Abstract : B.6
On the role of the Grenoble valley topography in vertical transport of mass and pollutants

Charles Chemel, Eric Chaxel, Jean-Pierre Chollet
Centre for Atmospheric and Instrumentation Research

The Grenoble urban area (France) is embedded within a highly complex alpine topography, which is somewhat similar to that of the Chambéry urban area (France). Indeed, the whole core of the city is located at the Y-shaped confluence of deep and steep-side valleys, which cannot be considered as secondary tributary valleys. The flow dynamics within this Y-shaped valley is thus rather specific. On top of that the uneven terrain of the Southern branch induces complex circulations, which may have significant impacts on both local (thermo) dynamics and air quality. Hence, the objectives of our work are (i) to investigate the onset, development and seasonal variability of the flow in the Southern part of the valley and (ii) to quantify its impact on the structure of the atmosphere as well as on air quality. A set of meteorological and air quality models using horizontal resolution down to 1 km, as well as existing data sets are used to detail the characteristics of that part of the valley for selected episodes in summer 2003 and winter 2005. During the summer episode the descending flow from the Southern part of the valley strongly interferes with the up-valley wind. As a consequence of this interaction, the air mass is pushed upwards. The resulting net vertical transport is found to have a significant impact on the high concentration of precursor gases in the free-atmosphere above. The winter episode is particularly stable and calm weather conditions lead to high concentrations of smog-forming pollutants and PM10 within the valley below a well-defined temperature inversion. Depending on the atmospheric conditions, the Southern branch of the valley either strengthens the slight down-valley wind or produces a recirculation zone within the valley. The main outcome of this study is that both the uneven terrain and the interaction of thermally-driven circulations in the Southern part of the valley can play a significant role in mass exchange with the free atmosphere.