ACPD Atmospheric Chemistry and Physics Discussions ACPD 1680-7375 Copernicus GmbH Göttingen, Germany 10.5194/acpd-11-18767-2011 TransCom model simulations of CH<sub>4</sub> and related species: linking transport, surface flux and chemical loss with CH<sub>4</sub> variability in the troposphere and lower stratosphere Patra P. K. 1 Houweling S. 2 3 Krol M. 2 3 4 Bousquet P. 5 Belikov D. 6 Bergmann D. 7 Bian H. 8 Cameron-Smith P. 7 Chipperfield M. P. 9 Corbin K. 10 Fortems-Cheiney A. 5 Fraser A. 11 Gloor E. 9 Hess P. 12 Ito A. 1 6 Kawa S. R. 8 Law R. M. 10 Loh Z. 10 Maksyutov S. 6 Meng L. 12 Palmer P. I. 11 Prinn R. G. 13 Rigby M. 13 Saito R. 1 Wilson C. 9 Research Institute for Global Change/JAMSTEC, 3173-25 Show-machi, Yokohama, 2360001, Japan SRON Netherlands Institute for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands Institute for Marine and Atmospheric Research Utrecht (IMAU), Princetonplein 5, 3584 CC Utrecht, The Netherlands Wageningen University and Research Centre, Droevendaalsesteeg 4, 6708 PB Wageningen, The Netherlands Universite de Versailles Saint Quentin en Yvelines (UVSQ), GIF sur YVETTE, France Center for Global Environmental Research, National Institute for Environmental Studies, 16-2 Onogawa, Tsukuba, Ibaraki, 305-8506, Japan Atmospheric, Earth, and Energy Division, Lawrence Livermore National Laboratory, 7000 East Avenue, Livermore, CA 94550, USA Goddard Earth Sciences and Technology Center, NASA Goddard Space Flight Center, Code 613.3, Greenbelt, MD 20771, USA Institute for Climate and Atmospheric Science, School of Earth and Environment, niversity of Leeds, Leeds, LS2 9JT, UK Centre for Australian Weather and Climate Research, CSIRO Marine and Atmospheric Research, 107–121 Station St., Aspendale, VIC 3195, Australia School of Geosciences, University of Edinburgh, King's Buildings, West Mains Road, Edinburgh, EH9 3JN, UK Cornell University, 2140 Snee Hall, Ithaca, NY 14850, USA Center for Global Change Science, Building 54, Massachusetts Institute of Technology, Cambridge, MA, 02139, USA 01 07 2011 11 7 18767 18821 This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. This article is available from http://www.atmos-chem-phys-discuss.net/11/18767/2011/acpd-11-18767-2011.html The full text article is available as a PDF file from http://www.atmos-chem-phys-discuss.net/11/18767/2011/acpd-11-18767-2011.pdf

A transport model intercomparison experiment (TransCom-CH<sub>4</sub>) has been designed to investigate the roles of surface emissions, transport and chemical loss in simulating the global methane distribution. Model simulations were conducted using twelve models and four model variants and results were archived for the period of 1990–2007. The transport and removal of six CH<sub>4</sub> tracers with different emission scenarios were simulated, with net global emissions of 513 ± 9 and 514 ± 14 Tg CH<sub>4</sub> yr<sup>−1</sup> for the 1990s and 2000s, respectively. Additionally, sulfur hexafluoride (SF<sub>6</sub>) was simulated to check the interhemispheric transport, radon (<sup>222</sup>Rn) to check the subgrid scale transport, and methyl chloroform (CH<sub>3</sub>CCl<sub>3</sub>) to check the chemical removal by the tropospheric hydroxyl radical (OH). The results are compared to monthly or annual mean time series of CH<sub>4</sub>, SF<sub>6</sub> and CH<sub>3</sub>CCl<sub>3</sub> measurements from 8 selected background sites, and to satellite observations of CH<sub>4</sub> in the upper troposphere and stratosphere. Most models adequately capture the vertical gradients in the stratosphere, the average long-term trends, seasonal cycles, interannual variations and interhemispheric gradients at the surface sites for SF<sub>6</sub>, CH<sub>3</sub>CCl<sub>3</sub> and CH<sub>4</sub>. The vertical gradients of all tracers between the surface and the upper troposphere are consistent within the models, revealing vertical transport differences between models. We find that the interhemispheric exchange rate (1.39 ± 0.18 yr) derived from SF<sub>6</sub> is faster by about 11 % in the 2000s compared to the 1990s. Up to 60 % of the interannual variations in the forward CH<sub>4</sub> simulations can be explained by accounting for the interannual variations in emissions from biomass burning and wetlands. We also show that the decadal average growth rate likely reached equilibrium in the early 2000s due to the flattening of anthropogenic emission growth since the late 1990s. The modeled CH<sub>4</sub> budget is shown to depend strongly on the troposphere-stratosphere exchange rate and thus to the model's vertical grid structure and circulation in the lower stratosphere. The 15-model median CH<sub>4</sub> and CH<sub>3</sub>CCl<sub>3</sub> atmospheric lifetimes are estimated to be 9.99 ± 0.08 and 4.61 ± 0.13 yr, respectively, with little interannual variability due to transport and temperature as noted by the ± 1 σ.

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