Abstract
Atmospheric circulation over mountainous regions is more complex than over flat terrain due to the interaction of flows on various scales: synoptic-scale flows, thermally-driven mesoscale winds and turbulent fluxes. In order to faithfully reconstruct the circulation affecting the dispersion and deposition of pollutants in mountainous areas, meteorological models should have a sub-kilometer grid spacing, where turbulent motions are partially resolved and the parametrizations of the sub-grid scale fluxes need to be evaluated. In this study, a modelling chain based on the Weather Research and Forecasting (WRF) model and the chemical transport model Flexible Air Quality Regional Model (FARM) is applied to estimate the pollutant concentrations at a 0.5 km horizontal resolution over the Aosta Valley, a mountainous region of the northwestern Alps. Two pollution episodes that occurred in this region are reconstructed: one summer episode dominated by thermally-driven winds, and one winter episode dominated by synoptic-scale flows. Three WRF configurations with specific planetary boundary layer and surface layer schemes are tested, and the numerical results are compared with the surface measurements of meteorological variables at twenty-four stations. For each WRF configuration, two different FARM runs are performed, with turbulence-related quantities provided by the SURface-atmosphere interFace PROcessor or directly by WRF. The chemical concentrations resulting from the different FARM runs are compared with the surface measurements of particulate matter of less than 10 µm in diameter and nitrogen dioxide taken at five air quality stations. Furthermore, these results are compared with the outputs of the modelling chain employed routinely by the Aosta Valley Environmental Protection Agency, based on FARM driven by COSMO-I2 (COnsortium for Small-scale MOdelling) at 2.8 km horizontal grid spacing. The pollution events are underestimated by the modelling chain, but the bias between simulated and measured surface concentrations is reduced using the configuration based on WRF turbulence parametrizations, which imply a reduced dispersion.
Highlights
Atmospheric transport processes over complex terrain are the result of a wide spectrum of flows acting and interacting at different spatial and temporal scales
Weather Research and Forecasting (WRF) correctly reproduces the daily cycle of the 10 m wind speed (Figure 5)
The scale-adaptive mass-flux scheme for momentum activated in the WRF MYNN2.5 configuration contributes mainly in unstable summer conditions
Summary
Atmospheric transport processes over complex terrain are the result of a wide spectrum of flows acting and interacting at different spatial and temporal scales. Mountain chains can modify synoptic-scale flows, trigger mesoscale winds and alter the properties of turbulent fluxes. The inhomogeneity of the terrain and land cover modifies the mesoscale fluxes and influence the spatio-temporal evolution of microscale fluxes [1,2]. The atmospheric circulation strongly influences pollutant concentrations, which depend on emissions, dispersion, deposition and chemical transformation phenomena. The summer convective conditions can enhance the pollutant dispersion, sometimes transporting pollution to high altitudes, whereas during winter, stable atmospheric conditions and inversion temperature profiles can strongly reduce dispersion phenomena [6]
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