Evaluating Multi-Scale CO2 Exchange of the Tropical and Temperate Troposphere in the ECMWF Integrated Forecasting System Using Observations and Large-Eddy Simulations

The diurnal cycle and vertical distribution of atmospheric CO2 in the lower troposphere is strongly influenced by multiple surface and atmospheric processes across different scales. For example, photosynthesis and CO2-assimilation at single leaves or the tree canopy, to the vertical and horizontal (turbulent) transport at scales of the boundary layer, clouds and large weather systems. Accurately capturing these multi-scale dynamics is essential for improving global estimates of the CO2 exchange in the lower troposphere. In this study, we evaluate the relative contribution of these multi-scale dynamics to the total CO2 exchange in the lower troposphere by examining how these processes are represented within the Integrated Forecasting System (IFS) of the ECMWF. More specifically, we assess their representation for three IFS horizontal resolutions: 25 km, 9 km (current operational resolution) and 4.4 km. We construct a framework with which we evaluate the CO2 budget in IFS through comprehensive sets of observations, simulations from a coupled surface-atmosphere mixed-layer model (CLASS) and large-eddy simulations (DALES). In this budget, we characterise tendencies of atmospheric CO2 into respective contributions from the different scales: (turbulent) diffusion, convective processes and large-scale dynamics. We focus on two comprehensive case studies across three ecosystems: the tropical clear-to-shallow moist convective Amazon rainforest and the temperate mid-latitudes with influences from large weather systems: the Cabauw (grass), and Loobos (pine forest) sites located in The Netherlands. Initial results reveal that the IFS accurately represents the vertical structure and diurnal evolution of the state variables (temperature, humidity and wind) within the lower troposphere for both case studies at the different model resolutions. However, larger uncertainties arise in the CO2 exchange, especially near the surface, as a result of discrepancies during the morning transition. Ongoing work focuses on investigating the underlying multi-scale dynamics to address these and other discrepancies in the CO2 exchange of the lower troposphere within IFS.