Multifractal analysis of hourly precipitation

The multifractal nature of the rainfall field is analysed using the methodo logy of singular measures. The analysis is applied to a long time series (5 4 years) of hourly rainfall intensities recorded at Valentia on the South-W est Coast of Ireland. The empirical probability distribution function sugge sts a hyperbolic intermittency with the divergence of the statistical momen ts being higher than the second order. The latter is in agreement with find ings of other authors for similar climatic regions (e.g. Sweden). The Fouri er transform statistics of the data are used to obtain the scaling range in which the data obey a power law with a coefficient of similar to 0.5. The scale invariance as identified by the spectral power law, ranges from 2 hou rs to about 24 hours. This is a narrower range than has been found for simi lar studies using continental sites where the range was found to be from 2 hours to about 3 days. Studies of Valentia rainfall using conventional stat istics suggest that two distinct periods, (1940-1975 and 1976-1993) are cle arly present. The second period is characterised by a greater annual rainfa ll depth than the first, and the increased depth was found to be concentrat ed primarily in the months of March and October. The intermittency analysis of the rain field of the two periods reveals two different K(q)-functions. The curvature of the K(q) convexity has been found to be larger for the se cond period suggesting lower intermittency or more frequent rain events. Th e intermittency function CI for the period 1976-1993 is shown to be quantit atively less than C-1 for the period 1940-1975, for die annual, March and O ctober time series verifying increasing precipitation since 1975. (C) 1999 Elsevier Science Ltd. All rights reserved.