Simulating the atmospheric CO<sub>2</sub> concentration across the heterogeneous landscape of Denmark using a coupled atmosphere-biosphere mesoscale model system

Abstract

Abstract. Surface heterogeneity can be challenging to fully encompass by modelling studies of CO 2 surface exchanges, especially when it comes to land-sea boarders. The relative importance of the marine and the terrestrial surface fluxes on the atmospheric CO 2 concentration were examined by developing a mesoscale modelling framework capable of simulating surface exchanges at a high spatiotemporal resolution. This study exploits the complexity of the Danish landscape and the many land-sea boarders found along the nation's 7,300 km of coastline. 5An atmospheric transport model, DEHM, with a horizontal spatial resolution of 5.6 km × 5.6 km constituted the basis of the modelling framework. A mechanistic biosphere model, SPA, was coupled to DEHM in order to simulate terrestrial surface exchanges applying a tiling approach with the seven most dominant land-use classes in Denmark to account for sub-grid heterogeneity. Detailed surface fields of pCO 2 were used to simulate the air-sea CO 2 exchange for the study region. Monthly mean diurnal cycles of surface water pCO 2 were imposed onto these, in order to include short-term variability in surface water 10 pCO 2 .The Danish biospheric fluxes simulated by the SPA-DEHM model system experienced an east-west gradient corresponding to the distribution of the land-use classes and their biological activity. The relative importance of the seven land-use classes varied throughout the year according to their individual growth patterns. A major contribution to the monthly net ecosystem exchange (NEE) through all seasons came from grasslands, while the influence from croplands increased from March to July. 15Grasslands had, on an annual basis, the largest impact on the biospheric net uptake with -1,423 GgC yr Impact from Roskilde fjord was difficult to detect in the simulated CO 2 concentrations. These difficulties in simulating the local impact from the Roskilde Fjord might arise from (i) the fjord not being adequately resolved in the constructed model system, 5(ii) the lack of a realistic representation of the surface water pCO 2 dynamics, or (iii) that the fjord is not in the simulated footprint and only had a modest impact on the simulated atmospheric CO 2 at the Risø tall tower.