nighttime EPEs in the YRD over the past 50 years,evaluated the historical simulation ability of the NEX-GDDP-CMIP6 extreme precipitation compared to observations,and illustrated the spatiotemporal characteristics and the occurrence probability in EPEs under 1.5,2.0,and 3.0℃warming in the future.This study also quantitatively analyzed the dynamic characteristics of population exposure to EPEs in the past and future periods.The main conclusions are as follows:(1)ERA extreme precipitation and total precipitation have a good reproducibility of the observed values in the YRD at the all-day and all-month scales,which can effectively reflect the spatial characteristics of the regional EPEs.ERA5 can accurately simulate the observed annual and seasonal changes in daily,daytime,and nighttime EPEs,but it needs to be improved in describing the long-term trends.The spatial distribution of the annual mean daily,daytime,and nighttime EPEs during 1961 to 2018 showed a significant gradient difference,with high(low)values in the south(north).Most stations showed an increasing trend in EPEs,especially in the central and eastern parts of the YRD.The extreme precipitation in the YRD is dominated by large-scale precipitation,and its ratio to convective precipitation is about 2:1.From April to October 2016(except August),the anomalously strong warm and wet southwestly water-vapor transport on the northwest side of the Western Pacific subtropical high merged with cold air,resulting in abnormally more EPEs in 2016 than any other years of the study period in the YRD.(2)The 24 NEX-GDDP-CMIP6 climate models can well simulate the spatial distribution of EPEs in the YRD during 1981-2014.Models are not effective in simulating interannual changes in EPEs,but they can better reflect the characteristics of the interannual variability,as well as the climatological amount and frequency.According to the overall evaluation,the models with the best performance in simulating extreme precipitation amount are INM-CM4-8,INM-CM5-0,EC-Earth3-Veg-LR,and Nor ESM2-LM,and in simulating extreme precipitation days are IPSL-CM6A-LR,Nor ESM2-MM,EC-Earth3,GFDL-CM4(gr1),and CMCC-CM2-SR5.The selection of optimal models is according to the extreme precipitation index of specific research.(3)Under the SSP2-4.5(SSP5-8.5)scenario,the YRD will achieve 1.5,2.0,and 3.0℃warming targets in 2033(2029),2046(2039),and 2084(2056).The annual and seasonal EPEs in the multi-model ensemble mean has a significant south-north gradient under 1.5,2.0,and 3.0℃ warming,but the value is generally higher in the southeast and northeast in summer.Compared with the historical period,the western and southern parts in winter/spring,the northern and southern parts in summer,and the southeastern parts in autumn showed large increase values in extreme precipitation under each warming target.EPEs increased with the rise of warming targets,and the increase in winter was larger than that in summer.Under the SSP2-4.5(SSP5-8.5)scenario during2015-2100,extreme precipitation amount and days increased significantly at the rates of 11.02(18.2)mm/decade and 0.42(0.59)days/decade,respectively.The extreme precipitation amount and days in the far future will increase more than that in the nearfuture compared to the historical period.The extreme precipitation during 2081-2100will have an amplification effect compared with the current level,which will show an increasing variability.The extreme precipitation amount in the return periods of 50 and100 years in the historical periods are more common during 2067-2100,and the corresponding return periods will shorten to 9-21 and 15-45 years,respectively.The southeast has a higher concentration of EPEs and a higher risk of flooding.The cumulative density function curve of extreme precipitation will shift to the right significantly from 2081-2100,and extreme precipitation will enter a new normal.(4)The YRD has experienced rapid urbanization and population growth during 1984-2018.The population exposure to EPEs in the historical period was higher in the eastern coastal cities,provincial capitals,northern,and western parts of the YRD.The population exposure to EPEs increased the fastest in the middle-eastern and southeastern parts.Population exposure to extreme precipitation days(amount)increased by 37%(40%),34%(39%),and 41%(41%),respectively,during 1984-1993to 2009-2018.The increase of population dominated the increase of population exposure during 1984-2018.In the future,the estimated number of total(urban)population will increase first and then decrease over time,and the peak will appear in2030(2040),whereas the rural population will continually decrease over time.Under SSP2-4.5 and SSP5-8.5 scenarios,the spatial distribution characteristics of population exposure to EPEs showed a trend of first increase and then decrease over time.Under the two scenarios,the increase of total population exposure to EPEs in the YRD(except Shanghai)during 2000-2050 is dominated by climate effects,while the increase of urban population exposure and the decrease of rural population exposure are both dominated by population effects.During 2050-2100,the decrease in population exposure will be dominated by the decrease in population(negative contribution).The results can provide important scientific information for the YRD to adapt to and mitigate the adverse effects of EPEs under climate change and strengthen climate resilience.It can also provide reference for other regions with similar characteristics to the YRD,which has important social and practical implications.