A Fixed-lag Ensemble Kalman Smoother to Estimate Surface Carbon Dioxide Exchange

The NOAA ESRL carbon cycle group uses an ensemble data assimilation system to estimate multiple years of emissions and uptake (fluxes) of carbon dioxide at the Earth's surface. Thereto, atmospheric observations of carbon dioxide mixing ratios are assimilated in a weekly cycle, constraining five weeks of past surface fluxes. This five week smoother window is needed because the time between the release of carbon dioxide at the surface and downwind sampling of the resulting signal at our relatively sparse observation network (~100 per cycle) is determined by slow atmospheric mixing processes. The `observation operator' for our problem thus links surface flux variations to atmospheric mixing ratios, and is a full tracer transport model of the global atmosphere with nested grids to focus on regions of special interest. We optimize a set of linear parameters (~1000 per cycle) that control the behavior of simplified `flux modules'. These contain physical descriptions of surface exchange from the oceans, biosphere, fires, and fossil fuel burning. The optimal set of parameters estimated in the assimilation, combined with the flux modules, yields a high resolution multiyear reanalysis of surface exchange for scientific studies. In addition to showing results from our assimilations, we will discuss some particularities of our system such as the lack of an appropriate dynamical model, the impracticality of geographical localization, and the resulting need for many (>100) ensemble members.