Influence of Liquid Water Content and Temperature on the Form and Growth of Branched Planar Snow Crystals in a Cloud

Abstract

ow liquid water content (LWC) and temperature affect the growth of branched planar snow crystals is poorly understood. To address this issue, a vertical supercooled cloud tunnel was used to grow 167 individual snow crystals for 10 min under nearly constant conditions. The LWC varied within 0.07?0.76 g m?3 and average temperature varied from ?12.4° to ?16.3°C, with the latter varying by at most 0.2°C per run.The crystal habits are divided mainly by temperature, warm to cold, into nine regions: sector above ?12.5°C; then broad branch to ?13.0°C; and then stellar, dendrite, and fern to ?14.5°C. Then the pattern reverses, with dendrite to ?14.8°C, stellar to ?15.7°C, broad branch to ?16.1°C, and finally sector. From ?13.3° to ?13.8°C, stellar changes to dendrite with LWC increase as well as with temperature decrease. From ?13.8° to ?14.5°C, dendrites coexist with ferns below an LWC of 0.25 g m?3, but only ferns exist at higher LWC. At other temperatures, a higher LWC does not produce greater side-branch development. Nevertheless, an increase of LWC produces heavier crystals (particularly crystals larger than 1 mm across), despite the crystal diameters being independent of LWC. Stellars at ?14.9°C have the maximum mass and diameter, whereas ferns at about ?14.3°C have the maximum basal-plane area and minimum fall speed. The apparent crystal density varies with temperature, with two local minima for stellars near ?13.3° and ?15.3°C and a local maximum for ferns near ?14.2°C.