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| Titel |
Origin of diversity in falling snow |
| VerfasserIn |
J. Nelson |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1680-7316
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| Digitales Dokument |
URL |
| Erschienen |
In: Atmospheric Chemistry and Physics ; 8, no. 18 ; Nr. 8, no. 18 (2008-09-26), S.5669-5682 |
| Datensatznummer |
250006392
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| Publikation (Nr.) |
 copernicus.org/acp-8-5669-2008.pdf |
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| Zusammenfassung |
| This paper presents a systematic way to examine the origin of variety in
falling snow. First, we define shape diversity as the logarithm of the number
of possible distinguishable crystal forms for a given resolution and set of
conditions, and then we examine three sources of diversity. Two sources are
the range of initial-crystal sizes and variations in the trajectory
variables. For a given set of variables, diversity is estimated using a model
of a crystal falling in an updraft. The third source is temperature-updraft
heterogeneities along each trajectory. To examine this source,
centimeter-scale data on cloud temperature and updraft speed are used to
estimate the spatial frequency (m−1) of crystal feature changes. For
air-temperature heterogeneity, this frequency decays as p−0.66, where
p is a measure of the temperature-deviation size. For updraft-speed
heterogeneity, the decay is p−0.50. By using these frequencies, the
fallpath needed per feature change is found to range from ~0.8 m, for
crystals near −15°C, to ~8 m near −19°C – lengths
much less than total fallpath lengths. As a result, the third source
dominates the diversity, with updraft heterogeneity contributing more than
temperature heterogeneity. Plotted against the crystal's initial temperature
(−11 to −19°C), the diversity curve is "mitten shaped", having a
broad peak near −15.4°C and a sharp subpeak at −14.4°C,
both peaks arising from peaks in growth-rate sensitivity. The diversity is
much less than previous estimates, yet large enough to explain observations.
For example, of all snow crystals ever formed, those that began near
−15°C are predicted to all appear unique to 1−μm resolution,
but those that began near −11°C are not. |
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