The IASI CH4 product has been validated with co-located ground-based NDACC Fourier Transform Infrared (FTIR) observations at 10 NDACC stations for the timeperiod 2011 to 2014. 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 between 4 and 17 km at 10 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 -2.31 and 0.18% with exception of the Thule station, where IASI is biased high with respect to NDACC by 4.04%.
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.
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.