Abstract
Abstract. This paper describes the global height-resolved methane (CH4) retrieval scheme for the Infrared Atmospheric Sounding Interferometer (IASI) on MetOp, developed at the Rutherford Appleton Laboratory (RAL). The scheme precisely fits measured spectra in the 7.9 micron region to allow information to be retrieved on two independent layers centred in the upper and lower troposphere. It also uses nitrous oxide (N2O) spectral features in the same spectral interval to directly retrieve effective cloud parameters to mitigate errors in retrieved methane due to residual cloud and other geophysical variables. The scheme has been applied to analyse IASI measurements between 2007 and 2015. Results are compared to model fields from the MACC greenhouse gas inversion and independent measurements from satellite (GOSAT), airborne (HIPPO) and ground (TCCON) sensors. The estimated error on methane mixing ratio in the lower- and upper-tropospheric layers ranges from 20 to 100 and from 30 to 40 ppbv, respectively, and error on the derived column-average ranges from 20 to 40 ppbv. Vertical sensitivity extends through the lower troposphere, though it decreases near to the surface. Systematic differences with the other datasets are typically < 10 ppbv regionally and < 5 ppbv globally. In the Southern Hemisphere, a bias of around 20 ppbv is found with respect to MACC, which is not explained by vertical sensitivity or found in comparison of IASI to TCCON. Comparisons to HIPPO and MACC support the assertion that two layers can be independently retrieved and provide confirmation that the estimated random errors on the column- and layer-averaged amounts are realistic. The data have been made publically available via the Centre for Environmental Data Analysis (CEDA) data archive (Siddans, 2016).
Highlights
Methane (CH4) is one of the most important long-lived greenhouse gases (GHG) in the atmosphere
Differences in direct comparisons between gas inversion and independent measurements from satellite (GOSAT) and Infrared Atmospheric Sounding Interferometer (IASI) are seen to be broadly similar to those found between MACC-II GHG and IASI (Fig. 6), which is consistent with the agreement found between the same GOSAT and MACC-II GHG datasets in Parker (2015)
At southern mid-latitudes IASI is higher than GOSAT. This bias is about a factor of 2 smaller than that seen in this region in the comparison to MACC-II GHG, implying that MACC-II GHG is negatively biased with respect to both IASI and GOSAT in this region
Summary
Methane (CH4) is one of the most important long-lived greenhouse gases (GHG) in the atmosphere. Observations in the SWIR, such as those from SCIAMACHY on Envisat (Buchwitz, 2005; Frankenberg, 2010) and GOSAT TANSO Fourier transform spectrometer (FTS) (Butz, 2010; Parker, 2011 and Yoshida, 2013), provide information on column-averaged methane with a vertical sensitivity, in cloud-free conditions over most land surfaces, which is close to uniform throughout the atmospheric column. Such measurements rely on surfacereflected sunlight, so observations are limited to daytime and predominantly over land (ocean reflectance being too low except in sun-glint geometry). By more extensively exploiting IASI measurements in the 7.8–8 micron range and modelling radiative transfer online, the RAL scheme reported here provides information extending into the lower troposphere and with near-global coverage
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