The turbulence structure and exchange processes in an Alpine valley: the MAP-Riviera project

Little is known at present concerning the turbulence structure and the associated turbulent exchange processes over highly complex topography such as an Alpine valley. With the exception of Large Eddy Simulation, numerical models of all scales essentially use turbulence parameteri-zations which have been derived under the assumption of flat and homogeneous terrain. However, due to the lack of appropriate field data, the validity of this basic constraint can hardly be assessed. As an attempt to fill in this gap the MAP-Riviera project was planned and realized by an international group of research teams. Its field part took place during summer/fall 1999 in the Riviera valley in southern Switzerland. It is embedded in the Mesoscale Alpine Programme (MAP), a major international effort to better understand meteorological and hydrological processes over an orographic ridge like the Alps. The Riviera valley is a relatively straight, mid-sized and u-shaped valley, about 1.5km wide at the bottom, roughly 2000m deep and with slopes of 30 – 35 degrees. On a cross-section through this valley, eight towers were deployed with one to six ‘turbulence levels‘ (sonic anemometers, fast-response hygrometers) up to 30m above ground. All together, twenty sonic anemometers were continuously operated between August and October 1999 on this cross-section. In addition, the radiation balance, precipitation and standard meteorological and surface hydrological variables were observed at most of the tower sites. During selected days (‘Flight Days’) a light research aircraft flew prescribed flight patterns on the cross-section as well as along the valley recording meteorological variables and chemical species at a 10 Hz temporal resolution. During ‘Flight Days’ supplementary instrumentation was operated within the valley, such as scintillometers, a radio sounding system, a tethered balloon system, SODARs and a passive microwave temperature profiler. With this a detailed three-dimensional picture of the mean and turbulence structure within and above the valley is available for the ‘Flight Days’. These were selected to cover days (or periods) of predominant mechanical turbulence, predominant thermally induced turbulence and ‘mixed’ days. An overview over the project will be given in terms of both, its research objectives and the layout of the field campaign. Also, selected preliminary results and an outlook to planned numerical studies will be presented.