Institute of Environmental Physics, Heidelberg University, Im Neuenheimer Feld 229, 69120 Heidelberg, Germany

now at: Department of Earth and Environmental Sciences, Geophysics Section, Ludwig-Maximilians University, Theresienstr. 41, 80333 Munich, Germany

Institute of Environmental Physics, Heidelberg University, Im Neuenheimer Feld 229, 69120 Heidelberg, Germany

now at: UFZ – Helmholtz Centre for Environmental Research, Permoserstr. 15, 04318 Leipzig, Germany

now at: WESS – Water and Earth System Science Competence Cluster, Keplerstr. 17, 72074 Tübingen, Germany

Alfred Wegener Institute for Polar and Marine Research – Research Unit Potsdam, Telegrafenberg A 43, 14473 Potsdam, Germany

Institute of Environmental Physics, Heidelberg University, Im Neuenheimer Feld 229, 69120 Heidelberg, Germany

Cold and Arid Regions Environmental and Engineering Research Institute, Chinese Academy of Sciences, 260 Donggang West Road, 730000 Lanzhou, China

Institute of Environmental Physics, Heidelberg University, Im Neuenheimer Feld 229, 69120 Heidelberg, Germany