Author:

Keywords:

Precipitation, Climate trends

Abstract:

The temporal clustering of rainfall extremes has long been a blurred phenomenon to researchers concerned with investigating temporal variability of rainfall intensities. There is also a growing concern that in addition to the clustering of the extreme rain storms, anthropogenic forcing may exacerbate the rainfall extremes. Based on a novel technique for the identification and analysis of changes in extremes, this paper affirms that rainfall extremes in northwestern Europe have oscillatory behaviour at multidecadal time scales. Moreover, it is shown that the recent upward trend in these extremes is partly related to a positive phase of this oscillation, which coincided with the climate change influence. The analysis is based on a worldwide unique dataset of 108 years of 10 minutes rainfall intensities at Uccle (Brussels), not affected by instrumental changes or measurement inhomogeneity. We also checked the consistency of the findings with long daily rainfall records in neighbouring regions of northwestern Europe. Rainfall extremes were extracted from the series by means of independence criteria. After calculation of empirical frequencies, rainfall quantiles (intensities of given frequency) were computed based on the full series and for block periods of 5, 10 and 15 years. The past 100 years showed for Brussels and neighbouring regions higher rainfall quantiles for the 1910s-1920s, the 1960s, and more recently during both winter and summer of the past 15 years. The increase in rainfall quantiles was due to both an increase in the number of extreme rainfall events and an increase in the intensity of the extreme events. The increases were found statistically significant at the 5% confidence level, and were found to be partly explained by persistence in atmospheric circulation patterns over the North Atlantic during periods of 10 to 15 years.