Extended Abstract (84K)P3.7
Temporal evolution and structure of gap flow in the Wipp Valley on 2 and 3 October 1999
Martin Weissmann, University of Innsbruck, Innsbruck, Austria; and R. Banta, G. Mayr, A. Gohm, and L. B. Nance
The MAP gap-flow experiment was organized to study southerly flow through the Brenner Pass, a major north-south gap in the Alps. To the north of the pass in Austria, the gap extends through the Wipp Valley from the Brenner Pass to Innsbruck. Southerly flow in the Wipp Valley is associated with foehn conditions. Gap flow in the Wipp Valley began as shallow foehn on 2 October 1999, deepened later on, and ended with a cold front passage in the afternoon of 3 October. We investigated the factors causing the deepening of the foehn layer on 3 October, and the three-dimensional flow structure on both days, including a wave-like structure of the jet layer in the valley and hydraulic features on the lee side of mountains east and west of the Wipp Valley. The detailed analysis of the flow structure on both days is based on the extensive dataset from instrumentation deployed during the MAP SOP, including 70 surface stations, the NOAA ETL Doppler Lidar, in situ aircraft measurements from the NOAA P3, rawinsondes, and pilot balloons. Atypically, this gap flow event began with slightly higher potential temperatures on the upstream side. On 2 October southerly flow (maximum wind speeds 25 m/s) was capped by a strong inversion at about 3000m MSL on the upstream side and 2400m MSL on the downstream side (in Innsbruck). The flow was mostly limited to the valley, but overflowing smaller mountains east and west of the Wipp Valley with heights of 2100 to 2300m MSL. Evidence of hydraulic flow structures (thinning and descending of jet layers, rotors) is found in the Lidar data. Also the outflow from Gschnitz Valley and Stubai Valley, two side valleys of the Wipp Valley, seems to have a significant impact on the flow in their exit regions, as seen in lower wind speeds and increased turbulence. The flow on 3 October in contrast was a deep foehn. The inversion rose to 3800m MSL at 0600 UTC and dissolved later on. The top of the foehn layer was about 4500m MSL, but the flow was not decoupled from the southwesterly wind above the shear layer.
Poster Session 3, Topographic Flows (with Coffee Break)
Thursday, 20 June 2002, 2:45 PM-4:30 PM
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