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

Shallow foehn flow, documented during the Mesoscale Alpine Programme on 6 Nov 1999, was interrupted by a cold front passage. The National Oceanic and Atmospheric Administration/Environmental Technology Laboratory deployed a Doppler lidar in Gedeir, Austria, approximately half-way between the city of Innsbruck and Brenner Pass. From this location in the Wipptal, a north/south oriented valley, the Doppler lidar measured southerly winds of 15 - 20 ms-1, 2 kmdeep for 19 hr before the cold front passage. Lidar measurements showed the evolution of the vertical structure of these winds with time, plus the reversal from southerly flow to northerly flow after the cold front passage ( 1200 UTC 6 Nov 1999).

The evolution of winds included a layer of 15 m s-1 winds mixing to thesurface from 1.5 km above ground level (AGL) during the early morning hours,then becoming elevated again. Several times during the night and morning hours before the front passage a wave structure was evident in the winds. These waves also brought the strongest winds closer to the surface. Flow above mountain top level had a westerly component most of the time, keeping the foehn flow confined to within 2 km above the surface. However, upper level winds became more southerly for a brief period, allowing the foehn flow to deepen.

The wind flow reversal associated with the cold front was detected about 20 min before the front passed the lidar site, at a range of 8 km to the north. Continuous vertical slices of radial winds and backscatter signal were taken by the lidar, documenting the progression of the front up the Wipptal. The turbulent interface between the foehn flow and cold air behind the front had a complex structure to it, with clouds associated with overturning winds. The vertical structure of the frontal head had the characteristics of a density current.

A sounding launched at 1200 UTC, just as the cold air entered Gedeir,indicated that the balloon encountered strong wave activity as it headed north, with a succession of rapid descents and ascents. A Richardson number of 0.05 calculated from balloon data indicated the potential for Kelvin-Helmholtz instability.

Once the front passed the lidar, vertical slices of radial velocity and backscatter were taken to the south, still documenting the progression of the front up the valley as it undercut the foehn flow. Once the lidar range dropped due to snow falling in Gedeir, lidar operations ceased. A second sounding was then launched from the lidar site to assess the post-frontal situation. This sounding showed that the foehn flow continued at upper levels, above the cold air.

In addition to the lidar measurements, data from an aircraft mission flown in the morning before the front will be presented. In a companion paper, a more in- depth analysis of the soundings and surface data will be presented.