Observed increase in extreme daily rainfall in the French Mediterranean
We examine long-term trends in the historical record of extreme precipitation events occurring over the French Mediterranean area. Extreme events are considered in terms of their intensity, frequency, extent and precipitated volume. Changes in intensity are analysed via an original statistical approach where the annual maximum rainfall amounts observed at each measurement station are aggregated into a univariate time-series according to their dependence. The mean intensity increase is significant and estimated at + 22% (+ 7 to + 39% at the 90% confidence level) over the 1961–2015 period. Given the observed warming over the considered area, this increase is consistent with a rate of about one to three times that implied by the Clausius–Clapeyron relationship. Changes in frequency and other spatial features are investigated through a Generalised Linear Model. Changes in frequency for events exceeding high thresholds (about 200 mm in 1 day) are found to be significant, typically near a doubling of the frequency, but with large uncertainties in this change ratio. The area affected by severe events and the water volume precipitated during those events also exhibit significant trends, with an increase by a factor of about 4 for a 200 mm threshold, again with large uncertainties. All diagnoses consistently point toward an intensification of the most extreme events over the last decades. We argue that it is difficult to explain the diagnosed trends without invoking the human influence on climate.
KeywordsPrecipitation Extreme events Climate change Detection and attribution Mediterranean
This work is dedicated to the memory of Valrie Jacq, who greatly contributed to the monitoring and statistical analysis of extreme precipitation events such as those studied in this paper. She also initiated the creation of several observed datasets used in this study. The authors thank Météo France for providing observed datasets. The authors gratefully acknowledge assistance from the French Ministère de l’environnement, de l’énergie et de la mer under the Extremoscope grant, which supported investigations of possible changes in extreme events. Part of this work has been supported by the Fondation STAE (through project Chavanada). This work is also a contribution to the MISTRALS/HyMeX programme.
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