Non-stationary frequency analysis of long-term convection permitting simulations reveals sub-daily extreme precipitation changes in central-southern Europe

Assessing the impact of climate change on extreme precipitation is a critical task for adapting flood mitigation strategies. Nowadays, Convection Permitting Models (CPM) are the state-of-the-art tool to simulate current and future extreme precipitation, as they explicitly represent convective processes, outperforming climate models with coarser resolutions. Nevertheless, CPM are computationally demanding and typically limited to decadal periods, making them potentially more affected by internal variability. Here, we exploit for the first time long-term CPM simulations over Central-Southern Europe to develop a non-stationary analysis of extreme precipitation. We use VHR-PRO_IT (Very High-Resolution PROjections over Italy), recent CPM projections with high spatiotemporal resolution (2.2 km and 1 h) and simulations spanning 90 years in two emission scenarios. We apply a non-asymptotic statistical approach (SMEV) with a non-stationary implementation using time as a covariate. This approach allows us to capture the transient change in precipitation extremes and in their distribution parameters, describing trends beyond the natural variability, which could affect short (decadal) simulations. We find a general increase of sub-daily extreme precipitation in a warmer climate, especially in the Mediterranean areas. Larger areas with statistically significant changes are found at shorter durations. Higher increases are expected with higher return periods (median value of about 3–4 % / decade at 100 yr return period), especially for longer durations. This can be explained by the increasing distribution tail heaviness found at those durations. Average expected changes in extreme precipitation are similar in the two scenarios, but larger non-stationary areas are expected in the RCP8.5.