Pengxiang Qiu, Georg-August-Universitaet Goettingen, Goettingen, Lower saxony, Germany, Rui Hu, Hohai University, School of Earth Science and Engineering, Nanjing, China, Huichen Yang, Georg-August-Universitaet Goettingen, Applied Geology, Goettingen, Germany, Quan Liu, Beijing meteorological service, Beijing weather modification office, Beijing, China and Thomas Ptak, Georg August University Goettingen, Goettingen, Germany