1. Department of the Geophysical Sciences, University of Chicago, Chicago, Illinois 60637, USA
2. Department of Physics, University of Chicago, Chicago, Illinois 60637, USA
3. Institute of Meteorology and Climate Research, Karlsruhe Institute of Technology, 76021 Karlsruhe, Germany
4. Physikalisch-Technische Bundesanstalt, 38116 Braunschweig, Germany