Abstract
Abstract. The successive occurrence of extreme precipitation events on sub-seasonal timescales can lead to large precipitation accumulations and extreme river discharge. In this study, we analyze the sub-seasonal clustering of precipitation extremes in Switzerland and its link to the occurrence and duration of extreme river discharge. We take a statistical approach based on Ripley's K function to characterize the significance of the clustering for each season separately. Temporal clustering of precipitation extremes exhibits a distinct spatiotemporal pattern. It occurs primarily on the northern side of the Alps in winter and on their southern side in fall. Cluster periods notably account for 10 %–16 % of seasonal precipitation in these two regions. The occurrence of a cluster of precipitation extremes generally increases the likelihood and duration of high-discharge events compared to non-clustered precipitation extremes, particularly at low elevations. It is less true in winter, when the magnitude of precipitation extremes is generally lower and much of the precipitation falls as snow. In fall, however, temporal clusters associated with large precipitation accumulations over the southern Alps are found to be almost systematically followed by extreme discharge.
Highlights
Switzerland’s climate, topography and high population density make floods one of the major natural disasters, accounting for instance for 71% of weather-related insurance claims over the 1973–2011 period (Swiss Re, 2012) and 36 % of total damages to buildings between 1995 and 2014 (BAFU, 2016)
Significant temporal clustering of precipitation extremes is mainly found in central Switzerland, along the Alpine ridge, at the 15–25 and 25–35 d timescales (Figs. 4a and A2b)
Results for the summer season show a complete absence of temporal clustering significance at all timescales (Figs. 4c and A2e, f)
Summary
Switzerland’s climate, topography and high population density make floods one of the major natural disasters, accounting for instance for 71% of weather-related insurance claims over the 1973–2011 period (Swiss Re, 2012) and 36 % of total damages to buildings between 1995 and 2014 (BAFU, 2016). Large precipitation accumulations may sometimes result from the occurrence of several extreme precipitation events in close succession. In contrast to persistent but moderate wet conditions, temporal clusters of extreme precipitation events involve more than 1 d of extreme precipitation. Such events can lead to extreme river discharge, flash flooding (Doswell et al, 1996) or mass movement, especially in urban and mountain areas (Guzzetti et al, 2007; Panziera et al, 2016). Clusters of extremes tend to be missing from risk models, which often rely on assumptions of independence in the timing of extreme precipitation occurrence (Priestley et al, 2018)
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