A look at the 30 October 1999 south foehn event in the Wipptal

Flow forced through topographic low points can produce strong terrain-induced surface winds referred to as gap winds. During the Special Observing Period (SOP) of the Mesoscale Alpine Programme (MAP), which took place between 7 September and 15 November, targeted observations of flow through the Brenner Pass in the central Alps were used to study the dynamics of gap winds. The ground-based instrumentation deployed in this target area included automated weather stations and microbarographs located along the axis of the gap, a UHF-wind profiler with RASS located at Brenner Pass, supplemental radiosondes upstream and downstream of the pass, and the NOAA/ETL Doppler lidar located in the Wipptal (the river valley to the north of Brenner pass and south of Innsbruck, Austria). In addition to the ground-based instrumentation, the gap flow project had four aircraft available for missions in the Brenner pass target area: NOAA P-3, NCAR Electra, INSU Fokker-27 ARAT, and DLR Dornier 228.

On 30 October 1999, a moderate shallow south foehn event developed in the Brenner Pass target area in response to increased southwesterly flow ahead of an approaching sharp North Atlantic trough. Two dual aircraft missions were flown during this south foehn event: a morning mission by the P-3 and Electra, and an afternoon mission by the P-3 and ARAT. The P-3 performed low-level flight patterns where the primary goal was the use of in situ measurements to document the flow characteristics in the gap. The Electra and ARAT flew upper-level flight patterns where the primary goal was the use of remote sensing instrumentation to document the structure of the flow in the gap. Dropsondes deployed by the Electra and supplemental radiosondes deployed upstream and downstream of the pass will provide important information on the stratification of the flow. The Doppler lidar coverage, which depends on the availability of scatterers, was outstanding during this event (8 km in the vertical and 20 km in the horizontal) due to the presence of Saharan dust over the target area. Using a variety of scanning strategies, the lidar was able to map out the spatial variability of the flow within the valley and monitor the vertical structure of the flow at fine time resolution over a 10 hour time period. We plan to present a preliminary analysis of the flow within the gap based on the P-3 in situ measurements, as well as an overview of the flow structures documented by the Doppler lidar during this gap flow event.