Cloud thermodynamic phase measured with a low-cost, ground-based, all-sky imaging polarimeter

Cloud thermodynamic phase, whether a cloud is composed of spherical water droplets or polyhedral ice crystals, is crucial for understanding the role of clouds in climate change, weather, and optical propagation. Clouds, covering approximately 60% of the earth's surface at any given time, still contribute some of the largest uncertainties in climate science. Cloud thermodynamic phase is also required to properly retrieve other cloud properties, including cloud optical depth and particle size distributions. Cloud phase remote sensing is often done with passively measured radiance ratios or lidar cross-polarization measurements, but recent research shows that the sign of the S1 Stokes parameter can be used to detect cloud thermodynamic phase with a ground-based polarimeter. Our group has been developing ground-based polarimetric imagers to determine cloud thermodynamic phase, with lidar cross-polarization detection used as ground truth. However, because the cloud polarization is small, often on the order of a percent, accurate classification requires high polarization sensitivity. This paper reports preliminary measurements indicating feasibility of using a low-cost, commercial division-of-focal-plane polarization imager for cloud thermodynamic phase remote sensing.