Autumnal precipitation distribution on the southern flank of the Alps: A numerical‐model study of the mechanisms

Abstract During the autumn there are frequent events of heavy precipitation on the southern slopes of the meso‐α‐scale Alpine chain. Moreover, the accompanying climatology exhibits a distinctive spatial distribution with some meso‐β‐scale regions of enhanced precipitation such as the ‘wet spot’ in the vicinity of Lago Maggiore. Possible mechanisms to account for the observed spatial distribution are established using a heuristic model‐based approach. It is shown using a mesoscale model that the response to a steady, horizontally uniform, and moisture‐laden flow impinging upon a comparatively realistic representation of the Alpine chain can reproduce both the overall meso‐α‐scale precipitation distribution and its meso‐β‐scale sub‐structure. The nature of the simulated flow regime (over, around or intermediate) is sensitive to the strength of the ambient geostrophic flow and its modification in the planetary boundary layer. Contrariwise, the simulated precipitation distribution in the Lago Maggiore ‘wet spot’ is comparatively insensitive to the changes in direction of the incident geostrophic flow from south to south‐west, and in strength in the range 10–30 m s −1 , although there is a marked change in the structure of the near‐Alpine flow. This systematic precipitation enhancement appears to be related to the relative strength and compensating effects of an easterly barrier jet tracking along the zonally elongated part of the Alps and an air stream channelled between the Maritime Alps and the Apennines. Further evidence to the above interpretation of the meso‐β‐scale precipitation distribution is given with simulations conducted using a smoothed and schematic representation of the Alpine chain. Likewise, the sensitivity to the presence of a Lago Maggiore‐like indentation of the topography is tested, and evidence is provided of a further ‘indentation‐induced’ precipitation enhancement at the local scale. Copyright © 2004 Royal Meteorological Society

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