Cloud thermodynamic phase detection using an all-sky imaging polarimeter

At any given time, clouds cover approximately 60% of the Earth's surface and they strongly influence weather and climate; however, they are one of the largest sources of uncertainty in climate models and predictions of atmospheric effects on remote sensing measurements. Knowing the cloud thermodynamic phase - whether a cloud is composed of ice crystals or liquid particles - is critical in these applications. Knobelspiesse et al. (Atmos. Meas. Tech., 8, 1537-1554, 2015) showed theoretically that the sign of the S-1 Stokes parameter can be used to detect cloud thermodynamic phase when observed with a ground-based passive polarimeter and demonstrated this principle with a zenith-viewing polarimeter. In this theory, a positive S1 value indicates a liquid cloud, while a negative S1 value indicates an ice cloud. In this paper, we report the use of our all-sky polarimeter, operating at 450 nm (10 nm band) to detect ice, liquid, and multi-layered clouds. The cloud thermodynamic phase was independently verified with a dual-polarization lidar pointed at the zenith.