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|Title:||Assessing 5 years of GOSAT Proxy XCH4 data and associated uncertainties|
Webb, Alex J.
Palmer, P. I.
Griffith, D. W. T.
|Publisher:||Copernicus Publications on behalf of the European Geosciences Union|
|Citation:||Atmospheric Measurement Techniques Discussions, 2015, 8, pp. 5937-5972|
|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 (a measure 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 (~ 0.3%), 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 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 measurements.|
|Rights:||Copyright © the authors, 2015. This is an open-access article distributed under the terms of the Creative Commons Attribution License ( http://creativecommons.org/licenses/by/3.0/ ), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.|
|Description:||This is a discussion paper currently under public peer review for publication in Atmospheric Measurement Techniques (AMT). The corresponding final paper will become available in AMT will be accessible through the publisher's link at http://www.atmos-meas-tech-discuss.net/8/5937/2015/amtd-8-5937-2015-discussion.html|
|Appears in Collections:||Published Articles, Dept. of Physics and Astronomy|
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