A Compact, Low Power Depolarization Backscattering Cloud Spectrometer for Water and Ice Discrimination

Mixed phase clouds, i.e. those that contain water droplets and ice crystals, make up a large fraction of the clouds that are found over the Arctic region. The radiative impact of these clouds on climate modulation is highly sensitive to the relative mix of water and ice, the size distribution of these hydrometeors and the shape and optical properties of the ice crystals. The properties of mixed phase clouds and the microphysical processes that underlie their formation and evolution remain largely un-documented, particularly in the Arctic. This is primarily due to the lack of instruments that can accurately discriminate water droplets from ice crystals as well as measure their optical properties and size distributions, especially in the size range from approximately 1 50 m where imaging techniques with optical array probes have factors of two or more uncertainties in sample volume and are unable to distinguish liquid from ice below about 30 m. A very small sensor is proposed for measuring the optical diameter and light depolarization of individual particles from 1 m to greater than 50 m from airborne platforms. This sensor will have the sensitivity and accuracy to discriminate not only water droplets from ice crystals, but will also identify volcanic ash and dust aerosol particles, a complementary but highly important capability.