Observations of the Temporal Evolution and Spatial Structure of the Gap Flow in the Wipp Valley on 2 and 3 October 1999

Abstract

The investigation of gap flow in the Wipp Valley (GAP project) is one of the objectives of the Mesoscale Alpine Programme (MAP). The valley runs south?north across the Brenner Pass, from Italy to Austria. The pass is the lowest one of the main Alpine ridge and is therefore a favorable location for a gap flow, which is called foehn. Based on the extensive dataset of MAP, this study gives a detailed analysis of foehn on 2 and 3 October 1999 [intensive observation period 5 (IOP 5)]. The foehn event began as a gap flow that was separated from midlevel winds by a strong temperature inversion during the night of 1?2 October 1999. On the next night (2? 3 October) the inversion dissipated, and the gap flow was combined with strong midlevel cross-ridge flow on 3 October 1999. This study shows that the existence of a temperature inversion above the gap flow has a crucial impact on the flow structure. Another emphasis of the study was the investigation of the small-scale flow structure downstream of the gap. Jumplike features and regions with flow reversals, which were interpreted as ?rollers? (reversed rotors), could be observed in the Wipp Valley. A jet layer with a wavy structure indicated a gravity wave in the southern part of the valley. In the northern part the flow showed strong asymmetry with wind speeds nearly twice as strong on the eastern side as on the western one.