Mesoscale organization and structure of orographic precipitation producing flash floods in the Lago Maggiore region

This article investigates the mesoscale precipitation mechanisms affecting the severest flood events observed from 2005 to 2012 in the Maggia River, located in the Lago Maggiore region on the southern side of the European Alps. High‐resolution volumetric radar data are used to describe the horizonta...

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Bibliographic Details
Published in:Quarterly journal of the Royal Meteorological Society 2015-01, Vol.141 (686), p.224-248
Main Authors: Panziera, L., James, C. N., Germann, U.
Format: Article
Language:English
Subjects:
Alpine radar
Freshwater
IMPRINTS
mesoscale flows
Meteorology
orographic convection
Precipitation
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Summary:This article investigates the mesoscale precipitation mechanisms affecting the severest flood events observed from 2005 to 2012 in the Maggia River, located in the Lago Maggiore region on the southern side of the European Alps. High‐resolution volumetric radar data are used to describe the horizontal and vertical structure of precipitation, while sounding data depict the airflow conditions. The events causing the nine highest peak discharge rates in the river in the last eight years are characterized by the presence of convection, which is generally absent in the storms that produced lower peak flow rates. During major floods, convective cells repeatedly propagate over the watershed over time. At large temporal scales, precipitation patterns assume the form of an elongated band of precipitation. The main reason for the formation of the band is the different direction of the low‐level and upper‐level flow in relation to the orientation of the Alpine orography. When pre‐frontal, moist and conditionally unstable air over the Po Valley is advected towards the Alpine chain by a southsoutheasterly low‐level jet, convection develops over the slopes west of Lago Maggiore where the low‐level flow is perpendicular to the terrain orientation. Convective cells are then transported towards the northeast by the upper‐level southwesterly steering flow, reaching maturity over the Maggia watershed. This mesoscale process can trigger and maintain moist convection for a long time over the same region. Statistical analyses show that the strength and stability of the upstream flow at low levels (
ISSN:0035-9009
1477-870X
DOI:10.1002/qj.2351