Assessing temporal clear-sky errors in assimilation of satellite CO₂ retrievals using a global transport model

Abstract. The Orbiting Carbon Observatory (OCO) and the Greenhouse gases Observing SATellite (GOSAT) will make global observations of the total column dry-air mole fraction of atmospheric CO₂ (X_CO2) starting in 2008. Although satellites have global coverage, X_CO2 retrievals will be made only a few times each month over a given location and will only be sampled in clear conditions. Modelers will use X_CO2 in atmospheric inversions to estimate carbon sources and sinks; however, if satellite measurements are used to represent temporal averages, modelers may incur temporal sampling errors. We investigate these errors using a global transport model. Temporal sampling errors vary with time and location, exhibit spatially coherent patterns, and are greatest over land and during summer. These errors often exceed 1 ppm and must be addressed in a data assimilation system by correct simulation of synoptic CO₂ variations associated with cloud systems.