Preprints
https://doi.org/10.5194/acp-2021-438
https://doi.org/10.5194/acp-2021-438
 
11 Jun 2021
11 Jun 2021
Status: this preprint was under review for the journal ACP but the revision was not accepted.

Introducing Ice Nucleating Particles functionality into the Unified Model and its impact on the Southern Ocean short-wave radiation biases

Vidya Varma1, Olaf Morgenstern1, Kalli Furtado2, Paul Field2, and Jonny Williams1 Vidya Varma et al.
  • 1National Institute of Water and Atmospheric Research (NIWA), Wellington, New Zealand
  • 2Met Office, Exeter, United Kingdom

Abstract. Insufficient reflection of short-wave radiation especially over the Southern Ocean region is still a leading issue in many present-day global climate models. One of the potential reasons for this observed bias is an inadequate representation of clouds. In a previous study, we modified the cloud micro-physics scheme in the Unified Model and showed that choosing a more realistic value for the capacitance or shape parameter of atmospheric ice-crystals, in better agreement with theory and observations, benefits the simulation of short-wave radiation over the Southern Ocean by brightening the clouds. However, attempts to modify the cloud phase by directly adjusting the micro-physics process rates like capacitance tend to affect both the hemispheres symmetrically whereas we seek to brighten only the high-latitude Southern Hemisphere clouds. In this study we implement a simple prognostic parametrisation whereby the heterogeneous ice nucleation temperature is made to vary three-dimensionally as a function of the mineral dust distribution in the model. As a result, those regions with less dust number density would have lower nucleation temperature compared to the default global value of −10 °C. By using mineral dust as an indicator for ice nucleating particles in the model, this parametrisation thus captures the impact of ice nucleating particles on the cloud distribution due to its general paucity over the Southern Ocean region. This approach thus improves the physics of the model with minimal complexity.

Vidya Varma et al.

Status: closed

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on acp-2021-438', Anonymous Referee #1, 14 Jul 2021
    • AC1: 'Reply on RC1', Vidya Varma, 27 Nov 2021
  • RC2: 'Comment on acp-2021-438', Anonymous Referee #2, 18 Aug 2021
    • AC2: 'Reply on RC2', Vidya Varma, 27 Nov 2021

Status: closed

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on acp-2021-438', Anonymous Referee #1, 14 Jul 2021
    • AC1: 'Reply on RC1', Vidya Varma, 27 Nov 2021
  • RC2: 'Comment on acp-2021-438', Anonymous Referee #2, 18 Aug 2021
    • AC2: 'Reply on RC2', Vidya Varma, 27 Nov 2021

Vidya Varma et al.

Vidya Varma et al.

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Short summary
We introduce a simple parametrisation whereby the immersion freezing temperature in the model is linked to the mineral dust distribution through a diagnostic function, thus invoking regional differences in the nucleation temperatures instead of the global default value of −10 °C. This provides a functionality to mimic the role of Ice Nucleating Particles in the atmosphere on influencing the short-wave radiation over the Southern Ocean region by impacting the cloud phase.
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