Articles | Volume 16, issue 21
Atmos. Chem. Phys., 16, 14003–14024, 2016
Atmos. Chem. Phys., 16, 14003–14024, 2016

Research article 11 Nov 2016

Research article | 11 Nov 2016

Seasonal variability of stratospheric methane: implications for constraining tropospheric methane budgets using total column observations

Katherine M. Saad1, Debra Wunch1,2, Nicholas M. Deutscher3,4, David W. T. Griffith3, Frank Hase5, Martine De Mazière6, Justus Notholt4, David F. Pollard7, Coleen M. Roehl1, Matthias Schneider5, Ralf Sussmann8, Thorsten Warneke4, and Paul O. Wennberg1 Katherine M. Saad et al.
  • 1Environmental Science and Engineering, California Institute of Technology, Pasadena, California, USA
  • 2Department of Physics, University of Toronto, Toronto, Ontario, Canada
  • 3Center for Atmospheric Chemistry, School of Chemistry, University of Wollongong, Wollongong, NSW, Australia
  • 4Institute of Environmental Physics, University of Bremen, Bremen, Germany
  • 5Institute for Meteorology and Climate Research, Karlsruhe Institute of Technology, IMK-ASF, Karlsruhe, Germany
  • 6Royal Belgian Institute for Space Aeronomy, Brussels, Belgium
  • 7National Institute of Water and Atmospheric Research, Omakau, New Zealand
  • 8Institute for Meteorology and Climate Research, Karlsruhe Institute of Technology, IMK-IFU, Garmisch-Partenkirchen, Germany

Abstract. Global and regional methane budgets are markedly uncertain. Conventionally, estimates of methane sources are derived by bridging emissions inventories with atmospheric observations employing chemical transport models. The accuracy of this approach requires correctly simulating advection and chemical loss such that modeled methane concentrations scale with surface fluxes. When total column measurements are assimilated into this framework, modeled stratospheric methane introduces additional potential for error. To evaluate the impact of such errors, we compare Total Carbon Column Observing Network (TCCON) and GEOS-Chem total and tropospheric column-averaged dry-air mole fractions of methane. We find that the model's stratospheric contribution to the total column is insensitive to perturbations to the seasonality or distribution of tropospheric emissions or loss. In the Northern Hemisphere, we identify disagreement between the measured and modeled stratospheric contribution, which increases as the tropopause altitude decreases, and a temporal phase lag in the model's tropospheric seasonality driven by transport errors. Within the context of GEOS-Chem, we find that the errors in tropospheric advection partially compensate for the stratospheric methane errors, masking inconsistencies between the modeled and measured tropospheric methane. These seasonally varying errors alias into source attributions resulting from model inversions. In particular, we suggest that the tropospheric phase lag error leads to large misdiagnoses of wetland emissions in the high latitudes of the Northern Hemisphere.

Short summary
Current approaches to constrain the global methane budget assimilate total column measurements into models, but model biases can impact results. We use tropospheric methane columns to evaluate model transport errors and identify a seasonal time lag in the Northern Hemisphere troposphere masked by stratospheric compensating effects. We find systematic biases in the stratosphere will alias into model-derived emissions estimates, especially those in the high Northern latitudes that vary seasonally.
Final-revised paper