Inverse modeling of GOSAT-retrieved ratios of total column CHsub4/sub and COsub2/sub for 2009 and 2010

摘要

This study investigates the constraint provided by greenhouse gas measurements from space on surface fluxes. Imperfectknowledge of the light path through the atmosphere, arising from scattering by clouds and aerosols, can heavily bias columnmeasurements retrieved from space. To minimize the impact of such biases, ratios of total column retrieved CH₄and CO₂ (X_ratio) have been used. We apply the ratio inversion method described in Pandey et al. (2015) to retrievals from the Greenhouse Gas Observing SATellite (GOSAT). The ratio inversion method uses the measured X_ratioas a weak constraint on CO₂ fluxes.In contrast, the more common approach of inverting proxy CH₄retrievals (Frankenberg et al., 2005) prescribes atmosphericCO₂fields and optimizes only CH₄fluxes.The TM5-4DVAR inverse modeling system is used to simultaneously optimize the fluxes of CH₄and CO₂for 2009 and2010. The results are compared to proxy inversions using model-derived-XCO₂mixing ratios (XCO₂model) from CarbonTrackerand MACC. The performance of the inverse models is evaluated using aircraft measurements from the HIPPO, CONTRAILand AMAZONICA projects.X_ratioand XCO₂modelare compared with TCCON retrievals to quantify the relative importance of errors in these componentsof the proxy XCH₄retrieval (XCH₄proxy). We find that the retrieval errors in X_ratio(mean = 0.61 %) are generally larger thanthe errors in XCO₂model(mean = 0.24 % and 0.01 % for CarbonTracker and MACC, respectively). On the annual time scale, the CH₄ fluxes from the different satellite inversions are generally in agreement with each other, suggesting that errors inXCO₂modeldo not limit the overall accuracy of the CH₄flux estimates. On the seasonal time scale, however, larger differencesare found due to uncertainties in XCO₂model, particularly over Australia and in the tropics. The ratio method stays closer tothe a prioriCH₄ flux in these regions, because it is capable of simultaneously adjusting the CO₂fluxes. Over Tropical SouthAmerica, comparison to independent measurements shows that CO₂fields derived from the ratio method are less realistic than those used in the proxy method. However, the CH₄fluxes are more realistic, because the impact of unaccounted systematicuncertainties is more evenly distributed between CO₂and CH₄. The ratio inversion estimates an enhanced CO₂release fromTropical South America during the dry season of 2010, which is in accordance with the findings of Gatti et al. (2014) andVanderlaan et al. (2015).The performance of the ratio method is encouraging, because despite the added non-linearity due to the assimilation of X_ratioand the significant increase in the degree of freedom by optimizing CO₂fluxes, still consistent results are obtained.