The Onset and Early Growth of Snow Crystals by Accretion of Droplets

Abstract
Snow crystals sampled from winter storms of the Sierra Nevada have been examined to determine their riming characteristics in terms of their dimensions and habits of growth. The field data show that accretion can begin on crystals that have grown for only 2–4 min. However, the onset of accretion is delayed over longer growth periods for many crystals, so considerable dispersion occurs in the size at the onset of riming on specific types oaf crystals. This dispersion, which occurs in individual snow showers as well as over the durations of whole storms, and results from interactions of various cloud processes, is very important in describing and modeling snow crystal growth. The minimum sizes of the observed crystals at the onset of accretion, in terms of major crystal dimensions, ranged from 115 to 320 μm. The minimum widths of different columnar types at the onset were very uniform (30–36 μm). Nevertheless, the basic columnar and planar habits show very systematic differences in minimum dimensions and durations of growth required for riming to begin; the measured sizes and growth times are somewhat less than those predicted by the more rigid of the current theories of accretion. The systematic differences among the various habits carry through to heavier stages of accretion. In dividually branched planar and radiating crystals and capped columns develop the highest rates of accretion and precipitate the most water in the form of rime. Total precipitation, of course, depends on the concentrations of the various crystals. Assuming that the laboratory evidence for the rime-droplet splintering mechanism can be applied in the field, significant needle and sheath production through such multiplication probably occurred in the Sierran cloud systems.

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