University of Michigan, Department of Atmospheric, Oceanic and Space Sciences, Ann Arbor, USA

Laboratoire de Météorologie Dynamique, CNRS/Institut Pierre Simon Laplace, 4, place Jussieu, 75005 Paris, France

now at: Max Planck Institute for Meteorology, Bundesstraße 53, Hamburg, Germany

University of Oslo, Department of Geosciences, Oslo, Norway

Research Institute for Applied Mechanics, Kyushu University, Fukuoka, Japan

Laboratoire d'Optique Atmosphérique, CNRS/Universite de Lille I, 59655 Villeneuve d'Ascq Cedex, France

now at: Hadley Centre, Met Office, FitzRoy Road, Exeter EX1 3PB, UK

University of Michigan, Department of Atmospheric, Oceanic and Space Sciences, Ann Arbor, USA

University of Oslo, Department of Geosciences, Oslo, Norway

University of Oslo, Department of Geosciences, Oslo, Norway

University of Oslo, Department of Geosciences, Oslo, Norway