27 Oct 2022
27 Oct 2022
Status: this preprint is open for discussion.

Technical Note: assessing predicted cirrus ice properties between two deterministic ice formation parameterizations

Colin Tully, David Neubauer, and Ulrike Lohmann Colin Tully et al.
  • Institute for Climate and Atmospheric Science, ETH Zurich, Zurich, Switzerland

Abstract. Determining the dominant ice formation mechanism in cirrus is still an open research question that impacts the abil- ity to assess the climate impact of these clouds in numerical models. Homogeneous nucleation is generally well understood. There is more uncertainty surrounding heterogeneous nucleation processes due to the complex physio-chemical properties of ice nucleating particles (INPs). In addition, determining whether a heterogeneous nucleation process follows a time-dependent (stochastic) or a time-independent (deterministic) approach increases the level of complexity in numerical modeling applica- tions. Kärcher and Marcolli (2021) introduced a new deterministic ice formation parameterization based on the differential activated fraction (AF), arguing that with explicit INP-budgeting this approach could help correct a potential over-prediction of the importance of heterogeneous nucleation within cirrus. We formulated a general circulation model (GCM)-compatible version of the differential AF parameterization and compared it to the method currently employed in the ECHAM6.3-HAM2.3 GCM that is based on cumulative AF, with implicit INP-budgeting. In a series of box model simulations that were based on the cirrus sub-model from ECHAM, we found that the cumulative approach likely under-predicts heterogeneous nucleation in cirrus as it does not account for INP concentration fluctuations across GCM timesteps. However, as the cases that we simulated in the box model were rather extreme, we extended our analysis to compare the differential and cumulative AF approaches in two simulations in ECHAM-HAM. We find that choosing between these two approaches impacts ice nucleation competition within cirrus in our model, but the climate impact is small and insignificant based on our five-year simulations. We argue that while our GCM-compatible differential AF parameterization is closer to first principles, the default approach based on cumulative AF is simpler and leads to more interpretability of the climate model results.

Colin Tully et al.

Status: open (until 24 Dec 2022)

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  • RC1: 'Comment on egusphere-2022-1057', Anonymous Referee #1, 23 Nov 2022 reply

Colin Tully et al.

Colin Tully et al.


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Short summary
A new method to simulate deterministic ice nucleation processes based on the differential activated fraction was evaluated against a cumulative approach. Box model simulations of heterogeneous-only ice nucleation within cirrus suggest that the latter approach likely underpredicts the ice crystal number concentration. Longer simulations with a GCM show that choosing between these two approaches impacts ice nucleation competition within cirrus but leads to small and insignificant climate effects.