Abstract : 1C.1
Reasons for the climatological precipitation gradient between the Alpine foreland and the northern Alps

Clemens Wastl, Günther Zängl
clemens@meteo.physik.uni-muenchen.de
Meteorologisches Institut der Universität München

This study examines the factors responsible for the 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 investigate the relative contribution of different precipitation types, precipitation events are classified into 4 groups. These are: Cold fronts, warm fronts, convection and a group carrying the remaining (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 (700hPa) is considered. For this investigation three selected regions in the Alps and three in the northern Alpine foreland, consisting of ten stations each, were compared.

Two mechanisms are found to be mainly responsible for the precipitation gradient towards the Alps. On the one hand, convective precipitation in connection with southwesterly flow is more abundant in the Alps than in the adjacent foreland. Showers and thunderstorms form primarily over the Alps and move into the Alpine foreland with the southwesterly flow, whereby they gradually decay. This mechanism is predominantly active in summer. On the other hand, frontal precipitation in connection with northwesterly and northerly winds is intensified at the northern Alpine rim due to orographic lifting. Also postfrontal precipitation occurring with northerly flow is more intense in the Alps than in the foreland. In contrast, fronts from exactly 270° produce more precipitation in the foreland than in the Alps. In this case, the wind blows parallel to the mountain range and lee effects related to upstream topography reduce the precipitation intensity in the Alps.