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