Definitions and Indices of Precipitation Extremes

Abstract

Definitions of extreme climatological and meteorological events may depend on the objective of the study, the type of data, or the researcher’s subjective opinion. This chapter discusses definitions of extreme climatic events and presents indices of precipitation extremes found in the literature. The spatial and seasonal variability in a range of these indices were analysed, and the results were used to select criteria for identifying extreme precipitation in Europe adopted in this study. These criteria were then applied in subsequent chapters. During the study period, the maximum daily precipitation totals in Europe ranged from 37.6 to 520 mm. An overwhelming majority of stations (89 %) recorded daily precipitation greater than 50 mm less than once per year. Because there is great variability between precipitation regimens in Europe, this study adopted a statistical definition of extreme precipitation events. The events were identified separately for each of the weather station and in each of the months using an empirical distribution of the daily precipitation totals. Precipitation totals exceeding or equal to the 95th percentiles of daily precipitation were selected as extreme. The 95th percentile (95P) was calculated from days with daily totals ≥1 mm during the period 1961–1990. The resulting spatial and seasonal variability of the threshold values corresponding to the 95P daily precipitation across Europe is similar to the spatial and seasons variability of the monthly totals.