Convective orographic precipitation in sheared flow: a conceptual model

This study presents a conceptual model for orographic convective development in unstable, vertically sheared flow. The model results from an exhaustive radar-based observational study of the precipitation structure within storms that produce flash floods in the Lago Maggiore region of the southern slopes of the European Alps.

Flash floods are frequently observed in the Maggia River catchment, a 900 km² watershed located in the Lago Maggiore region. During the heaviest floods, river discharge rates can increase by as much as 1400 cubic meters per second in less than 3 hours! Flash floods are due to training convective cells triggered by the orography, which are absent from storm events that produce lower peak flow rates. During the heaviest floods, individual convective cells repeatedly develop over the lower Alpine foothills, where southeasterly moist unstable low-level flow impacts the orography. Then, they are advected over the interior of the Alps by the upper-level southwesterly wind. Since the upper-level steering flow coincides with the southwest to northeast orientation of the orographic feature that most frequently initiates convection, the convective cells are naturally aligned into echo trains that repeatedly move across the Maggia catchment, where they produce excessive rainfall and river discharge rates. At large temporal scales, precipitation patterns assume the form of quasi-stationary, elongated bands of rainfall.

Other mountain ranges of the world may experience convective triggering mechanisms that are similar to those identified in this study as being responsible for frequent flash flooding in the Alps.