Offline Atmospheric Transport on a Global Mesh of Hexagons

We present a new version of the offline transport model from the LMDz atmospheric general circulation model. It paves the globe with hexagons and 12 pentagons of similar surface areas rather than with regular longitude-latitude rectangles. It is available in a complete, nonlinear version and in a linearized configuration for use in variational atmospheric inversions. It runs on graphics processing units like the previous version. The previous advection approach and physical parameterizations have been kept while the code has been restructured for better numerical efficiency. The change of mesh was made necessary by the evolution of the parent LMDz model, but the technical and scientific evaluation of the new version with a roughly constant number of cells shows some interesting advantages. This evaluation is based on an 11-year simulation of sulfur hexafluoride and on a 10-year atmospheric inversion assimilating column-averaged dry air mole fractions of carbon dioxide (CO₂) retrieved from measurements of NASA's second Orbiting Carbon Observatory. As it is used in variational inversion and at 90-km resolution with 79 layers in the vertical, we find that the new offline model is twice as fast. Further, it shows improved interhemispheric transport, some small improvements in terms of inferred (posterior) atmospheric concentrations and small differences in terms of inferred surface fluxes compared to the previous version. These assets allowed it to be commissioned in its 90-km configuration for the operational CO₂ inversions of the European Copernicus Atmosphere Monitoring Service.