Projet de thèse en Océan, Atmosphère, Hydrologie
Sous la direction de Chantal Staquet.
Thèses en préparation à Grenoble Alpes en cotutelle avec l'University of Hertfordshire, UK , dans le cadre de Terre, Univers, Environnement , en partenariat avec Laboratoire des Ecoulements Géophysiques et Industriels (laboratoire) depuis le 01-10-2013 .
See English version.
Wintertime Stable Boundary-Layer Processes in Alpine Valleys
Pooling processes associated with the stable boundary layer that develops in valleys are relatively common phenomena during wintertime, in particular for weak synoptic conditions. In urbanized Alpine valleys those phenomena have substantial impacts on society. This work focuses on the characterization of the variability of the pooling processes due to valley winds and orographic variations, both in idealized and real valleys, mainly by means of numerical modelling. This issue is addressed by first considering idealized valleys under dynamically-decoupled conditions. The comparison of an idealized three-dimensional valley opening on a plain with a two-dimensional valley (invariant in the along-valley direction) allows the quantification of the impact of the thermally-driven down-valley flow on the evolution of the valley boundary layer. Numerical results show that the boundary layer structure, heat and mass budget are strongly influenced by the development of the down-valley flow. Advection leads to a net cooling in the 2D valley and to a warming in the 3D valley, once the down-valley flow is fully developed. A quasi-steady state is reached in the 3D valley for which the divergence of the down-valley flow along the valley is balanced by the convergence of the downslope flows at the top of the cold-air pool, with no net contribution of subsiding motions far from the slope layer. When the valley opens on a narrower (pooling) or wider (draining) valley, the valley atmosphere reaches a quasi-steady state in a longer time. This allows deeper and colder boundary layer to develop. For the strong pooling case, an up-valley flow develops from the narrower to the wider valley during the evening transition, affecting the mass and heat budget of the valley during that period. Indeed, considering the heat budget of the valley system, the contribution of advection varies along the valley axis: it decreases for a pooling configuration and increases for a draining configuration. Consequently, for a pooling configuration, the heat transfer between the valley and the plain is reduced, thereby increasing the temperature difference between them. For the strong pooling case, this temperature difference can be explained by the topographic amplification factor once the down-valley flow has developed. As a last step, exploitation of vertical temperature profile data and ground-based PM10 concentration data collected during the Passy-2015 field campaign at very fine temporal resolution highlights the large hourly variability of the PM10 concentration during cold-air pooling events. This calls for numerical investigations of the meteorological processes governing the evolution of the valley atmosphere during those periods. The persistent cold-air pool event which takes place from the 9th to 14th of February 2015 in the valley of Passy is simulated. The role of the external forcing (i.e. the synoptic flow) and local forcings (i.e. thermally driven flows) in controlling the mass and heat budget of the valley is quantified contrasting a real-case simulation with a series of semi-idealized simulations.