1. Polar Terrestrial Environmental Systems, Alfred Wegener Institute (AWI) Helmholtz Centre for Polar and Marine Research, Telegrafenberg, 14473 Potsdam, Germany
2. Department of Physics, Freie Universität Berlin, Arnimallee 14, 14195 Berlin, Germany
3. Institute of Polar Sciences (CNR-ISP), National Research Council, Spianata S. Rainei 86, 98122 Messina, Italy