An Alternative Index for the Contribution of Precipitation on Very Wet Days to the Total Precipitation

Abstract

aily precipitation series from more than 1800 stations across Europe are analyzed for the fraction of the total precipitation due to very wet days: that is, days with precipitation amounts exceeding the 95th percentile. This fraction is calculated on a seasonal (3-monthly) basis for the period 1961?2010. A new index S95pTOT is introduced as an alternative to the frequently used index R95pTOT. Contrary to R95pTOT, which uses a fixed climatological 95th percentile, the new index assumes a separate 95th percentile for each year. Based on a Weibull distribution fit to the wet-day precipitation amounts, an analytical expression for S95pTOT is derived. It is shown that R95pTOT is strongly influenced by changes in the mean wet-day precipitation, whereas S95pTOT is more representative of changes in the distributional shape. The results for S95pTOT do not support the conclusion for a disproportional increase of extreme precipitation over northern Europe as was concluded from the trend in R95pTOT in earlier studies. Also, the contrast between trends in northern and southern Europe in winter is less pronounced for S95pTOT than for R95pTOT.