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

Abstract

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.

Selection of intense rainfall events based on intensity thresholds and lightning data in Switzerland

“…in the Graubünden region in the east of Switzerland where substantial blocking of precipitation is observed, and Lugano (LUG, 273 m a.s.l.) in the Tessin region to the south of the main Alpine divide which is significantly affected by Mediterranean weather patterns and orographic forcing, especially in the summer and fall seasons (Panziera and Germann, 2010). Schüepp and Gensler (1980).…”

The potential of radar-based ensemble forecasts for flash-flood early warning in the southern Swiss Alps

“…As precipitation in mountainous regions is influenced by orographic forcing, Panziera and Germann (2010) investigated the effects of orographic forcing on the rainfall patterns in the Lago Maggiore Region in southern Switzerland (Fig. 1).…”

NORA–Nowcasting of Orographic Rainfall by means of Analogues

“…Since the orographic forcing dominates the patterns of rainfall in the mountains (e.g. Houze et al, 2001;Panziera and Germann, 2010;Yuter et al, 2011), it can be exploited for nowcasting of precipitation. NORA takes advantage of not only the effects of the orographic forcing, but also the recent improvements of the Swiss weather radar precipitation estimates (Germann et al, 2006).…”

“…In Panziera and Germann (2010), from now on PG10, we analysed a large number of long-lasting orographic precipitation events occurring over the Lago Maggiore region, which is located in the southern part of the European Alps between Italy and Switzerland (Figure 1). In PG10 we demonstrated that the orographic forcing gives repeatability to the rainfall patterns observed in the mountains with certain environmental conditions.…”