Abstract
Abstract. This study uses two climate models and six scenarios of prescribed methane emissions to compare modelled and observed atmospheric methane between 1994 and 2007, for Cape Grim, Australia (40.7° S, 144.7° E). The model simulations follow the TransCom-CH4 protocol and use the Australian Community Climate and Earth System Simulator (ACCESS) and the CSIRO Conformal-Cubic Atmospheric Model (CCAM). Radon is also simulated and used to reduce the impact of transport differences between the models and observations. Comparisons are made for air samples that have traversed the Australian continent. All six emission scenarios give modelled concentrations that are broadly consistent with those observed. There are three notable mismatches, however. Firstly, scenarios that incorporate interannually varying biomass burning emissions produce anomalously high methane concentrations at Cape Grim at times of large fire events in southeastern Australia, most likely due to the fire methane emissions being unrealistically input into the lowest model level. Secondly, scenarios with wetland methane emissions in the austral winter overestimate methane concentrations at Cape Grim during wintertime while scenarios without winter wetland emissions perform better. Finally, all scenarios fail to represent a~methane source in austral spring implied by the observations. It is possible that the timing of wetland emissions in the scenarios is incorrect with recent satellite measurements suggesting an austral spring (September–October–November), rather than winter, maximum for wetland emissions.
Highlights
Methane (CH4) is an important greenhouse gas whose atmospheric concentration has more than doubled since the 18th century (MacFarling Meure et al, 2006), with considerable variations in its growth rate over recent decades (Rigby et al, 2008; Dlugokencky et al, 2009; Sussmann et al, 2012)
Both features mentioned above are most obvious in the Cubic Atmospheric Model (CCAM) results (Fig. 4b), where the model predicts exceptionally high methane concentrations in January and February 2003 for those tracers that include interannually varying biomass burning (BB, which includes interannually varying biomass burning emissions (WLBB) and EXTRA)
Forward modelling of a range of methane flux scenarios gives us the opportunity to compare those results with the measurement record at Cape Grim for the same period, offering insights into which of the flux scenarios appear to be most representative of southeastern Australian methane www.atmos-chem-phys.net/15/305/2015/
Summary
Methane (CH4) is an important greenhouse gas whose atmospheric concentration has more than doubled since the 18th century (MacFarling Meure et al, 2006), with considerable variations in its growth rate over recent decades (Rigby et al, 2008; Dlugokencky et al, 2009; Sussmann et al, 2012). To investigate non-baseline events, this study uses both observations at Cape Grim and forward model simulations with prescribed emissions. Observations made at Cape Grim are characterized by baseline periods when the wind is from the southwest (typically 30 % of the time), and air parcels have had long trajectories over the Southern Ocean. During these periods, concentrations are reasonably steady displaying underlying seasonal variations and long-term trends. The smooth curve fitted to the baseline data is subtracted from all CH4 observations to give a time series of residuals In most cases it is only the residuals from hours that have been flagged as non-baseline that are used for comparison with the model simulations. Other details can be found online in the TransCom-CH4 protocol (Patra et al, 2010)
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