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Atmospheric Chemistry and Physics An interactive open-access journal of the European Geosciences Union
doi:10.5194/acp-2016-892
© Author(s) 2016. This work is distributed
under the Creative Commons Attribution 3.0 License.
Research article
24 Oct 2016
Review status
A revision of this discussion paper was accepted for the journal Atmospheric Chemistry and Physics (ACP) and is expected to appear here in due course.
Sensitivity of local air quality to the interplay between small- and large-scale circulations: a Large Eddy Simulation study
Tobias Wolf-Grosse1, Igor Esau1, and Joachim Reuder2 1Nansen Environmental and Remote Sensing Center, Bergen, NO-5006, Norway
2Geophysical Institute, University of Bergen, Bergen, NO-5007, Norway
Abstract. We present an analysis of the interaction between a topographically forced recirculation of the large-scale flow above an urbanized coastal valley and a local breeze-like circulation. We found that such an interaction can enhance the stagnation inside the valley under cold air pool conditions. Analysis of a large dataset of air quality measurements in Bergen, Norway, revealed that the most extreme cases of recurring winter-time air pollution episodes are usually accompanied by an increased wind speed above the valley. The 10 m ERA-Interim wind-speed distribution against local NO2 measurements had a maximum at 3 m s−1 in contrast to a monotonic decrease, as it would be expected from theory developed for flat, homogenous surfaces. We conducted a set of 16 Large Eddy Simulation (LES) experiments with the PALM model to account for the realistic orography of the mountains surrounding the city. The simulations were driven by the typical circulation above the valley during observed air pollution episodes, and a heterogeneous combination of constant temperatures over water and a constant negative sensible surface heat flux over land. The LES revealed a strong steering of the local circulation during cold air pool conditions by a land-breeze between the warm sea and the cold land. This breeze circulation is counteracted by a recirculation of the flow above the valley. For certain combinations of both, this leads to a maximum in the local stagnation. Furthermore, a relatively small local water body acted as a barrier for the dispersion of air pollutants along the valley bottom, dispersing them vertically and hence diluting them. These findings have important implications for the air quality predictions over urban areas. Any prediction not resolving these, or similar local dynamic features, might not be able to correctly simulate the dispersion of pollutants in cities.

Citation: Wolf-Grosse, T., Esau, I., and Reuder, J.: Sensitivity of local air quality to the interplay between small- and large-scale circulations: a Large Eddy Simulation study, Atmos. Chem. Phys. Discuss., doi:10.5194/acp-2016-892, in review, 2016.
Tobias Wolf-Grosse et al.
Tobias Wolf-Grosse et al.

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Short summary
In wintertime the Bergen valley suffers from recurrent air pollution episodes caused by local temperature inversions. Under these conditions a balance between the local circulation presented as a flow of air from the cold land surface towards the warm fjord surface and the circulation forced by the flow above the valley can lead to enhanced stagnation inside the valley and hence accumulation of locally emitted pollutants. We have used an advanced modelling technique to study this interaction.
In wintertime the Bergen valley suffers from recurrent air pollution episodes caused by local...
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