1. a Alfred-Wegener-Institute for Polar and Marine Research , D-14473 Potsdam, Telegrafenberg A 43, Germany
2. b Deutscher Wetterdienst, Meteorological Observatory Lindenberg , D-15864 Lindenberg, Germany
3. c Deutscher Wetterdienst, Meteorological Observatory Hohenpeissenberg , Albin-Schwaiger-Weg 10, D-82383 Hohenpeissenberg, Germany
4. d Department of Applied Physics, University Athens, 33 Ippokratous Street , Athens, 106 80, Greece