Detection of atmospheric rivers affecting the Mediterranean and producing extreme rainfall over northern-central Italy

Abstract. Atmospheric rivers (ARs) have been recently identified also in the Mediterranean basin, and they have been shown to play an important role in intense precipitation events over northern Italy and the Alpine chain. In fact, as demonstrated by two recent severe events (27-29 October 2018; 02-03 October 2020), the synoptic pattern conducive to heavy rainfall in this area may favour an intense moisture transport from remote regions towards the Alps. In these events there was either a south-westerly moisture advection directly from the Atlantic Ocean Tropical area across the African continent, or a north-westerly transport over the Atlantic area, entering the Mediterranean in correspondence of the Gibraltar Strait.

In order to identify ARs in such a complex geographical area, a well-known algorithm of objective detection has been modified to take into account the peculiar morphology of the Mediterranean basin and, consequently, the peculiar shape of the organized water vapour transport, which may differ from that generally observed in the ARs over the open ocean. The two above-mentioned case studies have been used for testing the procedure.

Lastly, the algorithm has been applied in conjunction with some additional selection criteria for the identification only of the AR events that affected northern-central Italy in the last about 60-year. A climatological analysis is provided and the possible correspondence between the most intense identified ARs and extreme rainfall events is investigated. Exploiting a precipitation dataset for northern-central Italy (ArCIS), some areas turned out to be particularly exposed to extreme precipitation events in the presence of ARs.