Retrieval and validation of METOP/IASI methane
Abstract. A new global IASI methane product developed at the Royal Belgian Institute for Space Aeronomy (BIRA-IASB) is presented. The retrievals are performed with the ASIMUT-ALVL software based on the Optimal Estimation Method (OEM). This paper gives an overview of the forward model and retrieval concept. The usefullness of reconstructed Principal Component Compressed (PCC) radiances is highlighted. The retrieval uncertainty of the CH4 profiles is less than 4 % below 100 hPa (~ 16 km). The information content study carried out in this paper shows that most IASI pixels contain between 0.9 and 1.6 independent pieces of information about the vertical distribution of CH4, with a good sensitivity in the mid to upper troposphere. An extended validation with ground-based CH4 observations at 10 locations was carried out. IASI CH4 partial columns are found to correlate well with the ground-based data for 7 out of the 10 Fourier Transform Infrared (FTIR) stations with correlation coefficients between 0.71 and 0.96. Mean differences between IASI and FTIR CH4 range between −1.93 and 4.40 % and are within the systematic uncertainty. For 7 out of the 10 stations absolute differences are less than 1 %. The standard deviation of the difference lies between 1.40 and 3.99 % for all the stations.