Trend analysis of convective and stratiform precipitation over a Mediterranean area

Separating convective and stratiform rainfall is still today an open challenge, especially because of the difficulty in defining a clear distinction between the two rainfall regimes. At the same time, the study of convective rainfall is essential to better understand the dynamics that generate them and the proactive measures that could reduce the risk of important consequences (e.g., human life losses and economic damages). Furthermore, the shadow of a changing climate makes these aspects even more alarming, due to the possibility of an increase in the occurrence of such severe events. Sicily, whose climate could be considered as representative of the central Mediterranean area, has been often affected by high-intensity phenomena, especially during late summer-early autumn period. Starting from the 6-hour precipitation time series recorded in the period 2002-2020 by about a hundred of rain gauges homogeneously distributed over the island, an attempt to separate convective and stratiform precipitation at a monthly scale has been made. In particular, by aggregating the 6-hour events into a certain number of intensity classes for each month, it is possible to derive the total precipitation distribution which describes how the total precipitation depth tends to decrease as the intensity increases. This aspect allows to model the stratiform component as a negative exponential curve, obtaining the amount of convective and stratiform rainfall for each intensity class and, consequently, the monthly percentages of the two components. The results show that the average percentage of convective rainfall ranges from about 25-30% in the period January - April, then increasing and reaching the maximum in July – August (about 60%), and finally returning to about 25% in December, thus highlighting a seasonality of this variable. A trend analysis on the percentage of convective/stratiform rainfall detected for each month is carried out with the aim to verify if some statistically significant trends in the frequency of the convective/stratiform events have occurred over the region during the last two decades. The application of the pre-whitening Mann-Kendall trend test reveals a significant increasing in the percentage of convective rainfall, with a significance level equal to 0.1, for July and September, while an opposite pattern is detected in October.