Analysis of mountain-valley precipitation differences in the Alps

Abstract

This paper investigates altitudinal precipitation differences in the northern Alps, based on routine observations and simulations with the MM5 model for the time period between 1991 and 2000. The analysis is performed for four station pairs, consisting of a mountain station and a nearby valley station each. The results are discussed for three different temperature levels (snow line > 2500 m, 1000 m-2500 m and < 1000 m). The climatological precipitation distribution shows that the mountain stations usually receive substantially more precipitation than the valley stations, especially for northwesterly and northerly ambient flow in 700 hPa. However, the differences are regionally variable and indicate a strong influence of the local topography. Moreover, precipitation enhancement over mountains tends to be substantially more effective for low temperatures than for high temperatures. A more detailed investigation of some parameters affecting orographic precipitation enhancement is conducted for stratiform precipitation events. We find that the magnitude of orographic precipitation enhancement markedly increases with the wind speed at 700 hPa. Moreover, precipitation enhancement increases with the depth of the moist layer in the approaching airflow.