Influence of complex terrain and anthropogenic emissions on atmospheric CO2 patterns-a high-resolution numerical analysis

The accuracy of regional or mesoscale carbon budgeting by means of inverse modelling depends strongly on the ability of the atmospheric model to capture relevant atmospheric transport processes. In order to analyze the influence of terrain-induced flow dynamics and intense local anthropogenic emissions, we present high-resolution (forward) simulations of spatio-temporal CO2 variations using a recent biosphere-atmosphere model. The selected region is characterized by complex terrain and both rural and densely populated areas. The results indicate that, in situations with weak synoptic forcing, the nocturnal near-surface CO2 distribution is strongly affected by terrain-induced turbulent kinetic energy (TKE) above mountain ridges and by local convergent downslope winds. By increasing the grid spacing from approximate to 1 to approximate to 3 km, we show that, due to the smoothed model topography, the atmospheric flow causing the CO2 heterogeneity cannot be resolved any more. Finally, we quantify the influence of intense anthropogenic CO2 sources on atmospheric CO2 concentrations. A significant anthropogenic signal can be identified around and downstream of industrial and urban areas, especially in the morning but also within a well-mixed planetary boundary layer in the daytime. The results provide valuable information for including non-background CO2 observations in mesoscale inverse modelling studies using coarser resolutions than in this study.