The importance of transport model uncertainties for the estimation of CO<sub>2</sub> sources and sinks using satellite measurements

Abstract

Abstract. This study presents a synthetic model intercomparison to investigate the importance of transport model errors for estimating the sources and sinks of CO 2 using satellite measurements. The experiments were designed for testing the potential performance of the proposed CO 2 lidar A-SCOPE, but also apply to other space borne missions that monitor total column CO 2 . The participating transport models IFS, LMDZ, TM3, and TM5 were run in forward and inverse mode using common a priori CO 2 fluxes and initial concentrations. Forward simulations of column averaged CO 2 (xCO 2 ) mixing ratios vary between the models by σ =0.5 ppm over the continents and σ =0.27 ppm over the oceans. Despite the fact that the models agree on average on the sub-ppm level, these modest differences nevertheless lead to significant discrepancies in the inverted fluxes of 0.1 PgC/yr per 10 6 km 2 over land and 0.03 PgC/yr per 10 6 km 2 over the ocean. These transport model induced flux uncertainties exceed the target requirement that was formulated for the A-SCOPE mission of 0.02 PgC/yr per 10 6 km 2 , and could also limit the overall performance of other CO 2 missions such as GOSAT. A variable, but overall encouraging agreement is found in comparison with FTS Correspondence to: S. Houweling (s.houweling@sron.nl) measurements at Park Falls, Darwin, Spitsbergen, and Bremen, although systematic differences are found exceeding the 0.5 ppm level. Because of this, our estimate of the impact of transport model uncerainty is likely to be conservative. It is concluded that to make use of the remote sensing technique for quantifying the sources and sinks of CO 2 not only requires highly accurate satellite instruments, but also puts stringent requirements on the performance of atmospheric transport models. Improving the accuracy of these models should receive high priority, which calls for a closer collaboration between experts in atmospheric dynamics and tracer transport.

Diagnostic methods for atmospheric inversions of long-lived greenhouse gases

“…3.2) that could potentially be achieved by assimilating more observations over or downwind from poorly constrained regions as well as the effects of a more extensive observational network on the estimated spatial and temporal variability of fluxes (e.g., Butler et al, 2010;Saeki et al, 2013b;Kadygrov et al, 2015;Jiang et al, 2014;Peters et al, 2010). They can also be used to determine the value of episodic versus continuous observations (e.g., Peters et al, 2010). These sensitivity tests can also determine whether strong fluxes in some regions, such as the "dipoles" discussed in Sect.…”

“…The impact of the choice of flux regions, model grid resolution, model grid nesting, or model time step can all be explored (e.g., Rivier et al, 2010;Göckede et al, 2010a;Kim et al, 2014;Peters et al, 2010).…”

“…Inversions using alternative inventories or process-based models with different spatial and seasonal flux patterns as priors can be compared in terms of the spatial and temporal distributions of the posterior fluxes to assess the robustness of flux estimates (e.g., Kim et al, 2011;Göckede et al, 2010b;Bergamaschi et al, 2015;Corazza et al, 2011;Peters et al, 2010).…”

“…The impact of the choice of an estimate can be explored through sensitivity tests (e.g., Peylin et al, 2011;Peters et al, 2010).…”

“…In such studies, an offset is added to specific observations, and the results of the control inversion and the inversion with the offset can be compared to determine the effect of potential biases on the posterior flux field (e.g., Peters et al, 2010;Masarie et al, 2011).…”