Four-dimensional variational data assimilation for inverse modelling of atmospheric methane emissions: method and comparison with synthesis inversion J. F. Meirink1,*, P. Bergamaschi2, and M. C. Krol1,3,4 1Institute for Marine and Atmospheric research Utrecht (IMAU), University of Utrecht, Utrecht, The Netherlands 2European Commission Joint Research Centre, Institute for Environment and Sustainability (EC JRC IES), Ispra (VA), Italy 3Wageningen University and Research Centre (WUR), Wageningen, The Netherlands 4Netherlands Institute for Space Research (SRON), Utrecht, The Netherlands *now at: Royal Netherlands Meteorological Institute (KNMI), De Bilt, The Netherlands
Abstract. A four-dimensional variational (4D-Var) data assimilation system for inverse modelling
of atmospheric methane emissions is presented. The system is based on the TM5
atmospheric transport model. It can be used for assimilating large volumes of
measurements, in particular satellite observations and quasi-continuous in-situ observations,
and at the same time it enables the optimization of a large number of model parameters,
specifically grid-scale emission rates. Furthermore, the variational method allows to estimate uncertainties in
posterior emissions. Here, the system is applied to optimize monthly methane emissions
over a 1-year time window on the basis of surface observations from the
NOAA-ESRL network. The results are rigorously
compared with an analogous inversion by Bergamaschi et al. (2007), which was based
on the traditional synthesis approach. The posterior emissions
as well as their uncertainties obtained in both inversions show
a high degree of consistency. At the same time
we illustrate the advantage of 4D-Var in reducing aggregation errors by
optimizing emissions at the grid scale of the transport model.
The full potential of the assimilation system is exploited in
Meirink et al. (2008), who use satellite observations of column-averaged methane
mixing ratios to optimize emissions at high spatial resolution,
taking advantage of the zooming capability of the TM5 model.
Citation: Meirink, J. F., Bergamaschi, P., and Krol, M. C.: Four-dimensional variational data assimilation for inverse modelling of atmospheric methane emissions: method and comparison with synthesis inversion, Atmos. Chem. Phys., 8, 6341-6353, doi:10.5194/acp-8-6341-2008, 2008.