Correlations among the optical properties of cirrus‐cloud particles: Microphysical interpretation

Cirrus measurements obtained with a ground‐based polarization Raman lidar at 67.9°N in January 1997 reveal a strong positive correlation between the particle optical properties, specifically depolarization ratio δpar and extinction‐to‐backscatter (lidar) ratio Spar, for δpar < ∼40%, and an anticorrelation for δpar > ∼40%. Over the duration of the measurements, both particle properties vary systematically. This effect is particularly pronounced in the case of δpar, which decreases significantly with time. The analysis of lidar humidity and radiosonde temperature data shows that the measured optical properties stem from scattering by dry solid ice particles, while scattering by supercooled droplets, or by wetted or subliming ice particles, can be excluded. For the microphysical interpretation of the lidar measurements, ray‐tracing computations of particle scattering properties have been used. The comparison with the theoretical data suggests that the observed cirrus data can be interpreted in terms of size, shape, and growth of the cirrus particles, the latter under the assumption that the lidar measurements of consecutive cloud segments can be mapped on the temporal development of a single cloud parcel moving along its trajectory: Near the cloud top in the early stage of cirrus development, light scattering by nearly isometric particles that have the optical characteristics of hexagonal columns (short, column‐like particles) is dominant. Over time, the ice particles grow, and as the cloud base height extends to lower altitudes characterized by warmer temperatures they become morphologically diverse. For large Spar and depolarization values of ∼40%, the scattering contributions of column‐ and plate‐like particles are roughly the same. In the lower ranges of the cirrus clouds, light scattering is predominantly by plate‐like ice particles. This interpretation assumes random orientation of the cirrus particles. Simulations with a simple model suggest, however, that the positive correlation between Spar and δpar, which is observed for depolarization ratios <40% mainly at low cloud altitudes, can be alternatively explained by horizontal alignment of a fraction of the cirrus particle population.