An overview of the GAP flow measurements within the Mesoscale Alpine Program (MAP)

How a vertical constriction (pass) and a lateral contraction (gap) jointly influence the flow through and over it, is one of the objectives of MAP. The field measurement area chosen for this objective is the Brenner cross section, which passes over the lowest of all Alpine passes and has a topographically fairly simple downstream side for southerly flow. This paper provides an overview of the whole field campaign in this target area.

The instrumentation deployed for the field campaign was designed to give a finely resolved description of both mass and wind field. Two radiosonde locations on each side of the pass, three Doppler sodars, a wind profiler, and a scanning Doppler lidar provided vertical cross sections of these fields together with aircraft missions and a network of 70 surface stations strung along the valley floor and up the slopes. Locations were chosen using data gathered in ALPEX and during the preparatory phase I of MAP. Several research groups from Europe and the US provided the instrumentation.

The weather was very favorable for southerly gap flow and provided an above-average number of cases during the 10 weeks of the field measurements (climatologically about only every fourth year has as many cases). Unexpectedly many gap flow situations were "shallow" in the sense that the cross-Alpine flow was confined to below the peaks of the Alps.

A summary of all measurement days with aircraft missions will be given. The presentation concludes with samples of data from the various instruments for the case of October 30, 1999.