The IASI CH4 product has been validated with co-located ground-based NDACC Fourier Transform Infrared (FTIR) observatoins at 11 NDACC stations for the timeperiod 2010-2016. The NDACC FTIR profiles are smoothed with the IASI CH4 averaging kernel and the same a priori profile is used for the IASI CH4 and NDACC retrievals.
A barchart of the relative differences between IASI CH4 and smoothed NDACC partial columns (surface-40 km) at 11 NDACC stations is shown in Figure 1. The mean (IASI - NDACC)/NDACC (Δ) and standard deviation (σ) of the relative differences is given for each stations. Biases are between -1.25 and 0.23% with exception of the stations Altzomoni and Thule, which show a larger positive bias.
Figure 2 gives an example of the timeseries at Jungfraujoch of smoothed NDACC FTIR CH4 and IASI CH4 partial columns. The vertical bars represent the retrieval error. We see a clear agreement between the 2 products within the error bars of IASI CH4 and a good representation of the seasonal cycle by IASI and a mean relative difference of 0.15% and standard deviation of 3.18%.
Figure 1 : Barchart with relative differences (Δ) and standard deviation of the differences (σ) between IASI and smoothed NDACC 4-17km partial columns at 10 NDACC stations. The 3-letter abbreviation of each NDACC station is given on top or below each bar, the station names are given in the legend and its location visualised on the map.
Figure 2 : Timeseries IASI CH4 (blue) and smoothed NDACC (black) CH4 4-17km partial columns at Jungfraujoch for the period 2011-2014. The vertical bars represent the IASI CH4 retrieval error.