Warming-induced increase in liquid fraction amplifies sub-daily rainfall extremes in the Greater Alpine Region

Rising temperatures are increasing the liquid fraction of precipitation in mountainous regions. This change, added to other changes in dynamic and thermodynamic processes generating heavy precipitation, could determine a potential intensification of the flood regime, posing increasing hazards to the population. In this study we aim at quantifying the projected change in liquid sub-daily precipitation extremes in the Greater Alpine Region. We use an ensemble of convection-permitting climate models (CPM) provided by the CORDEX-FPS Convection project at 1 hour temporal resolution and remapped to 3 km spatial resolution, covering historical (1996-2005) and far future (2090-2099) time periods under the RCP8.5 scenario. Total precipitation extremes are estimated from the total precipitation time series by identifying the independent storms, extracting the ordinary events and using the Simplified Metastatistical Extreme Value (SMEV) approach. Temperature is then used to separate the liquid and solid fraction of the identified storms, and the liquid and solid precipitation extreme quantiles are estimated. The results for the historical period are validated using station-based statistics of liquid precipitation in the Eastern Italian Alps. Comparing future changes obtained for total, liquid and solid precipitation, our study shows a strong elevation-dependent signal of liquid precipitation extremes amplification over the domain across the entire range of precipitation severity, which is predominant at daily durations. On the contrary, at hourly duration no statistically significant signal of liquid precipitation amplification could be extracted. Advancing earlier results by Dallan et al. (2024), this study highlights that the changes in liquid precipitation are enhanced more at daily duration, typically affected by dynamic factors and processes, than at hourly duration, for which thermodynamics plays a major role. Obtaining robust estimates of these changes is crucial for better managing water resources and designing adaptation strategies. This is particularly important for infrastructures such as dams, which are often located at high elevation and so are strongly impacted by changes in the liquid-solid phase separation.