Global distribution of methane in the mid-troposphere as seen by IASI onboard three successive Metop platforms

Thanks to its continuous spectral coverage of the whole thermal infrared domain, the IASI sounder offers the possibility to monitor on the long term several essential climate variables, including mid-tropospheric columns of the 3 major greenhouse gases influenced by human activitie: carbon dioxide (CO₂), methane (CH₄) and nitrous oxide (N₂O).

To tackle the very small seasonal variability of these gases compared to their background values, combined to the strong dependence of IR radiances to atmospheric temperature and the simultaneous sensitivity of the channels to several gases, a non-linear inference scheme has been developed at LMD. Since 2007, mid-tropospheric columns of methane have been derived for both day and night conditions, over land and over sea. The retrieval scheme strongly relies on careful validation of level1c spectra, characterization of systematic radiative biases and severe cloud and aerosol screening. CH₄ fields are delivered on ‘near real time’ (D-1) basis to the Copernicus Atmosphere Monitoring Service (CAMS) and are assimilated in ECMWF C-IFS system, along with total columns from GOSAT, to produce forecast of vertical profiles of atmospheric concentration. Owing to its 20 year-program, IASI also participates to the establishment of long time series in the Copernicus Climate Change Service (C3S). The retrievals are thus used for a variety of purpose: assimilation to produce CH₄/CO₂ profile forecasts; estimation of surface fluxes using “top-down” atmospheric inversions; characterization of specific emissions such as biomass burnings.

In this talk we will present the latest development of the retrieval and application of methane. In particular, we will present the extension and validation of the retrieval to the high latitude regions achieved during the ESA MethaneCAMP project. By using AirCore 0-30 km profiles of methane concentration acquired at Sodankylä and Kiruna and several stations of the French AirCore network, we will also highlight the crucial need to better understand the variation of stratospheric methane in order to combine satellite-derived methane columns with simulations from atmospheric transport models. Finally, we will present long-term and interannual variability of methane as seen by IASI, with a focus of 2020-2021 methane anomaly and the characterization of specific emissions such as biomass burnings or NordStream leakage.