Marta Trodahl, University of Oslo, Geosciences, Oslo, Norway, Pål Erik Isachsen, University of Oslo, Oslo, Norway, Jonathan M Lilly, Theiss Research, La Jolla, CA, United States, Johan Nilsson, Stockholm University, Department of meteorology, Stockholm, Sweden and Nils Melsom Kristiansen, Norwegian Meteorological Institute, Ocean and Ice, Oslo, Norway