Preprints
https://doi.org/10.5194/acp-2021-511
https://doi.org/10.5194/acp-2021-511

  21 Jun 2021

21 Jun 2021

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

Aerosol-cloud interactions: The representation of heterogeneous ice activation in cloud models

Bernd Kärcher1 and Claudia Marcolli2 Bernd Kärcher and Claudia Marcolli
  • 1Institute of Atmospheric Physics, DLR Oberpfaffenhofen, Wessling, Germany
  • 2Institute for Atmospheric and Climate Science, ETH Zurich, Zurich, Switzerland

Abstract. The homogeneous nucleation of ice in supercooled liquid water clouds is characterized by time-dependent freezing rates. By contrast, water phase transitions induced heterogeneously by ice nucleating particles (INPs) are described by time-independent ice-active fractions depending on ice supersaturation (s). Laboratory studies report ice-active particle number fractions (AFs) that are cumulative in s. Cloud models budget INP and ice crystal numbers to conserve total particle number during water phase transitions. Here, we show that ice formation from INPs with time-independent nucleation behavior is overpredicted when models budget particle numbers and at the same time derive ice crystal numbers from s-cumulative AFs. This causes a bias towards heterogeneous ice formation in situations where INPs compete with homogeneous droplet freezing during cloud formation. We resolve this issue by introducing differential AFs, moving us one step closer to more robust simulations of aerosol-cloud interactions.

Bernd Kärcher and Claudia Marcolli

Status: open (until 02 Aug 2021)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on acp-2021-511', Anonymous Referee #1, 24 Jul 2021 reply

Bernd Kärcher and Claudia Marcolli

Bernd Kärcher and Claudia Marcolli

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Short summary
Aerosol-cloud interactions play an important role in climate change. Simulations of the competition between homogeneous droplet freezing and heterogeneous ice nucleation can be compromised by misapplication of ice-active particle fractions frequently derived from laboratory measurements or parameterizations. Our study frames the problem and establishes a solution that is easy to implement in cloud models.
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