This sub-project builds on the heritage of the GEMS and MACC rsearch projects to exploit and consolidate a well-documented processing chain that runs from a series of atmospheric observations of CH4 and CO2 to 4D concentration fields and 3D flux fields. A two step approach has been chosen and developed that relies on the ECMWF-devloped 12-h 4D-Var to extract the information about GHG concentrations in an optimal way, and on a multi-year-long variational inversion scheme to extend the analysis to the surface fluxes. Efforts have been made to homogenize the two steps, even though the transition is not seamless. The two-step approach allows the 4D concentrations to be analysed at much higher global spatial resolution (<2,000km2) than the fluxes (>10,000km2). Further it guarantees some consistency with the processing of the other species within MACC-II.
The GHG processing expolits a large variety of observations from the very accurate but sparse surface measurements to the global satellite observations of the vertical column. The constratints on accuracy precision for GHG measurements are much more stringent than for species that have not accumulated in the atmosphere. This explains why the remote sensing of GHGs has been developed only recently, using measurements either in the thermal infa-red spectral domain, with a peak sensitivity in the middle troposphere (AIRS, IASI, TES, GOSAT-TANSO) or in the solar infa-red domain (SCIAMACHY, GOSAT-TANSO, OCO-2) with a more uniform sensitivity to GHGs throughout the atmospheric column, including the boundary layer. The former instrument type is not well adapted to infer GHG surface fluxes as was demonstrated within GEMS, in contrast to the latter, but still has been providing nearly a decade of GHG free-tropospheric columns in some latitudes as we showed in GEMS. Systematic errors at various space and time scales have been hampered the inversion of CO2 surface fluxes from satellite retrievals up to now. They can be caused by uncertainties in the spectroscopy or by aliasing with other atmospheric signals like aerosols. However, the launch of GOSAT in January 2009 and the planned launch of OCO-2 have raised expectations for CO2 flux inversionsbecause these two instruments are the first ones to have been specifically designed for this purpose. They will be fully exploited by this project. It was shown in GEMS and MACC that the issue of biases in CH4 retrievals can be circumvented by anchoring the inversion with surface measurements. This finding has opened the possibility of a decadal monitoring of global CH4 fluxes from SCIAMACHY and GOSAT, in orbit since March 2002 and January 2009, respectively.
Since most of this observational data is not avaialble in real-time, we rely on a delayed mode production, that is far better constrained than a NRT one would be. Also, as part of the transition to an operational service, we will invest much effort in the efficiency and robustness of the processing chain from input data to flux and concentration fields. The service evolution will be guaranteed by testing of the various components of the system, particulary the transport algorithms and emission inventories.
MACC-II GHG will act in parallel and in collaboration with Phase 1 of ESA's Climate Change Initiative. According to the CCI statement of work (Section 2.5: CCI Programme Implementation), Phase 1 of the CCI defines and validates innovative approaches for continuously generating and updating a comprehensive and consistent set of Essential Cliamte Variable global satellite based data products in the long term. The GHG component of the CCI, GHG-cci, will select one retrieval algorrithm for CO2 concentrations and one for CH4 concentrations by August 2012 (Month 24 of GHG-cci) and will subsequently deliver a corresponding database in February 2013 (end of month 30 of GHG-cci) covering a test period of at least one year for CO2 and three years for CH4. The approach in MACC-II GHG differs from GHG-cci in that MACC-II production line has to run continuously from the start of the MACC-II project until the end. Therefore satellite retrievals will be generated within MACC-II GHG with the algorithms already developed within GEMS and MACC and will be delivered internally five months after real time to feed the MACC-II delayed-mode production stream. ESA's GHG-cci products may use diiferent algorithms and will be tested at the end of the MACC-II project in WP GHG.4. The articulation between MACC-II and GHG-cci will be acred for by IUP-UB, ULEIC, SRON, CNRS-LMD, CEA and EC-DG JRC who participate in the two projects.
FP7 GEOLAND2 and its follow-on are another international project with which close collabortaion will be sought. The Land Carbon Service of GEOLAND2 aims to set up pre-operational infrastructures for providing global and regional variables related to the terrestrial carbon cycle, in NRT, including CO2 surface fluxes. The GEOLAND2 approach relies on the assimilation of remotely-sensed observations of the terrestrial surfaces (LAI, FAPAR and soil humidity) within a surface model. It is complementary to the MACC-II approach that exploits measurements of the atmospheric concentrations. Ultimately, the GEOLAND2 CO2 surfacefluxes are expected to serve as prior fluxes for MACC-II GHG of for its own follow-on. This will be tested in WP GHG.4. ECMWF and CEA participate in the two projects and will liaise between them.