Breakout Summary Report


ARM/ASR User and PI Meeting

Ice Nucleation and Secondary Ice Production and Impacts on Mixed-Phase and Ice Clouds
26 October 2022
2:00 PM - 4:00 PM
Xiaohong Liu and Daniel Knopf

Breakout Description

Ice crystal formation in mixed-phase and ice clouds remains poorly understood and constrained in models of various scales, in part owing to uncertainty in both primary ice nucleation and secondary ice production. In the past decade, tremendous efforts have been undertaken in both measurement and modeling studies, particularly by the ARM/ASR-supported research, to address these challenges. The objectives of this breakout session are to (1) review progress on primary and secondary ice formation processes from ARM/ASR long-term observations, IOP deployments, and cloud-resolving and global model studies, and to (2) identify and discuss existing knowledge gaps and potential avenues for the use of ARM/ASR data sets or future field campaigns to advance understanding and constraints on model prediction of primary and secondary ice formation.

We envision the breakout session to be organized into two portions. In the first portion, we invite short contributions to report on how existing and potential future ARM/ASR measurements can be used to make specific advances in understanding and modeling ice formation. The second portion of the breakout will be used for discussions on main issues/remaining questions/new ideas to motivate future campaigns/group activities such as:

• What are the contributions of high-latitude dust, fertile soils, and marine and terrestrial biological aerosols to ambient INPs?

• Where and under what conditions is secondary ice production occurring? How to address it by lab, field (observational), and modeling studies?

• What is the relative importance of primary ice nucleation and secondary ice production for ice formation in different types of clouds (e.g., stratiform, convective, and cirrus clouds)?

• What are the impacts of ice formation processes on mixed-phase and ice cloud properties, cloud radiative effects, and cloud feedbacks?

Main Discussion

The breakout session was organized into two portions. In the first portion, we had eight talks (10 mins each) to report on how existing and potential future ARM/ASR measurements can be used to make specific advances in understanding and modeling ice formation. N. Hiranuma summarized findings concerning sources, atmospheric abundance, composition, and physical properties of INPs during different seasons at the Barrow, Alaska site. S. Burrows presented recent data on sources and variability of INPs at the ARM Southern Great Plains and focused on the contribution to INPs from agricultural dust. P. DeMott talked about the dominant role of biological INPs over arable regions. J. Creamean informed about the seasonal contrast of aerosol types that can initiate ice formation in central arctic clouds during MOSAIC and various sources of INPs in different seasons reflecting sea ice and open waters. E. Luke presented radar quantification of secondary ice occurrence via droplet freezing fragmentation at Utqiagvik. M. Diao provided evidence of secondary ice production (SIP) based on in situ and remote-sensing observations in the high latitudes and contrasted northern to southern hemispheric SIP incidences. G. McFarquhar combined modeling studies and observational campaigns to provide insight into SIP processes and highlighted the issue of detecting small ice crystals. X. Liu presented an investigation of the importance of SIP in comparison with primary ice production for high-latitude cloud properties.

            In the discussion portion of the session, issues, remaining questions, and challenges were discussed. These included:

  • Understanding and quantification of the contributions of terrestrial and maritime biogenic aerosols to ambient INPs.

  • Detection of small ice crystals in clouds and necessary instrument intercomparison for this measurement.

  • The need to conduct laboratory measurement of SIP under controlled environments. Development of a cloud chamber facility that enables this type of research.

  • Assessing environmental conditions that favor SIP processes (e.g., temperature, updraft, droplet size, etc.). Determining relative importance of different SIP mechanisms for given environmental conditions.

  • The importance of vertical variation of INPs to assess the representativeness of surface INP measurements for cloud formation.

Key Findings

  • Soil dust and biological aerosols can be important INPs over arable (continental) regions such as SGP.

  • There is strong evidence of SIP in the arctic and Southern Ocean clouds based on in situ and remote-sensing observations. Modeling studies suggest that all SIP processes need to be considered to achieve agreement with observations.




  • An instrument intercomparison for ice crystal number measurements in clouds would help to reduce the uncertainty associated with small ice crystals.

  • Conduct of airborne aerosol-ice formation closure study (similar to AEROICESTUDY).

  • Deployment of real-time and offline bio-aerosol collections in field studies.

  • Ambient INP measurements should be complemented with online aerosol particle size and composition analysis.

  • Parameterization development of soil dust and biogenic aerosol INPs for cloud and climate models to quantify their significance for ice crystal formation.



Future Plans


Action Items