The relation between airflow and orographic precipitation on the southern side of the Alps as revealed by weather radar

The relative importance of various environmental parameters in determining orographic precipitation patterns in the Lago Maggiore region in the southern part of the European Alps is investigated. We examine 58 long-lasting and widespread orographic precipitation events corresponding in total to 106 days of rainfall. The mesoscale winds are estimated by means of Doppler velocity radar measurements; air-mass stability is computed from both radiosoundings and pairs of ground stations located at different heights. High-quality radar-derived rain rates at the ground are used to characterize the precipitation field. The mesoscale flows are estimated within the layers with a mean wind velocity that correlates best with rainfall in the mountains; these layers are located around 1 km above sea level (asl) upstream of the Alps and around 3 km above the first alpine peaks. Upstream wind velocity has the largest impact on the intensity and frequency of precipitation in the mountains, while the direction of the wind determines the spatial distribution of precipitation. Unstable conditions cause more precipitation over the mountains compared with stable cases; however, differences in air-mass stability have a minor impact on the precipitation intensity compared with wind speed and direction. The intensity of the flow also dominates rainfall patterns in different Froude number airflows. This study builds the scientific framework necessary to develop a heuristic system for nowcasting orographic precipitation in the Alpine region by exploiting the presence of orographic forcing. The latter is shown to give repeatability to the rainfall patterns typically observed in the region with particular environmental conditions. Copyright (C) 2010 Royal Meteorological Society