Foehn flow in the Austrian Alps interrupted by a cold front passage: Part II

During the field phase of the Mesoscale Alpine Programme MAP in fall 1999, an intense network of automatic weather stations and atmospheric sounding systems was operated in the Wipp valley of Tyrol (Austria) to investigate southerly foehn winds on the lee side of the European Alps which are observed as pronounced gap flows especially over the Brenner pass. The 5th to 6th November 1999 event is chosen for a case study of the break down of this southerly flow caused by the passage of a cold front.

A general synoptic overview based on ECMWF analysis data and operational SYNOP surface station data will supply the background information for a mesoscale surface analysis of the event. High resolution data of the meso-network will show how the foehn flow in the Wipp valley is lifted from the valley bottom as the cold air penetrates from the northern Alpine foreland into the Inn and Wipp valley and moves further south over the Brenner pass in analogy to a density current. The sudden decay of the foehn flow near the valley bottom appears as rapid cooling, rising atmospheric pressure, and flow reversal. The approach of the cold front is delayed at higher altitudes, which is obvious when comparing data of mountain and valley stations. Data from a wind profiler at Innsbruck and a Doppler sodar at the Brenner pass will show the abrupt change in wind direction at the lowest levels of the valley atmosphere caused by the front passage. On a bigger spatial and temporal scale the passage of the front over the Alps is well observed by radiosounding data from several stations located northerly and southerly of the main Alpine crest between Munich and Verona.

A companion paper (Part I) focuses on the analysis of data measured with a Doppler lidar which was deployed by the National Oceanic and Atmospheric Administration / Environmental Technology Laboratory.