Evolution of the daytime atmospheric boundary layer structure in a deep Alpine valley

Processes leading to temperature inversion breakup exert an important influence on the vertical transport and mixing of pollutants. Pollutants trapped in the stable atmospheric boundary layer during nighttime are caught in the growing convective boundary layer (CBL). CBL develops from the valley floor and sidewalls due to surface heating. Thermally- generated upslope flows bring pollutants along the slopes and may dilute them at larger scale.

Alpine valley are particularly sensitive to air pollution due to emission sources generally concentrated close to the valley floor and local meteorology induced by complex terrain. The program POVA (`POllution des Vallées Alpines') was launched in May 2000. The general topics of the POVA program are the comparative studies of air quality and the modeling of atmospheric emissions and transport in the Chamonix and Maurienne valleys (France). This study benefits from an exceptional context, with the `Tunnel du Mont Blanc' (TMB) in the Chamonix valley being closed for nearly 3 years after a large accident in March 1999. The program includes several intensive field campaigns before and after the reopening of the TMB to heavy duty traffic, associated with 3D modeling.

In the present study, current highlights on the evolution of the daytime atmospheric boundary layer structure in the Chamonix valley are presented. Used data were collected from 25 June through 10 July 2003. The structure of the convective boundary layer was measured using a UHF radar profiler. Due to ground clutter and atmospheric echoes, radar profilers are often blind close to the surface. Furthermore, for a reliable prediction of chemical species concentrations, mixing-height evolution during morning and evening transition periods is one essential parameter. The lower layers of the atmosphere were documented using a small portable tethered balloon. Joined up with the wind profiler, the tethered balloon provided full vertical profiles.

Numerical simulations and their comparison with observations were performed during the whole field campaign. The ARPS (Advanced Regional Prediction System) model developed at the University of Oklahoma was used with several grid nesting levels. The Chamonix valley was finally resolved on a 93 E-W by 103 N-S grid with spacings of 300 m * 300 m in the horizontal directions. This grid encompassed a domain of about 25 km * 25 km. Results from ARPS were input in a meso-scale chemical-transport model (TAPOM).

Investigations of small scale flow fields were carried out, aiming at detailing valley wind systems. A peculiar emphasis was put on transition period to give a better understanding of the formation of the CBL. Figure 1 shows results from the model compared to measurements from the wind profiler in a location close to Chamonix center on July 8th, 2003. Wind direction reverses from down (50°) -- to up (230°) in the morning and the opposite at night. Wind speed culminates when being up-valley and peaks at 4-5 m/s at 1 PM. The figure suggests a mixed layer thickness of about 1200 m from both simulation and measurements.

Figure 1: Pattern wind structure evolution from ARPS compared to measurements from the wind profiler, Chamonix valley, July 8, 2003

The meteorological conditions during the studied period were favorable for formation of high ozone concentrations as high-pressure systems located over the area produced clear skies, weaker winds, hot temperatures. Peak ozone concentrations of about 80 ppbv were measured in surrounding cities. At night, the strong stability in the nocturnal boundary layer limited vertical mixing of momentum and surface-based pollutants to a shallow layer (i.e., 100 to 200 m thick) near the ground (see Figure 2). After daybreak (07H00 GMT), the surface warmed, resulting in the growth of the CBL. The CBL grew to an afternoon depth of 1200 m a.s.l., but further vertical growth was inhibited by the subsidence inversion.

Figure 2: Ozone concentration profile evolution from 0634 GMT (1) to 1133 GMT (10). Tethered-balloon data, Chamonix valley, July 8, 2003

Local thermally driven wind circulation within both valleys and transition periods were well simulated by the model. Available measurements were used to evaluate the model results. Both ground surface and upper-levels calculated winds showed good agreement with the observations. Numerical simulations with TAPOM are currently in progress. Preliminary results are encouraging.