This report presents the first (Tier 1) 9-km global nature run of the CO₂ Human Emission (CHE) project. The main purpose of this simulation is to provide boundary conditions to the regional simulations to be performed at higher resolution (down to 1 km for urban scale and 100m for point source simulations) as part of the CHE library of simulations. This library of simulations will allow to conduct observing system simulation experiments (OSSEs) to the CHE consortium and wider scientific community. The configuration of the Tier 1 nature run is the same as that of the Copernicus Atmosphere Monitoring Service (CAMS) CO₂ forecast with the Integrated Forecasting System (IFS) at the European Centre for Medium-range Weather Forecasts (ECMWF). This facilitates the timely delivery of the Tier 1 CHE nature run.

The meteorological aspects of the nature run have not been evaluated in this report because they are consistent with the ECMWF analysis and short-range forecasts which have been extensively investigated and evaluated in various ECMWF Technical Memoranda.

This report illustrates the capability of the nature run to represent the variability of CO₂ at different scales from seasonal and inter-hemispheric to synoptic, local and diurnal. Comparison with in situ and total column data shows a realistic variability of CO₂. The systematic errors are in the range of 1 to 2ppm for the total column on monthly timescales and less than 1ppm on global scales at baseline sites. These systematic errors are associated in large part to the prescribed and modelled surface fluxes which are not constrained by observations. The seasonal cycle, synoptic and diurnal cycle are all within the range of observed variability recorded by surface and total column observations. A preliminary evaluation of column-averaged CH₄ and CO also show a realistic representation of variability at synoptic and diurnal time-sales. An improved Tier 2 global nature run will be provided at the end of 2019 with improved fluxes and the newest NWP model version. In addition to the upgrades in the 9-km resolution nature run, the Tier 2 simulations will also be done using an ensemble approach to include information on uncertainties in the fluxes and transport.