Pressure Drag on the European Alps in Relation to Synoptic Events

Abstract

In addition to the frictional surface drag the pressure drag plays a significant role in mountainous terrain, such as the European Alps. The horizontal, two-dimensional pressure drag is computed for 4 horizontally and 3 vertically spaced sections of the Alps by applying Archimedes Law as the product of the local horizontal pressure gradient and the volume of the mountains. The computations cover the two month Special 0bservation Period (SOP) of the Alpine Experiment (ALPEX) from 1 March to 1 April 1982, with a three-hour resolution. This time series of the pressure drag vector is compared with the history of synoptic and subsynoptic weather events such as frontal passages lee, cyclogenesis, south- and north-foehn winds. Episodes of warm air advection leading to shallow south-foehn events, and the occurrence of lee cyclogenesis both produce very large drag vectors These drags have generally opposite directions and thus nearly cancel in a long term average. Rapid drag changes occur during frontal passage and reflect an eastward time lag of 6-12 hours.