MAPping foehn along the Brenner cross section

The Mesoscale Alpine Programme (MAP) is a concerted effort to explore the three-dimensional effects of complex topography on severe precipitation and windstorms (foehn). The field phase, which follows several years of numerical and theoretical investigations, will take place in fall 1999. One of the target areas is the Brenner cross section (Italy-Austria); it was chosen to study the effects of gaps on foehn flows. These gap flows have never been systematically investigated in the field. Only the concerted international effort will make it possible to gather the necessary data spanning several spatial scales, from the synoptic to the meso-gamma scale. This proposal is for the deployment of automatic weather stations, radiosoundings, operation of a Doppler sodar, car and cable-car based mobile measurements and missions of a small research aircraft. Advantage will be taken of the applicants` proximity to the target area by having a test phase before the international campaign starts so as to ensure optimal data quality, functioning logistics, and data retrieval and processing. The data will be used to answer questions about the applicability of theories explaining windstorms to the lee of simple mountains and the flow through lateral contractions in realistic, i.e. more complex terrain, the effects of wave breaking, hydraulic jumps, friction, non-stationarity, and three-dimensionality, with the ultimate goal of devising suitable theoretical concepts. A further application of the data set will be for the validation of research and operational numerical weather forecasting models and respective parameterizations.

In the fall of 1999 atmospheric scientists from all over the world came to the Alps to participate in the Mesoscale Alpine Programme (MAP), the largest-ever field campaign to study mountain meteorology. One question to which one had already been searching for an answer for one and a half centuries was how foehn worked. An answer was sought from measurements in the Brenner pass area in the central Alps. The collected data are unique and the analyses performed so far have brought scientist a lot closer to the answer. Current foehn researchers have the big advantage over their predecessors that they are not bound to the ground with their measurements. An international team of scientists from the USA, Canada, Great Britain, France, Italy, Germany, and Austria used instruments on both sides of the Brenner that could measure the foehn flow also above ground: research aircraft, weather balloons which radio measurements to a base station while rising, and the "big brothers" of the laser pistols used by police to spot speeding cars, which could measure the wind speed to a distance of 10 km from the laser device. Also modern versions of the traditional surface weather stations were used along the valley floor and up the slopes, 70 overall. The research group at the Department of Meteorology and Geophysics at the University of Innsbruck coordinated the measurement campaign in the Brenner area and undertook a large part of the observations and maintenance of the instruments. The largest chunk of funding came from the Austrian Science foundation FWF, project 13489. Many students had the opportunity to not only gain experience with a wide range of instruments but to also work side-by-side with established scientists, whom they had previously known only from journal articles. During the 70 day long measurement period the research aircraft caused the greatest excitement among the local population. One of them is normally used for hurricane research. People were impressed how this large 4-propeller plane flew as close as 300 m above ground through the turbulent foehn air in the narrow Wipp valley. The most important result to date is a very graspable one: Foehn and gap flow in the Brenner pass area closely resembles the flow of a river over a weir with the Alps crest being the weir. Upstream, the air is dammed like a river. The top of this upstream air is often formed by a cloud deck. When this air crashes down on the other side of the weir, the clouds dissolve and the air rushes down the valley. Just like various amounts of water can run through a riverbed, the depth of the air upstream of the weir can also vary. If it is barely deep enough to flow across the gaps and passes in the Alpine "weir" the flow is called "shallow foehn". The equivalent of a flood where air from far above the highest Alpine peaks descends on the other side is called "deep foehn". The collected data not only help scientists to understand the mechanisms leading to foehn and gap flow, they also have practical applications. The airport in Innsbruck for example lies in a foehn swath. The accompanying wind shear, where wind can reverse direction over a short distance, is hazardous for aircraft safety during take-off and landing. There are also larger airports, e.g. the Hong Kong International Airport, which are affected by gap flow jets. Results from this project are not only applicable to gap flow and foehn in the Brenner area but can be applied to other regions in the Alps as well as to any other mountain ranges in the world that have gaps and passes. This one of the reasons why scientists from all over the world participated in the observations.