Satellite Lidar Observations of Cirrus Cloud Properties: Differences in the High- and Midlatitude Regions

Aerosols, e.g. from air traffic emissions can increase the global cloudiness and exert impact on the properties of naturally-formed cirrus clouds. The densities of civil air traffic show a large difference in the high- and midlatitude regions, which may modify the properties of cirrus clouds at different latitudes differently. In this study, we analyze one-year lidar observations in 2014 with CALIPSO to determine the occurrence rates (ORs) and particle linear polarization ratios (PLDR) of cirrus clouds and further compare them with the ice particle sizes at midlatitudes (ML: 35–60° N; 30° W–30° E) and high latitudes (HL: 60–80° N; 30° W–30° E). The resulting ORs show a distinct seasonal variation with the largest values in winter and the smallest in summer, especially for the results at HL. The occurrence heights of cirrus clouds at HL are nearly 2 km lower than at ML. The derived histograms of cirrus PLDR follow a right-skewed distribution in all seasons. In different seasons, the latitudinal differences in PLDR are slightly different. To study the potential causes of the latitudinal difference in the PLDR values, the altitude profiles of the effective radii of ice crystals (Reff) within cirrus clouds are determined from the DARDAR CLOUD products. The results show larger values of Reff at ML than HL, except for summer. The larger ice crystals related with the specific aerosols acting as INPs at ML are presumed to contribute to the enhanced PLDR values of cirrus clouds.