Passive shortwave-infrared polarimetric sensing of cloud thermodynamic phase

Abstract

Cloud thermodynamic phase (CTP) classification is a vital piece of information for determining how clouds affect climate and electromagnetic wave propagation. We describe a passive shortwave infrared (SWIR) three-channel polarimeter for CTP remote sensing. This instrument adds polarimetric sensitivity to the traditional radiance ratio method that capitalizes on the difference in spectral absorption between liquid water and ice. The ground-based polarimeter was used to measure the first three Stokes parameters in three SWIR spectral bands alongside a dual-polarization lidar system for validation. Additionally, we operated the polarimeter in division-of-time and division-of-aperture modes to explore the severity of polarization artifacts that arose from fast-moving clouds. Although temporal smearing of polarization signatures was identified, the impacts were found to be minimal. Using a surface fitting technique, the radiance ratio method was directly compared with a method that combined radiance ratios with the S1 Stokes parameter. We found that the addition of polarimetry improved cloud phase classification ability from ∼73 % to 95%.