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https://doi.org/10.5194/acp-2020-253
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/acp-2020-253
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  08 Jun 2020

08 Jun 2020

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This preprint is currently under review for the journal ACP.

Sensitivity of mixed-phase moderately deep convective clouds to parameterisations of ice formation – An ensemble perspective

Annette K. Miltenberger1 and Paul R. Field2,3 Annette K. Miltenberger and Paul R. Field
  • 1Institute for Atmospheric Physics, Johannes Gutenberg-University, Mainz, Germany
  • 2Institute of Climate and Atmospheric Science, School of Earth and Environment, University of Leeds, United Kingdom
  • 3Met Office, Exeter, United Kingdom

Abstract. The formation of ice in clouds is an important processes in mixed-phase and ice-phase clouds. Yet, the representation of ice formation in numerical models is highly uncertain. In the last decade several new parameterisations for heterogeneous freezing have been proposed. It is so far unclear what the effect of choosing one parameterisation over another is in the context of numerical weather prediction. We conducted high-resolution simulations (Δx = 250 m) of moderately deep convective clouds (cloud to about ~ −18 °C over the southwestern UK using several formulations of ice formation and compare the resulting changes in cloud field properties to the spread of a initial condition ensemble for the same case.

The strongest impact of altering the ice formation representation is found in the hydrometeor number concentration and mass mixing ratio profiles. While change in accumulated precipitation are around 10 %, high precipitation rates (95th percentile) vary by 20. Using different ice formation representations changes the outgoing short-wave radiation by about 2.9 W m−2 averaged over daylight hours. The choice of a particular representation for ice formation has always a smaller impact then omitting heterogeneous ice formation completely. Excluding the representation of the Hallett–Mossop process or altering the heterogeneous freezing parameterisation has an impact of similar magnitude on most cloud macro- and microphysical variables with the exception of the frozen hydrometeor mass mixing ratios and number concentrations.

A comparison to the spread of cloud properties in a 10-member high-resolution initial condition ensemble shows that the sensitivity of hydrometeor profiles to the formulation of ice formation processes is larger than sensitivity to initial conditions. In particular, excluding the Hallet–Mossop representation results in profiles clearly different from any in the ensemble. In contrast, the ensemble spread clearly exceeds the changes introduced by using different ice formation representations in accumulated precipitation, precipitation rates, condensed water path, cloud fraction and outgoing radiation fluxes.

Annette K. Miltenberger and Paul R. Field

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Annette K. Miltenberger and Paul R. Field

Annette K. Miltenberger and Paul R. Field

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