Vapor Diffusional Growth of Free-Falling Snow Crystals between -3 and -23°C

Abstract

The characteristics of snow crystal growth by vapor diffusion at water saturation and in free fall were quantitatively investigated in a vertical supercooled cloud tunnel for periods up to 30 min at temperatures from -3 to -23°C.
The results obtained are as follows: 1) the basic growth habits were plates (>-4.0°C), columns (-4.0--8.1°C), plates (-8.1--22.4°C) and columns (<-22.4°C), respectively. At about -5.5, -12, -14.5, -16.5 and -18°C, crystal shapes were enhanced with time; 2) for an isometric crystal, the slope of a log-log plot between the crystal mass and the growth time showed the Maxwellian value of 1.5. The mass growth rate of a shape-enhanced crystal was larger than that of the isometric crystal, indicating more effective vapor transfer on the former; 3) in the case of shape-enhanced planar crystals grown at around -12, -14.5 and -16.5°C, ventilation effects became recognizable, whereas the effect was not evident for needle crystals grown at about -5.5°C. This suggests that the characteristic length of the flow field even around a needle crystal is along the a-axis. The ventilation effect became significant when the Reynolds number exceeded about 2 (sector) and 5 (dendrite); 4) linear relationships between the drag coefficient and the Reynolds number were found in log-log plots.