NASA - National Aeronautics and Space Administration

The detection of aerosol above clouds is critical for the estimate of both the aerosol and cloud radiative impacts. In this study, the authors present a new method to retrieve the aerosol properties over clouds that uses the multiangle polarization measurements of the Polarization and Directionality of Earth Reflectances (POLDER)–Polarization and Anisotropy of Reflectances for Atmospheric Sciences Coupled with Observations from a Lidar (PARASOL) instrument. The method is illustrated and applied to a case study exploiting the coincident observations from other passive and active sensors of the NASA A-Train satellite constellation. The case study is relative to an elevated biomass burning aerosol layer that originates from southern Africa and is then transported over low-level clouds extending over the Atlantic Ocean. It is shown that the comparison between the cloud-top heights retrieved with the different passive techniques developed for the A-Train sensors can be used to detect the presence of aerosols above clouds. The analysis of the PARASOL observations showed that the aerosols significantly affect the polarized light reflected by the clouds over the 808–1208 scattering angle range and in the rainbow region. A single scattering model permitted the reproduction of the polarization observations and the retrieval of an estimate of the aerosol layer optical thickness of 0.225 at 0.865 mm. The retrieved aerosol optical thicknesses over clouds agree quantitatively with the closest ones retrieved over clear-sky ocean (60.04 as a maximum departure), demonstrating the value of the method. This innovative technique based solely on passive measurements is expected to provide a better understanding of aerosol properties in regions where significant cloud cover usually prevents the retrieval of aerosol optical thickness. As such, this new retrieval method can provide significant and valuable information about the radiative impact of clouds and aerosols, especially where they can potentially interact strongly with each other.