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

Abstract

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.