Assessing 5 years of GOSAT Proxy XCH₄ data and associated uncertainties

Abstract. We present 5 years of GOSAT XCH₄ retrieved using the "proxy" approach. The Proxy XCH₄ data are validated against ground-based TCCON observations and are found to be of high quality with a small bias of 4.8 ppb (∼ 0.27 %) and a single-sounding precision of 13.4 ppb (∼ 0.74 %). The station-to-station bias (ameasure of the relative accuracy) is found to be 4.2 ppb. For the first time the XCH₄ / XCO₂ ratio component of the Proxy retrieval is validated (bias of 0.014 ppb ppm⁻¹ (∼ 0.30 %), single-sounding precision of 0.033 ppb ppm⁻¹ (∼ 0.72 %)). The uncertainty relating to the model XCO₂ component of the Proxy XCH₄ is assessed through the use of an ensemble of XCO₂ models. While each individual XCO₂ model is found to agree well with the TCCON validation data (r = 0.94–0.97), it is not possible to select one model as the best from our comparisons. The median XCO₂ value of the ensemble has a smaller scatter against TCCON (a standard deviation of 0.92 ppm) than any of the individual models whilst maintaining a small bias (0.15 ppm). This model median XCO₂ is used to calculate the Proxy XCH₄ with the maximum deviation of the ensemble from the median used as an estimate of the uncertainty. We compare this uncertainty to the a posteriori retrieval error (which is assumed to reduce with sqrt(N)) and find typically that the model XCO₂ uncertainty becomes significant during summer months when the a posteriori error is at its lowest due to the increase in signal related to increased summertime reflected sunlight. We assess the significance of these model and retrieval uncertainties on flux inversion by comparing the GOSAT XCH₄ against modelled XCH₄ from TM5-4DVAR constrained by NOAA surface observations (MACC reanalysis scenario S1-NOAA). We find that for the majority of regions the differences are much larger than the estimated uncertainties. Our findings show that useful information will be provided to the inversions for the majority of regions in addition to that already provided by the assimilated surface measurements.