Parameterization of Aerosol and Cirrus Cloud Effect on Reflected Sunlight Spectra Measured From Space: Application of the Equivalence Theorem

A novel approach is proposed to estimate the aerosol and cirrus cloud effect on reflected sunlight radiance spectra measured with spaceborne sensors. This method does not require detailed information on aerosol and cloud microphysical and optical properties. The proposed approach is based on application of the equivalence theorem and photon path length statistics with further parameterization of the of photon path-length probability density function (PPDF). A limited set of the parameters could permit their simultaneous retrievals with gas amount. Monte Carlo simulations were utilized to validate this parameterization for the vertically non- homogeneous atmosphere including aerosol layer and cirrus cloud. As a first approximation, the PPDF is shown to depend on four parameters. They can be interpreted as an effective cloud height, cloud relative reflectance and two additional factors to account for photon path-length distribution under the cloud. We have demonstrated that the total set of PPDF parameters can be retrieved using spectral radiance data in O2-A band for aerosol and cloud characterization. The PPDF retrievals are validated using direct simulation of the photon trajectories with Monte Carlo technique. To provide aerosol and cloud correction in the infrared channels that are utilized for CO2 retrievals, additional PPDF information from H2O saturated area of 2.0 &μm band is shown to be involved. The proposed approach to combine spectral radiance O2-A band and the saturated area for aerosol and cloud correction is described and tested by numerical simulations with synthetic effective transmittance. We demonstrated that the saturated area contains reliable information on the cloud relative reflectance that is the basic parameter to account for cloud and surface albedo. The aerosol and cloud correction according to this approach is shown to significantly reduce the error of effective transmittance modeling in the infrared channels.