Methane simulations at Cape Grim to assess methane flux estimates for South East Australia
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Date
2011-11-15
Journal Title
Journal ISSN
Volume Title
Publisher
Centre for Australian Weather and Climate Research
Abstract
A transport model intercomparison for methane (TransCom-CH4) has been run involving twelve
models (Patra et al., 2011). We contributed simulations using two climate models, CCAM and
ACCESS. The CCAM simulations were nudged to NCEP analysed meteorology, which allows
simulated atmospheric concentrations to be compared to observations on synoptic timescales.
The ACCESS simulations were forced only with observed sea surface temperatures and are
consequently not expected to match observed synoptic variations. The TransCom experiment
involved simulating six CH4 tracers (with different prescribed fluxes) along with SF6, radon and
methyl chloroform. We have analysed hourly model output for Cape Grim and find that the
magnitude of the non-baseline signal differs, especially in winter, dependent on the CH4 flux
scenario used. The magnitude of the non-baseline signal also varies between models, although
these differences can be reconciled when methane is scaled by model-simulated radon
concentration. Comparison with observed CH4, also scaled using radon, suggests that the CH4
flux scenario with little or no wetland emissions in winter matches the observations. The
observations also indicate an apparent extra source of CH4 in October-November not seen in the
model simulations. However this appears to be an artefact of this analysis method which
assumes that radon emissions are known (and in this case constant in space and time). We have
found that the discrepancy between model and observations in spring appears to be due to a
poor simulation of radon, rather than CH4. Observed radon shows a larger seasonality than
modelled radon, which suggests that temporal and spatial variations in radon flux need to be
considered. It would also be helpful to understand why the simulated CCAM and ACCESS
radon (and non-baseline CH4) concentrations differ in magnitude. Comparisons with Cape
Grim output from the other participating TransCom-CH4 models may provide some insight.
Description
Keywords
Methane, Simulation, Australia, Meteorology, Environmental transport, Ecological concentration, Ambient temperature, Radon
Citation
Loh, Z. M., Law, R. M., Corbin, K. D., Steele, L. P., Krummel, P. B., Fraser, P. J., & Zahorowski, W. (2011). Methane simulations at Cape Grim to assess methane flux estimates for South East Australia. Paper presented at the 5th CAWCR Annual workshop, Melbourne, Victoria, 15-16 November 2011.