Tripple Doppler radar analysis of the heavy rain event observed in Lago Maggiore region during MAP IOP 2B
Résumé
This paper presents analyses of data collected with three ground-based Doppler radars during the Mesoscale Alpine Programme (MAP) Intensive Observation Period (IOP) 2b from 1900 UTC 19 September until 1100 UTC 20 September 1999. During this period, the synoptic situation was characterized by the propagation of a deep upper-tropospheric trough towards the Alps, as often observed when heavy precipitation occurs on the southern slopes of the Alpine massif. A frontal cloud system with embedded convective cells passed over northern Italy in association with the trough moving rapidly eastwards. Ahead of the advancing cold front, a strong south to south-easterly low-level flow impinged on the mountains near Lago Maggiore and produced copious amounts of rain (>200 mm at several locations). An analysis of three-dimensional radar-derived wind and precipitation fields shows that the most intense precipitation occurred where and when the easterly component of the low- to mid-level flow was the strongest with, however, a preferential location on the southern slopes of the Alps. This phenomenon can be explained by enhanced confluence between the main southerly flow and an intensifying easterly flow, and/or more favourable orientation of the moist inflow perpendicular to the mountain slopes. In addition, the propagation of convective cells from the Po Valley toward the Alps led to enhanced precipitation over the mountainous area. Taking advantage of the approximately two-dimensional character of the event, the different terms of the water budget are calculated for a series of 25 successive vertical cross-sections to analyse the transformation of atmospheric moisture into surface rainfall.
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