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Atmospheric Chemistry and Physics An interactive open-access journal of the European Geosciences Union
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Discussion papers
https://doi.org/10.5194/acp-2019-150
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/acp-2019-150
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 01 Apr 2019

Submitted as: research article | 01 Apr 2019

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This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Chemistry and Physics (ACP).

No anomalous supersaturation in ultracold cirrus laboratory experiments

Benjamin W. Clouser1,2, Kara D. Lamb1,a, Laszlo Sarkozy2, Alexandra Nisenoff2, Jan Habig3, Volker Ebert4, Harald Saathoff3, Ottmar Möhler3, and Elisabeth J. Moyer2 Benjamin W. Clouser et al.
  • 1Department of Physics, University of Chicago, Chicago, IL, USA
  • 2Department of the Geophysical Sciences, University of Chicago, Chicago, IL, USA
  • 3Institute of Meteorology and Climate Research, Karlsruhe Institute of Technology, 76021 Karlsruhe, Germany
  • 4Physikalisch-Technische Bundesanstalt, 38116 Braunschweig, Germany
  • acurrently at: Cooperative Institute for Research in the Environmental Sciences, Boulder, CO, USA

Abstract. High-altitude cirrus clouds are climatically important: their formation freeze-dries air ascending to the stratosphere to its final value, and their radiative impact is disproportionately large. However, their formation and growth are not fully understood, and multiple in-situ aircraft campaigns have observed frequent and persistent apparent water vapor supersaturations of 5 %–25 % in ultracold cirrus (T < 205 K), even in the presence of ice particles. A variety of explanations for these observations have been put forth, including that ultracold cirrus are dominated by metastable ice whose vapor pressure exceeds that of hexagonal ice. The 2013 IsoCloud campaign at the AIDA cloud and aerosol chamber allowed explicit testing of cirrus formation dynamics at these low temperatures. A series of 28 experiments allow robust estimation of the saturation vapor pressure over ice for temperatures between 189 and 235 K, with a variety of ice nucleating particles. Experiments are rapid enough (~ 10 minutes) to allow detection of any metastable ice that may form, as the timescale for annealing to hexagonal ice is hours or longer over the whole experimental temperature range. We show that in all experiments, saturation vapor pressures are fully consistent with expected values for hexagonal ice and inconsistent with the highest values postulated for metastable ice, with no temperature-dependent deviations from expected saturation vapor pressure. If metastable ice forms in ultracold cirrus clouds, it appears to lack the defects and interfaces that are assumed to produce differences in vapor pressure from hexagonal ice.

Benjamin W. Clouser et al.
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Short summary
Previous measurements of water vapor in the upper troposphere and lower stratosphere (UT/LS) have shown unexpectedly high concentrations of water vapor in ice clouds, which may be due to an incomplete understanding of the structure of ice and the behavior of ice growth in this part of the atmosphere. Water vapor measurements during the 2013 IsoCloud campaign at the AIDA cloud chamber show no evidence of this ‘anomalous supersaturation’ in conditions similar to the real atmosphere.
Previous measurements of water vapor in the upper troposphere and lower stratosphere (UT/LS)...
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