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 (0800-1130 UTC), and an afternoon mission by the P-3 and ARAT (1300-1600 UTC). The NOAA/ETL ground-based Doppler lidar operated between 0600 and 1700 UTC. 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.

Analysis of the Doppler lidar data has revealed a strong temporal evolution of the vertical and horizontal structure of the gap flow during this particular event. The gap flow characteristics exhibited a number of changes, including depth, speed, and the number of layers in the flow within the valley. For example, the layered structured upstream of the Doppler lidar varied between a single thin layer less than 500 m deep, a single thick layer approximately 2-km deep, a layered wavy structure with two wind speed maxima in the vertical, and, near the end of the period, a layered wavy structure with three wind speed maxima in the vertical. During each of these periods, lidar scans in azimuth revealed strong horizontal variability in the gap flow in both the along-valley and cross-valley directions. This presentation will summarize the evolution of the kinematic flow field as documented by the Doppler lidar and relate the changes in the gap flow characteristics to the evolution of the upstream wind and stability profiles south of Brenner Pass, as documented by supplemental rawindsondes and dropsondes deployed from the Electra. The low-level flights by the P-3 will be used to supplement the Doppler lidar observations of the kinematic flow field within the gap, as well as providing information on the thermal characteristics of the gap flow.