Modelling CO 2 weather – why horizontal resolution matters

International audienceClimate change mitigation efforts require information on the current greenhouse gas atmospheric concentrations and their sources and sinks. Carbon dioxide (CO 2) is the most abundant anthropogenic greenhouse gas. Its variability in the atmosphere is modulated by the synergy between weather and CO 2 surface fluxes, often referred to as CO 2 weather. It is interpreted with the help of global or regional numerical transport models, with horizontal resolutions ranging from a few hundreds of kilometres to a few kilometres. Changes in the model horizontal resolution affect not only atmospheric transport but also the representation of topography and surface CO 2 fluxes. This paper assesses the impact of horizontal resolution on the simulated atmospheric CO 2 variability with a numerical weather prediction model. The simulations are performed using the Copernicus Atmosphere Monitoring Service (CAMS) CO 2 forecasting system at different resolutions from 9 to 80 km and are evaluated using in situ atmospheric surface measurements and atmospheric column-mean observations of CO 2 , as well as radiosonde and SYNOP observations of the winds. The results indicate that both diurnal and day-today variability of atmospheric CO 2 are generally better represented at high resolution, as shown by a reduction in the errors in si