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Abstract
A lidar-based study was performed to investigate the effect of atmospheric temperature on the heterogeneous freezing temperature of clouds. The underlying lidar dataset is based on approximately 2344 hours of measurements which were carried out between 1997 and 2008 at the Leibniz Institute for Tropospheric Research (IfT) in Leipzig, Germany. 2500 single cloud cases were identified from the measurements. According to their depolarization ratio, the clouds were classified as either water or Ice-containing clouds. Model and radiosonde data were used to determine the temperatures at the cloud boundaries. The statistical analysis revealed that the fraction of water clouds decreases strongly with decreasing
temperature. Between -10 and -15°C already 40% of all observed clouds contained detectable amounts of ice particles. At
temperatures below -25°C almost 100% of all clouds contained ice. This is in contradiction to the current understanding of
heterogeneous freezing mechanisms which are supposed to be active only at temperatures below -20°C. Cloud seeding, the so-called seeder-feeder mechanism, and aerosol particles, i.e. Saharan dust, acting as effective ice nuclei are possible effects to produce the observed discrepancies. Both effects were studied on the basis of the lidar data set and additional information about the dust load at Leipzig that was obtained from the dust model DREAM. It was found that cloud seeding as well as the availability of Saharan dust increase the fraction of ice clouds at temperatures above -20°C.