1. Finnish Meteorological Institute, Erik Palmenin aukio 1, FI-00560 Helsinki, Finland
2. School of Physical and Chemical Sciences, North-West University, PB X6001, Potchefstroom, 2520, Republic of South Africa
3. Centre for Atmospheric and Climate Physics Research and Centre for Climate Change Research, University of Hertfordshire, College Lane, Hatfield, AL10 9AB, United Kingdom