In-cloud Variability of LIDAR Depolarization of Polar and Midlatitude Cirrus

LIDAR depolarization is commonly used for discriminating liquid and ice particles. Since depolarization depends in a complicated manner on particle shape and size, in-cloud variability of depolarization has been used as an indicator of the microphysical homogeneity of cirrus. The comparison between midlatitude (Florence, Italy, 43.60°N) and polar (Dumont d’Urville, Antarctica, 66.68°S) cirrus showed a lower mean depolarization and a higher in-cloud uniformity of cloud depolarization for polar clouds in the (-80, -50°C) temperature range. A wider in-cloud variability of depolarization was observed in polar clouds at higher temperatures (-50, -30°C), reflecting the presence of supercooled liquid layers. The large in-cloud variability of depolarization in Florence cirrus could be explained with a microphysics that is dynamically and chemically perturbed as compared with the polar site. Aged jet contrails are, in fact, present in many Florence cirrus records.