Abstract
Two extratropical cyclones, passing along the Pacific Ocean side of Japan Island around February 8 and 14, 2014, brought heavy snowfall, serious damages, and frequent occurrence of avalanches over the Kanto-Koshin areas. During the snowfall periods of the two cyclones, we carried out radar observation, continuous measurements of size and fall speed of falling snows, and classification of snow crystals in Niigata district. The observations revealed that in the both cases the main snow types were aggregates of plate, closs plate, and bullet, which grow in the temperature lower than−20℃. This types was different from the types during ordinary winter monsoon. The radar observations also revealed that heights of top echoes were 5 km or more, which were higher than ordinary winter monsoon, 4 km or lower. It is also ascertained by the data analysis with meso-scale meteorological model that at high altitudes above 5 km temperature was lower than−20℃ and relative humidity was higher than 90% (ice super-saturation), and the high humidity region had expanded to lower altitude according to the lows approached, that satisfied the conditions for formation and maintenance of low temperature-type snow crystals. Moreover these low temperature-type snow crystals are suggested to relate to the frequent occurrence of avalanches in Kanto-Koshin areas, not only because similar events had reported in a previous article, but because glass-like snow including columnar crystals were observed near an avalanche area.
