Knowing the thermodynamic phase of a cloud–whether it is composed of spherical water droplets or polyhedral ice crystals–is critical in remote sensing applications and in climate studies. We recently showed that we can determine cloud phase with visible-wavelength sky polarimetry, and in this presentation we extend that method to shortwave infrared wavelength bands near 1.6 microns. We describe the instrument, a passive, three-channel polarimeter with spectral bands at 1550 nm, 1640 nm, and 1700 nm with approximate width of 40 nm and how we are using it in experiments to discriminate between liquid-water and ice clouds. This portable polarimeter measures scattered sunlight using polarizers orientated at 0° , 45‡ , and 90° with respect to the solar vertical scattering plane. It has a 4.9° field-of-view and a motorized, computer-controlled pan-and-tilt mount that controls the positioning of the polarimeter so that it can measure any point in the sky.

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Martin Jan Tauc, Carol L. Baumbauer, Benjamin Moon, Andrew M. Abel, David W. Riesland, Laura M. Eshelman, Wataru Nakagawa, and Joseph A. Shaw, "Cloud thermodynamic phase detection with a 3-channel shortwave infrared polarimeter," Proc. SPIE 10655, Polarization: Measurement, Analysis, and Remote Sensing XIII, 106550O (Presented at SPIE Commercial + Scientific Sensing and Imaging: April 17, 2018; Published: 14 May 2018); https://doi.org/10.1117/12.2305171.