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| Titel |
The validity of the kinetic collection equation revisited – Part 3: Sol–gel transition under turbulent conditions |
| VerfasserIn |
L. Alfonso, G. B. Raga, D. Baumgardner |
| 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 ; 13, no. 2 ; Nr. 13, no. 2 (2013-01-16), S.521-529 |
| Datensatznummer |
250017585
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| Publikation (Nr.) |
 copernicus.org/acp-13-521-2013.pdf |
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| Zusammenfassung |
| Warm rain in real clouds is produced by the collision and coalescence of an
initial population of small droplets. The production of rain in warm cumulus
clouds is still one of the open problems in cloud physics, and although
several mechanisms have been proposed in the past, at present there is no
complete explanation for the rapid growth of cloud droplets within the size
range of diameters from 10 to 50 μm. By using a collection kernel
enhanced by turbulence and a fully stochastic simulation method, the formation of a runaway
droplet is modeled through the turbulent collection process. When the
runaway droplet forms, the traditional calculation using the kinetic
collection equation is no longer valid, since the assumption of a continuous
distribution breaks down. There is in essence a phase transition in the
system from a continuous distribution to a continuous distribution
plus a runaway droplet. This transition can be associated
to gelation (also called sol–gel transition) and is proposed here as a mechanism for the formation of
large droplets required to trigger warm rain development in cumulus clouds.
The fully stochastic turbulent model reveals gelation and the formation of a
droplet with mass comparable to the mass of the initial system. The time
when the sol–gel transition occurs is estimated with a Monte Carlo method
when the parameter ρ (the ratio of the standard deviation for the
largest droplet mass over all the realizations to the averaged value)
reaches its maximum value. Moreover, we show that the non-turbulent case
does not exhibit the sol–gel transition that can account for the
impossibility of producing raindrop embryos in such a system. In the context
of cloud physics theory, gelation can be interpreted as the formation of the "lucky
droplet" that grows at a much faster rate than the rest of the population
and becomes the embryo for runaway raindrops. |
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