This report defines the necessary components for a multi-scale and multi-species data assimilation (DA) system that targets anthropogenic CO2 emissions. This DA system will use multiple streams of observations, including satellite observations. Out of the many viable options to serve as the basis for such a system on the global scale, we see a hybrid 4d-VAR-ensemble approach, implemented in an online transport model, and operated within a Numerical Weather Prediction environment, as a fundamental building block. On top of this a DA system should use multiple tracers, be adoptable to long- and short windows, and optimize both the atmospheric state as well as surface fluxes.
Such a system does currently not yet exist, and we recommend a number of concrete research and development needs, including to:
- Allow mass-conserving transport in the operational Integrated Forecast System (IFS) of ECMWF
- Improve the treatment of background covariances and building of long-window information transfer in the IFS DA system
- Develop Fossil Fuel DA models and capacity (FFDAS) for global and regional scales
- Expand Biospheric Carbon Cycle DA models and capacity (CCDAS) for global and regional scales
- Invest in multi-tracer transport+source modelling on all scales
- Improve the seamless coupling of regional DA systems to the global IFS
- Investigate plume-based methods for fast DA, also in a plume-in-grid approach
Many of these developments are ongoing in the community and to facilitate their uptake in an MVS for anthropogenic CO2, we see an important role for:
- a prototype MVS built around the IFS and focusing on available high-resolution CO and NO2 satellite data
- a multi-scale integration tool that allows local- and regional scale DA systems to feed into the global analyses.