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Roger F. Reinking


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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Sergey Y. Matrosov, Roger F. Reinking, and Irina V. Djalalova


Single pristine planar ice crystals exhibit some flutter around their preferential horizontal orientation as they fall. This study presents estimates of flutter and analyzes predominant fall attitudes of pristine dendritic crystals observed with a polarization agile Ka-band cloud radar. The observations were made in weakly precipitating winter clouds on slopes of Mt. Washington, New Hampshire. The radar is capable of measuring the linear depolarization ratios in the standard horizontal–vertical polarization basis (HLDR) and the slant 45°–135° polarization basis (SLDR). Both HLDR and SLDR depend on crystal shape. HLDR also exhibits a strong dependence on crystal orientation, while SLDR depends only weakly on orientation. The different sensitivities of SLDR and HLDR to the shape and orientation effects are interpreted to estimate the angular flutter of crystals. A simple analytical expression is derived for the standard deviation of angular flutter as a function of the HLDR to SLDR ratio assuming perfect radar system characteristics. The flutter is also assessed by matching theoretical and observed depolarization patterns as a function of the elevation of the radar’s beam. The matching procedure is generally more robust since it accounts for the actual polarization states and imperfections in the radar hardware. The depolarization approach was used to estimate flutter of falling pristine dendrites that were characterized by Reynolds numbers in a range of approximately 40–100. Using the matching approach, this flutter was found to be about 9° ± 3°, as expressed by the standard deviation of the crystal minor axes from the vertical direction. The analytical expression provides a value of flutter of about 12°, which is at the high end of the estimate obtained by the matching procedure. The difference is explained by the imperfections in the polarization states and radar hardware, so the analytical result serves as an upper bound to the more robust result from matching. The values of flutter estimated from the experimental example are comparable to estimates for planar crystals obtained in laboratory models and by individual crystal sampling.

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