Assessment of polarization sensitivity of Aqua-MODIS using co-incident POLDER-3 polarized measurements over marine stratocumulus clouds

Over the MODIS mission, normalized trends of Earth-view digital numbers (EV-dn) over desert calibration sites (PICS) have shown biases in several Terra MODIS reflective solar bands (RSBs) related to the changes in polarization sensitivity. The MODIS Characterization Support Team (MCST) corrects for these effects at the Level 1B stage in Collection 7 with monthly coefficient updates from NASA Ocean Biology Processing Group (OBPG). However, upcoming orbit changes for both MODIS instruments may require more frequent updates and new algorithms. In this work, we present a vicarious calibration algorithm with the potential to characterize polarization sensitivity using a single MODIS granule, across a range of angles-of-incidence (AOI). Marine stratocumulus cloud targets off the coast of South America, measured in near-real-time by Aqua-MODIS and the polarimeter POLDER-3, are used. These clouds strongly polarize light in optical wavelengths, are spatially uniform over wide areas, and are present year-round. After geo-registering both data to similar to 50 km superpixels, we find the polarized reflectance fit that best matches the cloud microphysics of the POLDER-3 target at 0.865 mu m. We then interpolate the fit to the Aqua-MODIS target geometry. We derive polarization sensitivity coefficients for Aqua-MODIS Band 2 (0.858 mu m) at a range of AOI using the POLDER-3 retrieval results for six different matchups in 2005. The results suggest that cloud development in the time between Aqua-MODIS and POLDER-3 measurements (similar to 3 min) and simultaneous nadir overpass (SNO) distance are the main error contributions, combined with relatively low polarization sensitivity for Aqua Band 2. Even so, the derived sensitivity coefficients agree with pre-launch values within uncertainty. Therefore, simultaneous, co-incident radiometer and polarimeter data are optimal, such as from OCI and HARP-2/SPEXone on the upcoming NASA Plankton Aerosol Cloud and ocean Ecosystem (PACE) mission.