Carbon dioxide retrieval from reflected sunlight spectra in the presence of cirrus cloud: model studies

The results of model study for CO 2 retrievals from numerically synthesized GOSAT (Greenhouse gases Observing SATellite) observation data are presented. The GOSAT is scheduled to be launched in 2008 to monitor column amounts of CO 2 and CH 4 . A nadir-looking Fourier-Transform Spectrometer (FTS) of Short Wavelength Infrared (SWIR, 1.6 μm and 2 μm) and 0.76 μm oxygen A-band regions will be mounted on GOSAT. To assess CO2 sources and sinks, the monthly averaged CO2 column amounts estimated by satellite-based measurements should have a precision of within 1% or better to provide an advantage over existing ground-based measurement networks. This study focuses on CO 2 retrievals in the presence of cirrus clouds. An important feature of this problem is to apply radiance data measured in several spectral channels. In particular, 1.58 μm spectral band was utilized for CO 2 total column amount retrievals. The cloud correction was performed using an original approach that is based on the application of the equivalence theorem with parameterization of photon path-length probability density function (PPDF). The PPDF parameters were estimated using nadir radiance in the oxygen A-band and in the H 2 O-saturated area of the 2.0 μm spectral band.