Sensitivity of local air quality to the interplay between small- and
large-scale circulations: a Large Eddy Simulation study
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.