Targeting and impacts of AgI cloud seeding based on rain chemical composition and cloud top phase characterization

During four winters when cloud seeding took place, precipitation samples were collected at three stations in the catchment area of the Sea of Galilee (target stations) and in one station west of the seeding line (control station). Chemical analyses were carried out on more than 4000 rain samples in order to determine the major and trace metal compositions and enrichment factors of Ag (EF) with respect to Al, where Al is used as a tracer for natural dust. In addition, satellite images were analyzed to characterize the cloud phase and the temperature of the tops of the rain clouds using the EUMETSAT second generation geostationary satellite. Our results show that the seeding agent (AgI) arrives to the target stations, as indicated by significantly higher EFAg values of Ag there compared to the control station. Furthermore, we found higher EFAg values in precipitation samples from mixed-phase clouds compared to precipitation from warm or fully glaciated clouds. This difference was observed only at the target stations. Therefore, our results are consistent with the hypothesis that AgI contributes actively to precipitation formation processes in mixed-phase clouds, where ice content is controlled by ice nuclei (IN) concentration. This is in accordance with the conventional wisdom that AgI should be mostly active in such clouds, but not in clouds that are already naturally glaciated or in warm clouds, and it supports previous statistical studies which claimed that cloud seeding enhanced rainfall in Northern Israel. (c) 2012 Elsevier B.V. All rights reserved.