Atmospheric CO₂ modeling at the regional scale: Application to the CarboEurope Regional Experiment

[1] The CarboEurope Regional Experiment Strategy (CERES) experiment took place in May and June 2005 in France and offers a comprehensive database on atmospheric CO2 and boundary layer processes at the regional scale. One "golden" day of CERES is interpreted with the mesoscale atmospheric model Meso-NH coupled on-line with the Interactions between Soil, Biosphere and Atmosphere, CO2-reactive (ISBA-A-gs) surface scheme, allowing a full interaction of CO2 between the surface and the atmosphere. The rapid diurnal cycle of carbon coupled with water and energy fluxes is parameterized including, e. g., plant assimilation, respiration, anthropogenic emissions, and sea fluxes. During the analyzed day, frequent vertical profiles and aircraft transects revealed high spatial and temporal variabilities of CO2 concentrations within the boundary layer at the regional scale: a 10-ppm gradient of CO2-mixing ratio is observed during the day by the aircraft measurements. The Meso-NH model proved able to simulate very well the CO2 concentration variability as well as the spatial and temporal evolution of the surface fluxes and the boundary layer in the domain. The model is used to explain the CO2 variability as a result of two complementary processes: ( 1) the regional heterogeneity of CO2 surface fluxes related to the land cover ( e. g., winter crops versus a pine forest) and ( 2) the variability of mesoscale circulation across the boundary layer: development of the sea breeze in the western part of the domain and dominating wind flow in the eastern part of the domain.