1. Geophysical Institute, University of Bergen, Postbox 7803, 5020 Bergen, Norway
2. Department of Atmospheric and Oceanic Sciences, University of Colorado, Boulder, Colorado 80309, USA
3. Manteuffelstraße 59, 10999 Berlin, Germany
4. Royal Netherlands Meteorological Institute, De Bilt, The Netherlands
5. Geosciences Department, Ecole Normale Supérieure and PSL Resaerch University, Paris, France
6. Department of Atmospheric and Oceanic Sciences, University of California, Los Angeles, CA 90095-1565, USA