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| Titel |
Experimental evidence for millisecond activation timescales using the Fast IN Chamber (FINCH) measurements |
| VerfasserIn |
U. Bundke, R. Jaenicke, H. Klein, B. Nillius, B. Reimann, T. Wetter, H. Bingemer |
| Konferenz |
EGU General Assembly 2009
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 11 (2009) |
| Datensatznummer |
250027933
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| Zusammenfassung |
| Abstract:
Ice formation in clouds is a subject of great practical and fundamental importance since the
occurrence of ice particle initializes dramatic changes in the microphysical structure of the
cloud, which finally ends in the formation of precipitation. The initially step of ice formation
is largely unknown. Homogenous nucleation of ice occurs only below -40 °C. If an ice
nucleus (IN) is present, heterogeneous nucleation may occur at higher temperature. Here
deposition freezing, condensation and immersion freezing as well as contact freezing are
known. Also growth rates of ice particles are known as function of crystal surface
properties, temperature and super saturation. Timescales for homogenous freezing
activation in the order of 0.01 seconds and nucleation rates have been measured
by Anderson et al. (1980) and Hagen et al., (1981) using their expansion cloud
chamber.
This contribution of deposition mode freezing measurements by the ice nucleus counter
FINCH presents evidence that the activation timescale of this freezing mode is in the order of
1E-3 seconds. FINCH is an Ice Nucleus counter which activates IN in a supersaturated
environment at freezing temperatures. The activation conditions are actively controlled by
mixing three gas flows (aerosol, particle-free cold-dry and warm-humid flows).See Bundke et
al. 2008 for details. In a special operation mode of FINCH we are able to produce
a controlled peak super saturation in the order of 1 ms duration. For several test
aerosols the results observed in this particular mode are comparable to normal mode
operations, where the maximum super saturation remains for more than a second,
thus leading to the conclusion that the time for activation is in the order of 1ms or
less.
References:
R.J. Anderson et al, ”A Study of Homogeneous Condensation Freezing
Nucleation of Small Water Droplets in an Expansion Cloud Chamber, Journal of
the Atmospheric Sciences, Vol. 37, 2508-2520, 1980
U.Bundke
et al., ”The fast Ice Nucleus chamber FINCH”, Atmospheric Research, Volume
90, Issues 2-4, 180-186, DOI:10.1016/j.atmosres.2008.02.008, 2008
D.E. Hagen et al., ”Homogenous Condensation Freezing Nucleation Rate
Measurements for Small Water Droplets in an Expansion Cloud Chamber”,
Journal of the Atmospheric Sciences, Vol 38, 1236-1243, 1981
Acknowledgments:
This work was supported by the German Research Foundation: SFB 641 “Tropospheric Ice
Phase” TP A1, SPP1294 BU1432/3-1, JA344/12-1, by the Helmholtz Association:
VI-233 “Aerosol Cloud Interactions” and by and by the EU FP6 Infastructure Project
EUSAAR. |
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