1. Institute of Physics and Meteorology, University of Hohenheim, Garbenstrasse 30, 70599 Stuttgart, Germany;
2. Helmholtz-Center Potsdam, German Research Center for Geosciences, Telegrafenberg A17, 14473 Potsdam, Germany;
3. WESS - Water & Earth System Sciences, c/o University of T¨ubingen, Center for Applied Geosciences, Keplerstrasse 17, 72076 T¨ubingen, Germany