The observed evolution of sub-daily to multi-day heavy precipitation in Switzerland

Heavy precipitation events and their changes due to climate change affect many aspects of daily life in the Alpine region. In this study, we revisit the long-term (1901-2022) evolution of daily and multi-day heavy precipitation intensity and frequency, discuss trends for sub-daily to multi-day events in the recent period 1981-2022, and investigate possible elevation dependencies in the complex topography of Switzerland. Station measurements from the dense operational network of MeteoSwiss from all parts of the country and elevation levels are analyzed. We find that daily maximum precipitation and the frequency of precipitation events exceeding the 99th daily percentile have increased since 1901, with a peak in the 1980s and some decline thereafter. For the more recent period 1981-2022, positive trends in summer heavy precipitation intensity are found for short (10 minutes to 3 hours) events, but no changes are found in the frequency of these events. For longer events (one to five days), however, decreases in intensity and frequency are found, especially for the winter half-year. We hypothesize that the opposing trends on long and short time scales are caused by the superposition of thermodynamics (i.e. the main forcing of anthropogenic climate change) and internal variability of atmospheric dynamics. We also observe a small negative elevation dependence of the long-term trends up to 2300 m. For the 1981-2022 trends, no strong elevation dependence is found for sub-daily events. For daily events we find small opposing negative summer and positive winter elevation dependencies. The reason for these trends remains unclear. Our results underline the need to better investigate the interplay between climate change, internal variability of large-scale dynamics and elevation for heavy precipitation in the complex Alpine terrain. Longer observational records with high spatial and temporal resolution will help to answer this open question.