Downscaling the probability of heavy rainfall over the Nordic countries

Abstract. We used empirical-statistical downscaling to derive local statistics for 24-hr and sub-daily precipitation over the Nordic countries, based on large-scale information provided by global climate models. The local statistics included probabilities for heavy precipitation and intensity-duration-frequency curves for sub-daily rainfall. The downscaling was based on estimating key parameters defining the shape of mathematical curves describing probabilities and return-values, namely the annual wet-day frequency f_w and the wet-day mean precipitation μ. Both parameters were used as predictands representing local precipitation statistics as well as predictors representing large-scale conditions. We used multi-model ensembles of global climate model (CMIP6) simulations, calibrated on the ERA5 reanalysis, to derive local projections for future outlooks. Our analysis included an evaluation of how well the global climate models reproduced the predictors, in addition to assessing the quality of downscaled precipitation statistics. The evaluation suggested that present global climate models capture essential covariance, and there was a good match between annual wet-day frequency and wet-day mean precipitation derived from ERA5 and local rain gauges in the Nordic region. Furthermore, the ensemble downscaled results for f_w and μ were approximately normally distributed which may justify using the ensemble mean and standard deviation to describe the ensemble spread. Hence, our efforts provide a demonstration for how empirical-statistical downscaling can be used to provide practical information on heavy rainfall which subsequently may be used for impact studies. Future projections for the Nordic region indicated little increase in precipitation due to more wet days, but most of the contribution comes from increased mean intensity. The west coast of Norway had the highest probabilities of receiving more than 30 mm/day precipitation, but the strongest relative trend in this probability was projected over northern Finland. Furthermore, the highest estimates for trends in 10-year and 25-year return-values were projected over western Norway where they were high from the outset. Our results also suggested that future precipitation intensity is sensitive to future emissions whereas the wet-day frequency is less sensitive.