24 Jun 2022
24 Jun 2022
Status: this preprint is currently under review for the journal ACP.

Impact of formulations of the nucleation rate on ice nucleation events

Peter Spichtinger1, Patrik Marschalik1,a, and Manuel Baumgartner1,2,b Peter Spichtinger et al.
  • 1Institute for Atmospheric Physics, Johannes Gutenberg University Mainz, Germany
  • 2Zentrum für Datenverarbeitung, Johannes Gutenberg University Mainz, Germany
  • anow at: DB Systel GmbH, Berlin, Germany
  • bnow at: German Weather Service (DWD), Offenbach, Germany

Abstract. Ice formation in cold temperature regimes is most probably dominated by homogeneous freezing of aqueous solution droplets. The nucleation rate as derived from laboratory experiments can be represented as a function of water activity. For idealized nucleation events as modelled with a state-of-the-art ice microphysics, the impact of different approximations of the nucleation rate on the resulting ice crystal number concentrations and maximum supersaturation ratios is investigated. The nucleation events are sensitive to the slope of the nucleation rate but only weakly affected by changes in its absolute value. This leads to the conclusion that details of the nucleation rate are less important for simulating ice nucleation in bulk models, if the main feature of the nucleation rate (i.e. its slope) is represented sufficiently. The weak sensitivity on the absolute values of the nucleation rate suggests that the amount of available solution droplets also does not crucially affect nucleation events. The use of just one distinct nucleation threshold for analysis and model parameterisation should be reinvestigated. The frequently used thresholds corresponding to a very high nucleation rate value is not reached in many nucleation events with low vertical updrafts. In contrast, the maximum supersaturation and thus the nucleation thresholds reached during an ice nucleation event depend on the vertical updraft velocity or cooling rate. This feature might explain some high supersaturation values during nucleation events in cloud chambers and suggests a reformulation of ice nucleation schemes used in coarse models based on a fixed nucleation threshold.

Peter Spichtinger et al.

Status: open (extended)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on acp-2022-434', Anonymous Referee #1, 15 Jul 2022 reply
  • CC1: 'Comment on acp-2022-434', Bernd Kärcher, 19 Jul 2022 reply

Peter Spichtinger et al.

Peter Spichtinger et al.


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Short summary
We investigate the impact of the homogeneous nucleation rate on idealized nucleation events. As long as the slope of the rate is represented sufficiently well, the resulting ice crystal number concentrations are not crucially affected. Even a change in the prefactor over orders of magnitude does not change the results. However, the maximum supersaturation during nucleation events shows strong changes. This quantity should be used for diagnostics instead of the popular nucleation threshold.