Depolarization of lidar returns by small ice crystals: An application to contrails

Abstract

Measurements of the lidar linear depolarization ratio δ can be a powerful remote sensing technique for characterizing the microphysics of contrail particles. Since young contrails often consist of relatively small ice crystals, the quantitative interpretation of lidar measurements requires accurate theoretical computations of δ for polydisperse, randomly oriented nonspherical particles with size parameters ranging from zero to at least several tens, thus ruling out most of the currently available numerical techniques. In this paper we use the recently improved T-matrix method and compute δ for polydispersions of randomly oriented ice spheroids, circular cylinders, and Chebyshev particles with sizes typical of young contrails. We show that ice crystals with effective radii as small as several tenths of a micron can already produce δ exceeding 0.5 at visible wavelengths. This may explain the frequent occurrence of large δ values for very young contrails. We also show that observed increases of δ with the contrail's age can be explained either by a rapid increase of the particle size parameter from essentially zero to about 5 or by assuming that the contrail particles originate as perfect spheres and then acquire a certain degree of asphericity.