Article ID: 2016-027
CO2 concentrations in the upper troposphere were retrieved from thermal infrared spectra as observed by the only spaceborne hyperspectral sounder launched in the 1990s: the Interferometric Monitor for Greenhouse gases (IMG) onboard the Advanced Earth Observing Satellite (ADEOS). First, the effective optical path difference of IMG was evaluated because the actual instrumental line shape function of interferometer component has not been evaluated for technical reasons in orbit. CO2 retrieval method was based on maximum a posteriori (MAP) retrieval method and on procedures to decrease errors that obstruct CO2 signal detection. For retrieval analysis, ERA-40 re-analysis meteorological data were used as temperature field data. A method of selecting effective channels for CO2 retrieval was used to remove the channels with high temperature dependency and to reduce errors in estimating water vapor, ozone, and surface temperature. Furthermore, uncertainties of the temperature and other error factors, which cannot be removed through channel selection, were evaluated and optimized by treating them as components of measurement errors in MAP retrieval. CO2 retrieval noises of MAP retrieval were estimated respectively as 2.5% and 2.0% at pressure levels of 500 hPa and 300 hPa. CO2 concentrations retrieved from IMG data were compared with aircraft measurement data. Results showed that the random error in IMG retrieval was smaller than that estimated as a posteriori error of MAP retrieval. No significant biases were shown compared with the margin of random errors. The CO2 retrieval method was applied to IMG data measured in April, 1997. Although assuming uniform CO2 concentration as a priori, latitudinal gradient of zonal mean concentration was consistent with climatological features presented by previous studies at pressure levels of 500 hPa and 300 hPa. Those results suggest that thermal infrared observation by IMG is effective for evaluating the upper tropospheric CO2 concentration in the 1990s.
