Most heavy-rain events over the southside Alps occur in automn, when the first large cyclonic systems over western Europe drive humid and unstable air-masses from the warm Mediterranean towards the orography. The precipitation over the upwind slopes can locally reach dramatically high amounts when these conditions remain steady (24h or more), as confirmed by measurements collected during the MAP Special Observing Period.

In order to study the precipitation distributions obtained over the Alps with such large-scale conditions, mesoscale numerical simulations are performed with an idealized stationary background flow with a dominant southerly component, the complex orography of the Alpine area being kept. The prescribed vertical structure of the atmosphere far upstream has a conditionnally unstable boundary layer characterized by high values of both CAPE and CIN.

The simulated rain distributions obtained in stationary regime with a uniform upstream flow are in qualitative agreement with the heavy-precipitation climatology in October over the Alps. This indicates that the slopes with a maximum of precipitation also correspond to a maximum of orographic lifting of the incident flow. In particular, the Lago Maggiore forms a low area with a diameter of approximate 100km surrounded by high massives and embedded within the global concavity of the Alpine arc. There, in addition to the orographically induced convergence of the flow at the scale of the entire chain, the particular configuration of the topography at the local scale forces the horizontal convergence of the wind towards the Gotthard pass. The weak sensitivity of the precipitation to the wind direction contributes further to the climatological maximum observed at that place.

The configuration of an incident southerly wind-jet impinging on the Alps is also investigated. The jet has a longitudinal width of 300km and mimics a low-level jet ahead of a cold front. The flow regime over or around the Alps thus varies locally with longitude, due to the varying wind-speed. The convective activity over the Laggo Maggiore area is found to remain remain strong independently of the meridian position of the jet (whereas it is much more variable for otherAlpine areas).