Yoshimasa Matsumura, The University of Tokyo, Atmosphere and Ocean Research Institute, Kashiwa, Japan, Naoya Kanna, Hokkaido University, Arctic Research Center, Sapporo, United States, Shin Sugiyama, Hokkaido University, Institute of Low Temperature Science, Sapporo, Japan and Shigeru Aoki, Hokkaido University, Sapporo, Japan