Cloud Chamber Studies on the Linear Depolarisation Ratio of Small Cirrus Ice Crystals

Abstract. Space-borne lidar, in combination with other remote sensing instrumentation, has been used to infer vertical profiles of ice cloud properties from A-train satellites, and more recently, also from the newly launched EarthCARE mission. However, accurately retrieving ice crystal microphysical properties from lidar signals requires a thorough understanding of their relationship to backscattering characteristics. Cloud chambers can be used to study the link under a controlled environment. This study investigates the link between the linear depolarisation ratio in the near-backscattering direction (178°) and the ice microphysical properties for 47 cloud experiments at cirrus temperatures between −75 and −39 °C. Predominantly small (diameter<70 µm) columnar and irregularly shaped ice crystals were grown under distinct conditions of supersaturation with respect to ice. A statistical and visual analysis of size, shape and morphological complexity reveals that more than 40 % of the columnar particles exhibit hollowness on the basal facets. Ice crystals larger than 10 µm show depolarisation ratios below 0.3, which is lower than typical values observed in mid-latitude cirrus but in agreement with polar cirrus observations. Two temperature-dependent depolarisation ratio–size modes were found and successfully reproduced with ray tracing simulations of hollow columns incorporating surface roughness, hollowness and internal scattering. These results are important for the interpretation of the linear depolarisation ratio of small ice crystals in active remote sensing or can be used for evaluating the performance of state-of-the-art optical particle models, especially for small size parameters below 100.