Via Vetoio
Silvio Davolio (CNR – ISAC, Bologna): “Atmospheric river in the Mediterranean and heavy precipitation over northern Italy.”
Abstract
Atmospheric rivers (ARs) are defined as long, narrow regions of enhanced horizontal moisture transport in the atmosphere, typically several thousand kilometers long and only a few hundred kilometers wide, often associated with an extratropical cyclone. They play a key role in modulating the global water cycle, accounting for most of the meridional water vapor transport outside of the tropics. When an AR makes landfall and is forced upward, water vapor is released in the form of precipitation, sometimes leading to destructive flooding. In the last decade, the impact of ARs has become evident also over Europe, where extreme precipitation have been associated with an anomalous amount of moisture flowing over the Atlantic Ocean. Only recently, the presence of ARs across the Mediterranean has been documented and connected to heavy rainfall over southern Europe and Italy in particular. The favourable large-scale environment leading to heavy precipitation events on the southern side of the Alps is well-known: an upper-level trough over the western Mediterranean basin inducing a meridional transport of warm and moist air. Additionally, the mesoscale thermodynamic characteristics of the impinging moist flow and its interaction with the orography modulate the distribution and the intensity of the rainfall. The present study shows that an intense AR, superimposed on the these dynamical mechanisms, contributes critically to turning a heavy rainfall event into a devastating flood. To this aim, a detection algorithm has been adapted to the peculiar morphology of the Mediterranean and applied to identify ARs during some of the most severe weather events affecting the Alpine region in the last decades. Moreover, some diagnostic tools, such as an algorithm for the computation of the atmospheric water budget, have been employed to compare and investigate such AR events. Finally, high-resolution numerical simulations have been exploited to disentangle how much rainfall can be attributed directly to the presence of the AR and to improve our understanding of the complex interaction between large-scale flows and mesoscale mechanisms during heavy Alpine rainfall events.