Observations of flow through a mountain pass

During the Special Observing Period (7 September - 15 November 1999) of the Mesoscale Alpine Programme (MAP), 35 automatic surface stations were deployed in the Wipp valley area. The primary aim was to observe Foehn flows in the vicinity of the major north-south valley system crossing the alpine range. The study formed part of the Dynamics of Gap Flow (GAP) project within MAP. The site locations included (i) 19 sites along the Wipptal valley floor, (ii) 5 sites in the Inn valley running perpendicular to the Wipptal, (iii) 7 sites on a profile up the sides of the Wipp valley, and (iv) 4 other mountain top sites.

All surface stations include wind, temperature and humidity measurements with averaging periods in the range 30 seconds to 10 minutes. At 19 sites there were also microbarographs measuring pressure to an absolute accuracy of 0.25 hPa and a resolution of 0.005 hPa. Near-surface turbulent momentum flux profiles were measured using three-axis ultrasonic anemometers. Pairs of these were mounted at 6.5 m and 13 m at 3 locations along the Wipptal valley floor. Data were recorded at 1 Hz.

The aims of the project were to investigate: (1) how the measured surface drag is related to wind speed, boundary layer turbulent stresses and flow separation/hydraulic jumps; (2) which forces are causing the near-surface acceleration along the Wipptal Valley; (3) how the thermodynamic stratification of the atmosphere influences the behaviour of the gap flow; and (4) the relationship between the gap flow and the flow at mountain top level.

Preliminary analysis of the observations will be presented. The observed flow types include shallow and deep Foehn events and buoyancy driven flows (drainage and convective).