Riming growth process contributing to the formation of snowfall in orographic areas of Japan facing the Japan Sea

Heavy snowfall occurs every year in orographic areas of Japan facing the Japan Sea. The riming growth process contributing to the formation of snowfall was studied by radar and microphysical analyses. The observational results can be summarized as follows. Radar analyses showed that snow clouds which formed over the Japan Sea moved from the sea toward the land, and changed from the developing stage to the mature stage and then to the dissipating stage as they moved inland. When the clouds reached an orographic area, they developed again due to topographic updraft. The microphysical measurements showed that rime mass increased with an increase in snowfall intensity, and with an increase in wind speed when the mass flux of ice crystals was subtracted. Besides, it was supposed from the figure of wind speed vs. riming proportion relationship that rime mass increased with an increase in wind speed even if the advection effect of cloud droplets is subtracted. These indicate in the case of a strong wind that falling ice crystals can wash out many cloud droplets in clouds where cloud droplets increased, resulting in a snowfall of high riming proportion and high intensity. Thus, the riming growth process contributes significantly to the formation of orographic snowfall. The observational results also showed that the deposition and aggregation growth processes contributed to the formation of orographic snowfall. It was shown that the liquid water content in snow clouds depends on not only the production rates of clond droplets due to topography-induced updraft, but also the mass flux of ice crystals in clouds. The observational results showed that two cases in which there are only a small mass flux of ice crystals, as compared with the amounts of cloud droplets, have a high potential for artificial seeding experiments.