Formation of Poly-Crystalline Snow Crystals by Riming Process

[摘要] Freely falling water droplets 20-100μm in diameter were frozen by collision with ice crystals at temperatures between -10 and -20°C. Crystalline nature of frozen droplets depended on the nature of the crystal faces to which they accreted. Frozen droplets on the prism face were single-crystalline with the same crystal orientation as the ice crystal. Frozen droplets on the basal face were also single-crystalline with the same orientation as the ice crystal at temperatures above -13°C, and were poly-crystalline at temperatures below -19°C.Two types of the growth of frozen droplets were observed. In most cases of single-crystalline frozen droplets, circular crystal faces appeared and the droplets grew to snow crystals with well-known crystal habit as shown by Nakaya's diagram. In the case of poly-crystalline frozen droplets and several single-crystalline droplets frozen at temperatures below about -17°C, steps parallel to the basal faces were formed or the spherical surface of the droplets became rugged in shape. Some of the steps or a part of the rugged surface grew into plate-like crystals. Such plate-like growth occurred even at temperatures below -24°C and seems to be related to the formation of snow crystals of a combination of columns and plates.

[学科分类] 大气科学