The Australian Community Climate and Earth System Simulator (ACCESS) is a coupled landocean-atmosphere model being developed for a wide variety of applications. One key area of research with ACCESS is the carbon cycle, in particular atmospheric carbon dioxide (CO2) and methane (CH4) concentrations resulting from prescribed surface fluxes. ACCESS derives its atmospheric model from the UK Met Office Unified model (UM), which included both a specific atmospheric CO2 tracer (used for climate-carbon feedback studies) as well as other generic atmospheric tracers, developed for stratospheric transport studies. For our applications, the CO2 and generic tracers within the UM required several modifications. The use of the CO2 tracer was made more flexible, with less assumed coupling with other model components (e.g. radiation). For atmospheric tracers, associated surface fluxes were added for the first twenty tracers, and a methodology to more easily initialize the tracers was implemented. To participate in a model intercomparison, new routines were added to simulate the atmospheric loss of CH4 and methyl chloroform (MCF) without explicitly modelling chemistry, as well as radioactive decay of radon. Investigations of the tracers revealed that the atmospheric mass was not being conserved, and a simple mass mixing ratio fixer has been developed to ensure tracer conservation, taking into account any global mixing ratio change resulting from surface fluxes. The new capabilities in ACCESS are used to investigate the model transport using atmospheric CO2 concentrations, indicating that the inter-hemispheric mixing may be too slow and that the night-time and winter stable boundary layers apparently mix too slowly, particularly using the “sharpest” stable boundary layer parameterization. Analysis of resulting CH4 concentrations for the model inter-comparison, revealed a problem with the transport in the top level of the model, which impacted the atmospheric concentrations throughout the atmosphere. Case studies using various model set-ups indicated that a simple fix is to set the top level equal to the neighbouring lower level; however, further investigation into the mixing at the top of the atmosphere should be conducted to fully diagnose the problem.
Collection(s) and Series: CAWCR Technical Report- No. 035
Format: Digital (Free), Hard copy (ill., charts, maps)