Enhanced Summer Convection Explains Observed Trends in Extreme Subdaily Precipitation in the Eastern Italian Alps

Understanding past changes in precipitation extremes could help us predict their future dynamics. We present a novel approach for analyzing trends in extremes and attributing them to changes in the local precipitation regime. The approach relies on the separation between intensity and occurrence of storms. We examine the relevant case of the Eastern Italian Alps, where significant trends in extreme precipitation were reported. The model is able to reproduce the observed trends at all durations between 15 min and 24 hr, and allows us to quantify trends in extreme return levels. Despite the significant increase in storm occurrence and typical intensity, the observed trends can be only explained considering changes in the tail heaviness of the intensity distribution, that is the proportion between heavy and mild events. Our results suggest that the observed changes are caused by an increased proportion of summer convective storms. Plain Language Summary Quantifying past trends in extreme rainfall is important because it can help us understand future changes caused by global warming. Climate scientists and hydrologists use specific statistical models to do so, but interpreting the results is complicated because extremes are rare and the structure of the models is not linked to the local meteorology. We use a new statistical model that allows to better understand the mechanisms behind the trends we detect. We find that rainfall extremes in the Eastern Italian Alps increased over the past decades and we associate this change to an increased proportion of summer thunderstorms. Key Points We present a method for analyzing extreme precipitation trends based on the separation of storm intensity and occurrence frequency Our approach reproduces observed trends in annual maxima and allows to quantify trends on rare return levels Observed trends in the Eastern Italian Alps are explained by an increased proportion of heavy convective storms in the summer