Simulation of atmospheric CO₂ over Europe and western Siberia using the regional scale model REMO

Abstract

The spatial distribution and the temporal variability of atmospheric CO₂ over Europe and western Siberia are investigated using the regional atmospheric model, REMO. The model, of typical horizontal resolution 50 km, is part of a nested modelling framework that has been established as a concerted action during the EUROSIBERIAN CARBONFLUX project. In REMO, the transport of CO₂ is simulated together with climate variables, which offers the possibility of calculating at each time step the land atmosphere CO₂ fluxes as driven by the modelled meteorology. The uptake of CO₂ by photosynthesis is calculated using a light use efficiency formulation, where the absorbed photosynthetically active solar radiation is inferred from satellite measurements. The release of CO₂ from plant and soil respiration is driven by the simulated climate and assumed to be in equilibrium with photosynthesis over the course of one year. Fossil CO₂ emissions and air–sea fluxes within the model domain are prescribed, whereas the influence of sources outside the model domain is computed from as a boundary condition CO₂ fields determined a global transport model. The modelling results are compared against pointwise eddy covariance fluxes, and against atmospheric CO₂ records. We show that a necessary condition to simulate realistically the variability of atmospheric CO₂ over continental Europe is to account for the diurnal cycle of biospheric exchange. Overall, for the study period of July 1998, REMO realistically simulates the short-term variability of fluxes and of atmospheric mixing ratios. However, the mean CO₂ gradients from western Europe to western Siberia are not correctly reproduced. This latter deficiency points out the key role of boundary conditions in a limited-area model, as well as the need for using more realistic geographic mean patterns of biospheric carbon fluxes.