Articles | Volume 11, issue 24
Atmos. Chem. Phys., 11, 12813–12837, 2011
Atmos. Chem. Phys., 11, 12813–12837, 2011

Research article 19 Dec 2011

Research article | 19 Dec 2011

TransCom model simulations of CH4 and related species: linking transport, surface flux and chemical loss with CH4 variability in the troposphere and lower stratosphere

P. K. Patra1, S. Houweling2,3, M. Krol2,3,4, P. Bousquet5, D. Belikov6, D. Bergmann7, H. Bian8, P. Cameron-Smith7, M. P. Chipperfield9, K. Corbin10, A. Fortems-Cheiney5, A. Fraser11, E. Gloor9, P. Hess12, A. Ito1,6, S. R. Kawa8, R. M. Law10, Z. Loh10, S. Maksyutov6, L. Meng12,*, P. I. Palmer11, R. G. Prinn13, M. Rigby13, R. Saito1, and C. Wilson9 P. K. Patra et al.
  • 1Research Institute for Global Change/JAMSTEC, 3173-25 Show-machi, Yokohama, 2360001, Japan
  • 2SRON Netherlands Institute for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands
  • 3Institute for Marine and Atmospheric Research Utrecht (IMAU), Princetonplein 5, 3584 CC Utrecht, The Netherlands
  • 4Wageningen University and Research Centre, Droevendaalsesteeg 4, 6708 PB Wageningen, The Netherlands
  • 5Universite de Versailles Saint Quentin en Yvelines (UVSQ), GIF sur Yvette, France
  • 6Center for Global Environmental Research, National Institute for Environmental Studies, 16-2 Onogawa, Tsukuba, Ibaraki, 305-8506, Japan
  • 7Atmospheric, Earth, and Energy Division, Lawrence Livermore National Laboratory, 7000 East Avenue, Livermore, CA 94550, USA
  • 8Goddard Earth Sciences and Technology Center, NASA Goddard Space Flight Center, Code 613.3, Greenbelt, MD 20771, USA
  • 9Institute for Climate and Atmospheric Science, School of Earth and Environment, University of Leeds, Leeds, LS2 9JT, UK
  • 10Centre for Australian Weather and Climate Research, CSIRO Marine and Atmospheric Research, 107–121 Station St., Aspendale, VIC 3195, Australia
  • 11School of Geosciences, University of Edinburgh, King's Buildings, West Mains Road, Edinburgh, EH9 3JN, UK
  • 12Cornell University, 2140 Snee Hall, Ithaca, NY 14850, USA
  • 13Center for Global Change Science, Building 54, Massachusetts Institute of Technology, Cambridge, MA, 02139, USA
  • *now at: Department of Geography and Environmental Studies Program, Western Michigan University, Kalamazoo, MI 49008, USA

Abstract. A chemistry-transport model (CTM) intercomparison experiment (TransCom-CH4) 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. All but one model transports were driven by reanalysis products from 3 different meteorological agencies. The transport and removal of CH4 in six different emission scenarios were simulated, with net global emissions of 513 ± 9 and 514 ± 14 Tg CH4 yr−1 for the 1990s and 2000s, respectively. Additionally, sulfur hexafluoride (SF6) was simulated to check the interhemispheric transport, radon (222Rn) to check the subgrid scale transport, and methyl chloroform (CH3CCl3) to check the chemical removal by the tropospheric hydroxyl radical (OH). The results are compared to monthly or annual mean time series of CH4, SF6 and CH3CCl3 measurements from 8 selected background sites, and to satellite observations of CH4 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 (IAVs) and interhemispheric (IH) gradients at the surface sites for SF6, CH3CCl3 and CH4. The vertical gradients of all tracers between the surface and the upper troposphere are consistent within the models, revealing vertical transport differences between models. An average IH exchange time of 1.39 ± 0.18 yr is derived from SF6 time series. Sensitivity simulations suggest that the estimated trends in exchange time, over the period of 1996–2007, are caused by a change of SF6 emissions towards the tropics. Using six sets of emission scenarios, we show that the decadal average CH4 growth rate likely reached equilibrium in the early 2000s due to the flattening of anthropogenic emission growth since the late 1990s. Up to 60% of the IAVs in the observed CH4 concentrations can be explained by accounting for the IAVs in emissions, from biomass burning and wetlands, as well as meteorology in the forward models. The modeled CH4 budget is shown to depend strongly on the troposphere-stratosphere exchange rate and thus on the model's vertical grid structure and circulation in the lower stratosphere. The 15-model median CH4 and CH3CCl3 atmospheric lifetimes are estimated to be 9.99 ± 0.08 and 4.61 ± 0.13 yr, respectively, with little IAV due to transport and temperature.

Final-revised paper