Abstract
AbstractDuring 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
Highlights
Autumnal events of heavy precipitation on the Alpine south side are related to the approach aloft of a narrow meridionally elongated trough with an associated co-aligned surface cold front
In both cases, the barrier wind is the dominating low-level flow feature over the Lago Maggiore (LM) area and precipitation is found preferably over the approximately eastfacing slopes. These two aspects of the simulated distribution pertain to observed distributions deduced by Houze et al (2001). Their radar-data-based climatology for two autumnal seasons over the LM area indicated that: (i) the blocking of the low layers induces a region of moderate precipitation largely extending upstream from the Alps, while in flow-over regimes the precipitation is concentrated over the windward slopes; (ii) upstream conditions for blocked regimes are well correlated with the easterly orientation of the low-level wind and in turn with precipitation located over the east-facing slopes
The indentation favours intense precipitation since: (i) the slope at the location of the wet spot is quasi-perpendicular to the moisture-laden wind; (ii) its steepness is greater than in any other settings without indentation. These simple geometric arguments are supplemented by the fact that both horizontal velocity fields in the real-topography (Fig. 12(a)) and barrier + indentation (Fig. 12(c)) experiments show a significant acceleration of the barrier wind just at the entrance of the indentation
Summary
Autumnal events of heavy precipitation on the Alpine south side are related to the approach aloft of a narrow meridionally elongated trough (sic potential-vorticity streamer) with an associated co-aligned surface cold front. The higher frequency of heavy precipitation over given regions, such as the LM area, might result from local topography-induced flow regimes that are set up for a range of large-scale upstream flow conditions. It comprises a horizontally uniform and stationary moist airflow impinging upon the topography, with the strength and direction assigned as case-dependent parameters. The flow and precipitation regimes in two MAP cases resulting from either conditionally unstable (IOP2B) or moist stable (IOP8) upstream conditions are discussed in Medina and Houze (2003) and Rotunno and Ferretti (2003) Their main conclusions converge with those inferred in the present study using wind and not stability as the case-dependent parameter. The present EM configuration suffices for our goals, including the study of flow dynamics and precipitation over meso-β-scale topographic features, and its low computational cost permits a wide exploration of flow regimes and a range of sensitivity experiments
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