The influence of an atmospheric river on a heavy precipitation event over the western Alps

On 2-3 October 2020, a heavy precipitation event severely affected northern Italy and in particular the western Alps, with rainfall amount exceeding 600 mm over 24 h. This event was associated with an upper-level trough over the western Mediterranean basin, a large-scale configuration typical of heavy precipitation phenomena on the southern side of the Alps, since it induces a northward transport of large amounts of moisture impinging on the orography. The present study shows that a relevant amount of moisture moved towards the Mediterranean basin in the form of an atmospheric river (AR), a long and narrow filament-shaped structure crossing the whole Atlantic Ocean, characterized in the present case by a maximum Integrated Vapour Transport exceeding 1000 kg m s. Therefore, in addition to the local contribution from the Mediterranean Sea, a relevant amount of moisture moved from the Tropics towards the Mediterranean, feeding the precipitation systems. The presence of an AR represented a distinguishing aspect of the event, superimposed on the well-known dynamic-thermodynamic mechanisms of heavy precipitation over the Alps. High-resolution numerical simulations and diagnostic tools have been exploited to investigate in detail how the transport of water vapour associated with the AR has influenced the dynamics and favoured the severity of the heavy precipitation processes. The results disclose the role of the AR and add further details to the theoretical framework of heavy precipitation mechanisms in the Alpine area, improving our understanding of the complex interaction between large-scale flows and mesoscale dynamics during extreme precipitation episodes. Due to the relatively fast evolution of the synoptic disturbance, the typical mesoscale mechanisms would have led only to an ordinary intense rainfall event. The contribution of the AR turned the event into a devastating flood.