Ensemble-based satellite-derived carbon dioxide and methane column-averaged dry-air mole fraction data sets (2003–2018) for carbon and climate applications
Authors: Reuter Maximilian , Buchwitz Michael , Schneising Oliver , Noël Stefan , Bovensmann Heinrich , Burrows John P. , Boesch Hartmut , Di Noia Antonio , Anand Jasdeep , Parker Robert J. , Somkuti Peter , Wu Lianghai , Hasekamp Otto P. , Aben Ilse , Kuze Akihiko , Suto Hiroshi , Shiomi Kei , Yoshida Yukio , Morino Isamu , Crisp David , O'Dell Christopher W. , Notholt Justus , Petri Christof , Warneke Thorsten , Velazco Voltaire A. , Deutscher Nicholas M. , Griffith David W. T. , Kivi Rigel , Pollard David F. , Hase Frank , Sussmann Ralf , Té Yao V. , Strong Kimberly , Roche Sébastien , Sha Mahesh K. , De Mazière Martine , Feist Dietrich G. , Iraci Laura T. , Roehl Coleen M. , Retscher Christian , Schepers Dinand 
Published: February 2020

Abstract. Satellite retrievals of column-averaged dry-air mole
fractions of carbon dioxide (CO2) and methane (CH4), denoted
XCO2 and XCH4, respectively, have been used in recent years to
obtain information on natural and anthropogenic sources and sinks and for
other applications such as comparisons with climate models. Here we present
new data sets based on merging several individual satellite data products in
order to generate consistent long-term climate data records (CDRs) of these
two Essential Climate Variables (ECVs). These ECV CDRs, which cover the time
period 2003–2018, have been generated using an ensemble of data products
from the satellite sensors SCIAMACHY/ENVISAT and TANSO-FTS/GOSAT and (for
XCO2) for the first time also including data from the Orbiting Carbon Observatory 2 (OCO-2) satellite. Two types of products have been generated:
(i) Level 2 (L2) products generated with the latest version of the
ensemble median algorithm (EMMA) and (ii) Level 3 (L3) products obtained
by gridding the corresponding L2 EMMA products to obtain a monthly
5∘×5∘ data product in Obs4MIPs (Observations for Model Intercomparisons Project) format. The L2 products consist of daily NetCDF (Network Common Data Form) files, which contain in addition to the main
parameters, i.e., XCO2 or XCH4, corresponding uncertainty
estimates for random and potential systematic uncertainties and the
averaging kernel for each single (quality-filtered) satellite observation.
We describe the algorithms used to generate these data products and present
quality assessment results based on comparisons with Total Carbon Column
Observing Network (TCCON) ground-based retrievals. We found that the
XCO2 Level 2 data set at the TCCON validation sites can be
characterized by the following figures of merit (the corresponding values
for the Level 3 product are listed in brackets) – single-observation random
error (1σ): 1.29 ppm (monthly: 1.18 ppm); global bias: 0.20 ppm (0.18 ppm); and spatiotemporal bias or relative accuracy (1σ): 0.66 ppm
(0.70 ppm). The corresponding values for the XCH4 products are single-observation random error (1σ): 17.4 ppb (monthly: 8.7 ppb); global
bias: −2.0 ppb (−2.9 ppb); and spatiotemporal bias (1σ): 5.0 ppb (4.9
ppb). It has also been found that the data products exhibit very good
long-term stability as no significant long-term bias trend has been
identified. The new data sets have also been used to derive annual XCO2
and XCH4 growth rates, which are in reasonable to good agreement with
growth rates from the National Oceanic and Atmospheric Administration (NOAA)
based on marine surface observations. The presented ECV data sets are
available (from early 2020 onwards) via the Climate Data Store (CDS,
https://cds.climate.copernicus.eu/, last access: 10 January 2020) of the Copernicus Climate
Change Service (C3S, https://climate.copernicus.eu/, last access: 10 January 2020).