Supercooled Cloud Tunnel Studies on the Growth of Snow Crystals between-4 and-20°C

Abstract

Snow crystal growth in free fall by vapor diffusion under water saturation and riming was studied up to 30 min in a supercooled cloud tunnel.
The results showed that: 1) the growth rates were pronouncedly highest along a-axis at-15°C and along c-axis at-5.5°C. Each peak coincided with the growth of dendrites and needles, respectively; 2) polycrystalline snows, consisting of spatial dendrites and spatial plates, were observed very frequently below-15°C; 3) the variation of fall velocity with growth time could be expressed by two straight lines, except for the temperature zone of dendritic crystal growth, where a curve and straight lines were fitted; 4) the empirical relations between crystal fall velocity and growth time under constant temperatures were divided into two groups. In the higher fall velocity group, isometric crystals grew and were rimed; in the lower fall velocity group, shape enhancement occurred during continuing vapor diffusional growth. The crystals in the former group dropped from 700 to 800m, and those in the latter only 300 to 400m in 30 min; 5) isometric crystals grown at around-10°C changed into graupel. The growth mode was represented by three stages: vapor diffusional, transitional and graupel growths. The crystal mass was roughly proportional to t^1/2, t² and t⁶ in each corresponding stage. In the vapor diffusional and graupel growth stages, the particles followed Stokes' and Newton's laws of resistance, respectively.