The Monitoring Atmospheric Composition and Climate Interim Implementation
(MACC-II) delayed-mode (DM) system has been producing an atmospheric methane
(CH4) analysis 6 months behind real time since June 2009. This analysis
used to rely on the assimilation of the CH4 product from the SCanning
Imaging Absorption spectroMeter for Atmospheric CHartographY (SCIAMACHY)
instrument onboard Envisat. Recently the Laboratoire de
Météorologie Dynamique (LMD) CH4 products from the Infrared
Atmospheric Sounding Interferometer (IASI) and the SRON Netherlands Institute
for Space Research CH4 products from the Thermal And Near-infrared Sensor
for carbon Observation (TANSO) were added to the DM system. With the loss of
Envisat in April 2012, the DM system now has to rely on the assimilation of
methane data from TANSO and IASI. This paper documents the impact of this
change in the observing system on the methane tropospheric analysis. It is
based on four experiments: one free run and three analyses from respectively
the assimilation of SCIAMACHY, TANSO and a combination of TANSO and IASI
CH4 products in the MACC-II system. The period between December 2010 and
April 2012 is studied. The SCIAMACHY experiment globally underestimates the
tropospheric methane by 35 part per billion (ppb) compared to the HIAPER
Pole-to-Pole Observations (HIPPO) data and by 28 ppb compared the Total
Carbon Column Observing Network (TCCON) data, while the free run presents an
underestimation of 5 ppb and 1 ppb against the same HIPPO and
TCCON data, respectively. The assimilated TANSO product changed in October
2011 from version v.1 to version v.2.0. The analysis of version v.1 globally
underestimates the tropospheric methane by 18 ppb compared to the
HIPPO data and by 15 ppb compared to the TCCON data. In contrast, the
analysis of version v.2.0 globally overestimates the column by 3 ppb.
When the high density IASI data are added in the tropical region between
30° N and 30° S, their impact is mainly positive but more
pronounced and effective when combined with version v.2.0 of the TANSO
products. The resulting analysis globally underestimates the column-averaged
dry-air mole fractions of methane (xCH4) just under 1 ppb on
average compared to the TCCON data, whereas in the tropics it overestimates
xCH4 by about 3 ppb. The random error is estimated to be less
than 7 ppb when compared to TCCON data
