Novel methods for inferring future changes in extreme rainfall over Northern Europe

Abstract

Empirical studies show that observed frequencies nP for a daily precipitation amount P can be approximated by nP ∝ e mP , with negative values for the exponential coefficient m. The para- meter m describes the mean (μ) or any quantile for variables following such exponential distributions. The coefficient m varies from location to location, and exhibits a systematic relationship with local mean temperature and precipitation as well as other geographical parameters. A general linear model can be utilised to model μ directly from local climate conditions and geography. Estimates of m and μ from local climatic conditions allow an estimation of extreme values in the form of high per- centiles. Given changes in the mean local climate, it is possible to infer changes in the upper per- centiles. A new multi-model ensemble of the most recent climate simulations, carried out for the Intergovernmental Panel on Climate Change fourth assessment report (IPCC AR4), has been sub- jected to empirical-statistical downscaling, and provides best estimates for the continuing trends in mean temperatures and precipitation in northern Europe. These scenarios are used in conjunction with (1) the established relationship between the exponential coefficient m on the distribution func- tion, and (2) local mean temperature and precipitation for 2050, to infer changes in the 95th per- centiles of the rainfall for 2050. Two new independent analyses point towards an increase in the number of extreme precipitation events and a slight change in the number of rainy days over large parts of the Nordic countries. The projected increase was found to be sensitive to to the choice of predictors used to model the geographical dependency, rather than to the choice of method.