Atmos. Chem. Phys. Discuss., 12, 12623-12662, 2012
www.atmos-chem-phys-discuss.net/12/12623/2012/
doi:10.5194/acpd-12-12623-2012
© Author(s) 2012. This work is distributed
under the Creative Commons Attribution 3.0 License.
Review Status
This discussion paper has been under review for the journal Atmospheric Chemistry and Physics (ACP). Please refer to the corresponding final paper in ACP.
Time dependence of immersion freezing
A. Welti1, F. Lüönd1,2, Z. A. Kanji1, O. Stetzer1, and U. Lohmann1
1ETH Zurich, Institute for Atmospheric and Climate Science, Switzerland
2Federal Office of Metrology, Bern, Switzerland

Abstract. The time dependence of immersion freezing was studied for temperatures between 236 K and 243 K. Droplets with single immersed, size-selected 400 nm and 800 nm kaolinite particles were produced at 300 K, cooled down to supercooled temperatures typical for mixed-phase cloud conditions, and the fraction of frozen droplets with increasing residence time was detected. To simulate the conditions of immersion freezing in mixed-phase clouds we used the Zurich Ice Nucleation Chamber (ZINC) and its vertical extension, the Immersion Mode Cooling chAmber (IMCA). We observed that the frozen fraction of droplets increased with increasing residence time in the chamber. This suggests that there is a time dependence of immersion freezing and supports the importance of a stochastic component in the ice nucleation process. The rate at which droplets freeze was observed to decrease towards higher temperatures and smaller particle sizes. Comparison of the laboratory data with four different ice nucleation models, three based on classical nucleation theory with different representations of the particle surface properties and one singular, suggest that the classical, stochastic approach combined with a distribution of contact angles is able to reproduce the ice nucleation observed in these experiments most accurately. Using the models to calculate the increase in frozen fraction at typical mixed-phase cloud temperatures over an extended period of time, yields an equivalent effect of −1 K temperature shift and an increase in time scale by a factor of ~10.

Citation: Welti, A., Lüönd, F., Kanji, Z. A., Stetzer, O., and Lohmann, U.: Time dependence of immersion freezing, Atmos. Chem. Phys. Discuss., 12, 12623-12662, doi:10.5194/acpd-12-12623-2012, 2012.
 
Search ACPD
Discussion Paper
    XML
    Citation
    Final Revised Paper
    Share