Preprints
https://doi.org/10.5194/acp-2021-320
https://doi.org/10.5194/acp-2021-320

  21 Apr 2021

21 Apr 2021

Review status: this preprint is currently under review for the journal ACP.

New investigations on homogeneous ice nucleation: the effects of water activity and water saturation formulations

Manuel Baumgartner1,2,a, Christian Rolf3, Jens-Uwe Grooß3, Julia Schneider4, Tobias Schorr4, Ottmar Möhler4, Peter Spichtinger1, and Martina Krämer1,3 Manuel Baumgartner et al.
  • 1Institute for Atmospheric Physics, Johannes Gutenberg University, Mainz, Germany
  • 2Zentrum für Datenverarbeitung, Johannes Gutenberg University, Mainz, Germany
  • 3Forschungszentrum Jülich GmbH, Institute of Energy and Climate Research 7 – Stratosphere, Jülich, Germany
  • 4Institute of Meteorology and Climate Research, Karlsruhe Institute of Technology, Karlsruhe, Germany
  • anow at: Deutscher Wetterdienst, Offenbach, Germany

Abstract. Laboratory measurements at the AIDA cloud chamber and airborne in-situ observations suggest that the homogeneous freezing thresholds at low temperatures are possibly higher than expected from the so-called “Koop-line”. This finding is of importance, because the ice onset relative humidity affects the cirrus cloud coverage and, at the very low temperatures of the tropical tropopause layer, together with the number of ice crystals also the transport of water vapor into the stratosphere. Both, the appearance of cirrus clouds and the amount of stratospheric water feed back to the radiative budget of the atmosphere. In order to explore the enhanced ice onset humidities, we re-examine the entire homogeneous ice nucleation process, ice onset and nucleated crystal numbers, by means of a two-moment microphysics scheme embedded in the trajectory based model (CLaMS-Ice) as follows: the well-understood and described theoretical framework of homogeneous ice nucleation yet includes certain formulations of the water activity of the freezing aerosol particles and the saturation vapor pressure of water with respect to liquid water. However, different formulations are available for both parameters. Here, we present extensive sensitivity simulations testing the influence of three different formulations for the water activity and four for the water saturation on homogeneous ice nucleation. We found that the number of nucleated ice crystals is almost independent of these formulations but is instead sensitive to the size distribution of the freezing aerosol particles. The ice onset humidities, also depending on the particle size, are however significantly affected by the choices of the water activity and water saturation, in particular at cold temperatures  205 K. From the CLaMS-Ice sensitivity simulations, we here provide combinations of water saturation and water activity formulations suitable to reproduce the new, enhanced freezing line.

Manuel Baumgartner et al.

Status: open (until 16 Jun 2021)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse

Manuel Baumgartner et al.

Manuel Baumgartner et al.

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Short summary
An important mechanism for the appearance of ice particles in the upper troposphere at low temperatures is homogeneous nucleation. This process is commonly described by the “Koop-line”, predicting the humidity at the freezing. However, laboratory measurements suggest that the freezing humidities are above the Koop-line, motivating the present study to investigate the influence of different physical parameterizations on the homogeneous freezing with the help of a detailed numerical model.
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