Analysis of the climatological precipitation gradient between the Alpine foreland and the northern Alps

Abstract

This study investigates the climatological precipitation gradient between the northern Alpine rim and the Alpine foreland, based on routine observations and reanalysis-driven simulations with the MM5 model for the time period of 1991-2000. To estimate the relative contribution of different precipitation types, we distinguish between cold fronts, warm fronts, convection and a class carrying unclassified events. Convective precipitation in connection with fronts is attributed to the respective frontal class. Unclassified events predominantly consist of postfrontal upslope precipitation and quasi-stationary fronts. In addition, the wind direction at Alpine crest level (700 hPa) is considered. Our analysis indicates that summertime convection and orographic lifting make the largest climatological contribution to the precipitation gradient towards the Alps. Convective precipitation occurs predominantly in connection with southwesterly flows and is more abundant in the Alps than in the adjacent foreland because convection is primarily triggered over the Alps. Orographic lifting is most active with northwesterly and northerly winds and intensifies both frontal and postfrontal precipitation. The climatological importance of orographic lifting is much larger in winter than in summer. A reversed precipitation gradient with systematically more precipitation in the foreland than in the Alps is found for fronts connected with a wind direction of exactly 270°. Under these circumstances, the wind blows parallel to the mountain range and lee effects related to upstream topography reduce the precipitation intensity in the Alps.