Case studies of frontal passages in a mountain valley with direct access to the bavarian pre-Alpine region results from the German Front Experiment 1987

Abstract

The orographic impact on cold fronts is investigated in the Loisach River Valley area by means of two events obtained during the German Front Experiment 1987. The discussion is focussed on the frontal passages in the Garmisch area traced from continuous recordings of three meteorological/ air-chemical stations at different heights, viz: valley floor (735m), wank peak station (1780m) and Zugspitze station (2964m); the analysis also includes some aerological data (pibal ascents at Farchant near Garmisch) and sensitive pressure data from temporary stations in the Bavarian “foreland”, respectively. In the case of the west-east (or parallel to the mountains) moving front A (October 8, 1987), with prefrontal foehn, the following features were observed: i) formation of large (up to 7hPa/100 km up-valley directed pressure gradients after the frontal passage in the foreland, ii) intrusion of cold air from the foreland into the valley (lasting for several hours, filling the whole valley and even traceable at crest height), iii) destruction of upper level foehn current during the “intrusion-phase”, iv) retardation of the front (progressing rapidly in the foreland but being retarded by the mountains located to the west). In the case of the high-reaching, southward (or perpendicular to the mountains) moving D-front (December 18/19, 1987), such striking orographic modifications were not observed. In that case the mountains located to the west obviously did not act as noticeable barrier and, therefore, the flow around and over the mountains prevented the for ation of large pressure gradients between the foreland and the valley (and an intrusion into the valley). In both events it was confirmed that trace gas measurements provide a very promising tool to localize atmospheric flows indirectly.