Assessing 5 years of GOSAT Proxy XCH4 data and associated uncertainties

International audience ; We present 5 years of GOSAT XCH 4 retrieved using the "proxy" approach. The Proxy XCH 4 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 (a measure of the relative accuracy) is found to be 4.2 ppb. For the first time the XCH 4 / XCO 2 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 XCO 2 component of the Proxy XCH 4 is assessed through the use of an ensemble of XCO 2 models. While each individual XCO 2 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 XCO 2 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 XCO 2 is used to calculate the Proxy XCH 4 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 XCO 2 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 XCH 4 against modelled XCH 4 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 ...