Preprints
https://doi.org/10.5194/acp-2021-801
https://doi.org/10.5194/acp-2021-801

  29 Sep 2021

29 Sep 2021

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

Assessing the potential for simplification in global climate model cloud microphysics

Ulrike Proske1, Sylvaine Ferrachat1, David Neubauer1, Martin Staab2, and Ulrike Lohmann1 Ulrike Proske et al.
  • 1Institute for Atmospheric and Climate Science, ETH Zürich, Zürich, Switzerland
  • 2Max-Planck-Institut für Gravitationsphysik (Albert-Einstein-Institut), Hannover, Germany

Abstract. Cloud properties and their evolution influence Earth's radiative balance. The cloud microphysical (CMP) processes that shape these properties are therefore important to be represented in global climate models. Historically, parameterizations in these models have grown more detailed and complex. However, a simpler formulation of CMP processes may leave the model results mostly unchanged while enabling an easier interpretation of model results and helping to increase process understanding. This study employs sensitivity analysis on an emulated perturbed parameter ensemble of the global aerosol-climate model ECHAM-HAM to illuminate the impact of selected CMP cloud ice processes on model output. The response to the phasing of a process thereby serves as a proxy for the effect of a simplification. Aggregation of ice crystals is found to be the dominant CMP process in influencing key variables such as the ice water path or cloud radiative effects, while riming of cloud droplets on snow influences mostly the liquid phase. Accretion of ice and snow and self-collection of ice crystals have a negligible influence on model output and are therefore identified as suitable candidates for future simplifications. In turn, the dominating role of aggregation suggests that this process has the greatest need to be represented correctly. A seasonal and spatially resolved analysis employing a spherical harmonics expansion of the data corroborates the results. This study introduces a new framework to evaluate a processes' impact in a complex numerical model, and paves the way for simplifications of CMP processes leading to more interpretable climate models.

Ulrike Proske et al.

Status: open (until 10 Nov 2021)

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

Ulrike Proske et al.

Ulrike Proske et al.

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Short summary
Cloud microphysical processes shape cloud properties and are therefore important to be represented in climate models. Their parameterization has grown more complex, making the model results more difficult to interpret. Using sensitivity analysis we test how the global aerosol-climate model ECHAM-HAM reacts to changes to these parameterizations. The model is sensitive to the parameterization of ice crystal aggregation, but e.g. not to self-collection, suggesting that it may be simplified.
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