Mean and extreme precipitation regime in North and Central Italy – between stability and change

The work was carried out on the gridded precipitation dataset of the ARCIS consortium (hereafter ARCIS) that was produced spatializing a daily dataset of 1762 stations coming from 7 regions of Northern Italy and three regions of Central Italy for the period 1961-2023, with a pixel of 4.7 x 4.7 km. The mean yearly precipitation value from 1961 to 2023 shows a large interannual variability. On the other hand, the trend in frequency and intensity of extreme events (daily events above 50 mm) exhibit high space and time variability. Highlighting areas with significant or not significant trends is crucial for designing hydraulic infrastructures and managing the impacts of natural risks such as floods and landslides. Overall, results reveal that 83% and 86% of the areas do not show significant increases respectively in frequency and intensity while 11% and 15% show significant increases and 2% and 3% show significant decreases. Moreover, the spatial pattern of the areas affected by increasing frequency and intensity, highlights the presence of some regions characterized by increasing trend in North Italy at the beta mesoscale (20-200 km) which is the theatre of phenomena like mesocyclones or low-level jets. Finally, it should be noted that the 1991-2023 analysis of seasonal variations highlights a frequency increase of extreme phenomena in autumn (from October to December with a more marked increase in November), a weak decrease from January to April and a marked decrease in August. These variations can be interpreted as the result of the interaction of the synoptic circulation with mesoscale effects triggered by orography and sea surface temperature.