Methane variability in the upper troposphere and lower stratosphere and their relevance for emission inversions constrained by satellite observations

Observation-based constraints on methane (CH4) surface emissions can be obtained from measurements of atmospheric CH4 concentrations. Small variations in mixing ratio can be traced back to variations in surface fluxes via inverse modeling. However, surface emissions are not the only source of CH4 concentration variations in the atmosphere. Atmospheric variability in CH4 – at all altitudes – has to be well simulated by the forward model that is used in inversions based on satellite-borne observations of the total column mixing ratio (CMR) of CH4. In this study we examine the CH4 variability in the upper troposphere and lower stratosphere (UTLS region) between ~6-25 km altitude. By comparing global chemistry-transport model simulations with in-situ aircraft observations of CH4 it is shown that in the UTLS region over Europe variability in winter is predominantly controlled by atmospheric processes. In summer, and at lower latitudes, CH4 surface emissions are effectively transported to the UTLS region by convection and, subsequently, quasi-horizontally transported over large distance. Simulated CH4 variations in the UTLS critically depend on timing, location and the strength of convective uplift in combination with the assumed distribution and seasonal variability of the emissions. Other processes contributing to the observed variability in the UTLS are downward transport from the stratosphere, variations in tropopause height and long-range transport. Satellite-based CH4 vertical profile information has the potential to reduce model uncertainties and provide additional constraints for CH4 emission estimates based on variations in total CMR.