Modeling polarized solar radiation for CLARREO inter-calibration applications

The Climate Absolute Radiance and Refractivity Observatory (CLARREO) is a high-priority
NASA Decadal Survey mission recommended by the National Research Council in 2007.
The CLARREO objectives are to conduct highly accurate decadal climate-change observa-
tions and to provide a non-orbit inter-calibration standard for relevant Earth observing
sensors.The inter-calibration approach is based on providing highly accurate spectral
reflectance measurements from the CLARREO Reflected Solar Spectrometer (RSS) as the
reference for existing sensors and to monitor and characterize their response function
parameters including gain, offset, non-linearity, optics spectral response, and sensitivity to
polarization of light. The inter-calibration of instrument sensitivity to polarization
requires on-orbit knowledge of polarization state of light as function of observed scene
type and viewing geometry. In this study, we validate polarization parameters calculated
with the adding-doubling radiative transfer model (ADRTM) for developing the Polariza-
tion Distribution Models (PDMs). These model results are compared with observations
from the Polarization and Anisotropy of Reflectances for Atmospheric Science instrument
coupled with Observations from a Lidar (PARASOL) data. Good agreement between model
results and satellite data is shown for both liquid water clouds and ice clouds. Difference
between model results and satellite measurements for clear-sky oceans is explained as
due to the presence of undetected clouds, that are super-thin or whose spatial and
temporal mean optical depth is small, in the PARASOL clear-sky scenes. These results
demonstrate that the ADRTM provides a reliable approach for building spectral PDMs for
the inter-calibration applications of the CLARREO mission.