1. Institute for Marine and Atmospheric Research Utrecht (IMAU), Universiteit Utrecht, Princetonplein 5, NL-3508 TA Utrecht, The Netherlands;
2. Max Planck Institut für Meteorologie, Bundesstrasse 55, D-20146 Hamburg, Germany;
3. Royal Netherlands Meteorological Institute (KNMI), Wilhelminalaan 10, NL-3730 AE De Bilt, The Netherlands