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| Titel |
Homogeneous Freezing of Water Droplets and its Dependence on Droplet Size |
| VerfasserIn |
Thea Schmitt, Ottmar Möhler, Kristina Höhler, Thomas Leisner |
| Konferenz |
EGU General Assembly 2014
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250086589
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| Publikation (Nr.) |
 EGU/EGU2014-483.pdf |
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| Zusammenfassung |
| The formulation and parameterisation of microphysical processes in tropospheric clouds,
such as phase transitions, is still a challenge for weather and climate models. This includes
the homogeneous freezing of supercooled water droplets, since this is an important process in
deep convective systems, where almost pure water droplets may stay liquid until
homogeneous freezing occurs at temperatures around 238 K. Though the homogeneous ice
nucleation in supercooled water is considered to be well understood, recent laboratory
experiments with typical cloud droplet sizes showed one to two orders of magnitude smaller
nucleation rate coefficients than previous literature results, including earlier results from
experiments with single levitated water droplets and from cloud simulation experiments at
the AIDA (Aerosol Interaction and Dynamics in the Atmosphere) facility. This
motivated us to re-analyse homogeneous droplet freezing experiments conducted
during the previous years at the AIDA cloud chamber. This cloud chamber has a
volume of 84m3 and operates under atmospherically relevant conditions within
wide ranges of temperature, pressure and humidity, whereby investigations of both
tropospheric mixed-phase clouds and cirrus clouds can be realised. By controlled
adiabatic expansions, the ascent of an air parcel in the troposphere can be simulated.
According to our new results and their comparison to the results from single levitated
droplet experiments, the homogeneous freezing of water droplets seems to be a
volume-dependent process, at least for droplets as small as a few micrometers in diameter. A
contribution of surface induced freezing can be ruled out, in agreement to previous
conclusions from the single droplet experiments. The obtained volume nucleation rate
coefficients are in good agreement, within error bars, with some previous literature data,
including our own results from earlier AIDA experiments, but they do not agree with
recently published lower volume nucleation rate coefficients. This contribution will
show the results from the re-analysis of AIDA homogeneous freezing experiments
with pure water droplets and will discuss the comparison to the literature data. |
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