Mesoscale organization and structure of orographic precipitation producing flash floods in southern Switzerland

In this study, high-resolution volumetric radar data are used to investigate the environmental conditions and mesoscale precipitation mechanisms influencing the heaviest flood events observed from 2005 to 2012 in the Maggia River, located in southern Switzerland. Three-dimensional radar reflectivity data are used to describe the horizontal and vertical structure of precipitation, while sounding data and model reanalysis data depict the airflow. The events causing the nine highest discharge rates in the river over the 8-year period are characterized by the presence of strong convection, which is generally absent from storm events that produce lower peak flow rates. During the heaviest floods, individual convective cells propagate repeatedly across the watershed, while at larger spatial scales, precipitation patterns assume the form of quasi-stationary, elongated bands of precipitation.

Detailed analyses of eight years of high-resolution radar, sounding, mesonet and model data reveal that this persistent convective behavior results from the variation of the direction of the impinging airflow with height in relation to the orientation of the orography. When prefrontal, moist and conditionally unstable air over the Po Valley of northern Italy is advected towards the Alpine chain by a southeasterly low-level jet, convection develops where the first steep rise of the Alpine terrain is roughly perpendicular to the flow. Convective cells are then transported towards the northeast, parallel to the barrier by the southwesterly steering flow at mid and upper levels, reaching maturity over the Maggia watershed. These processes can trigger multiple training convective cells and maintain moist convection for an extended period of time over the same region, leading to flash flooding.