Synoptic variations in atmospheric CO2 at Cape Grim: a model intercomparison

A 'TransCom' model intercomparison is used to assess how well synoptic and diurnal variations of carbon dioxide (CO2) and 222Rn (radon) can be modelled at the coastal site, Cape Grim, Australia. Each model was run with prescribed fluxes and forced with analysed meteorology for 2000-2003. Twelve models were chosen for analysis based on each model's ability to differentiate baseline CO2 concentrations from non-baseline CO2 (influenced by regional land fluxes). Analysis focused on non-baseline events during 2002-2003. Radon was better simulated than CO2, indicating that a spatially uniform radon land flux is a reasonable assumption and that regional-scale transport was adequately captured by the models. For both radon and CO2, the ensemble model mean generally performed better than any individual model. Two case studies highlight common problems with the simulations. First, in summer and autumn the Cape Grim observations are sometimes influenced by Tasmanian rather than mainland Australian fluxes. These periods are poorly simulated. Secondly, an event with an urban plume demonstrates how the relatively low spatial resolution of the input CO2 fluxes limits the quality of the simulations. Analysis of periods with below baseline concentration indicates the possible influence of carbon uptake by winter crops in southern mainland Australia.