The turbulence structure in an Alpine valley

During the observational phase of the MAP-Riviera project (August to October, 1999; objectives and setup are presented elsewhere in this Conference), data on the thermal and dynamical structure of the atmosphere within and above an Alpine valley were collected under a variety of synoptic conditions by means of ground-based instruments, radiosondes and a light research aircraft. As a case study, the present contribution considers the spatial structure and temporal evolution of the turbulence field associated to a well-developed valley- and slope-wind system. The momentum flux and the turbulent kinetic energy (TKE) are examined first. It is shown that the maxima in the momentum flux and TKE fields occur close to the ground but also in relation to the core of the valley-wind system, indicating mechanical generation of turbulence away from the surface. Next, the turbulent heat and moisture transfer are discussed with respect to the surface energy balance, the vertical stratification and vertical moisture distribution. The data suggest a different response of the heat and moisture flux to changes in the external conditions. It is shown that, within the valley atmosphere, the heat transfer, which is positive around noon, becomes negative much earlier than the moisture flux as the sunset is approached.