Trends in the daily intensity of precipitation in Italy from 1951 to 1996

An analysis of 67 sites of daily precipitation records over the 1951–1996 period for Italy is presented. Seasonal and yearly total precipitation (TP), number of wet days (WDs) and precipitation intensity (PI) are investigated, and the trends both for the single station records, and for some different area average series are studied. PI is analysed by attributing precipitation to ten class-intervals, removing the influence of variations in the number of WDs to yield changes in the underlying shape of the WD amount distribution. The results show that the trend for the number of WDs in the year is significantly negative throughout Italy, stronger in the north than in the south: this trend is mainly a result of the winter. Moreover, they show that there is a tendency toward an increase in PI. This increase is globally less strong and significant than the decrease in the number of WDs. It is not concentrated in one specific season, but changes from area to area, and is generally weak in winter, due to a significant decrease of winter TP. In northern Italy, the increase in PI is mainly owing to a strong increase in the heaviest events, while in central–southern Italy, it depends on a larger part of the distribution of WD amounts. The analysis of the evolution of the class-interval contributions shows that the positive trend of the heaviest events starts in the 1970s, as does the negative trend of lightest events.