Breakout Summary Report

Building on outcomes from the 2023 ASR Workshop on New Directions in Ice Processes Research, this session focuses on advancing the understanding of ice processes in clouds, from primary ice production (PIP) and secondary ice production (SIP) to ice crystal growth and evolution. These processes are critical to cloud dynamics, precipitation, and radiative properties but remain key sources of uncertainty in models.

This session will review findings from the 2023 workshop, identify barriers to progress, and explore actionable pathways forward. Discussions will emphasize integrating observations, laboratory studies, and modeling to address critical gaps, including:

• Observational challenges for measuring small-scale processes such as PIP and SIP.
• Integration of PIP and SIP mechanisms into models and evaluating their impacts.
• Best practices for connecting ARM facility data to cloud system studies.

Goals and Outcomes:

• Share insights from the workshop to inform broader ARM/ASR community efforts.

• Develop recommendations for new ARM observational priorities and modeling strategies.

• Identify actionable steps for leveraging ARM datasets to improve representation of ice processes in models.

Fundamental understanding and accurate model representations of cloud ice formation processes are critical for modeling cloud evolution, precipitation, and radiative balance.  This breakout session built on the outcomes of the 2023 ASR Workshop on New Directions in Ice Processes Research, with a focus on advancing understanding of ice formation in clouds, from primary ice production (PIP) to secondary ice production (SIP). The session began with an overview of the key workshop outcomes provided by Dr. Susannah Burrows, followed by presentations by Dr. Russell Perkins addressing the INP – ice crystal number discrepancy within the scope of DOE SAIL campaign data and by Dr. Christina McCluskey describing efforts of merging observations with models.  Each presentation was followed by discussion, with significant time reserved for an overall discussion at the end of the session.

Despite substantial progress in understanding atmospheric ice processes in recent decades, several key knowledge gaps persist that require further research. While these topics were discussed initially at the workshop with a select group of researchers, this breakout session enabled additional feedback on atmospheric ice process research limitations from a broader community.

For primary ice production (PIP), multiple theoretical approaches exist; there is a need for a careful intercomparison of these approaches with targeted laboratory experiments. Another important area of research is bio-INPs, which are an important contributor to the warm-temperature INP population.  Studies that advance understanding of the emissions, transport and associated aging and efficacy of INPs remain a priority.

Secondary ice production (SIP) was another major topic of discussion.  Field observations indicate that SIP is widespread in the atmosphere.  SIP involves multiple mechanisms by which new ice crystals are formed from previously present ice: at least eight have been identified, including rime splintering, shattering during droplet freezing, and collisional breakup of ice. Coordinated laboratory, field, and modelling efforts are needed to quantify the relative importance of these SIP mechanisms in specific cloud regimes.

Limitations of current instrumentation also contribute to the challenges in advancing understanding of the ice phase: in particular, there are challenges surrounding the measurement of small ice crystals, large INPs, water vapor supersaturation, and high-resolution vertical winds (in both time and space).

The translation of microscale aerosol and cloud processes into parameterizations of their macroscale effects is a long-standing challenge, as is the coordinated integration of multiple observational datasets into modelling efforts at multiple scales.  New machine learning methods and advances in computational power open doors to innovation on both challenges.

Several current challenges associated with atmospheric ice processes were mentioned, including:

·       Orders of magnitude difference between INPs and ice concentrations.  The community faces challenges in linking these due to gaps in both measurements and process-level understanding.

·       Measuring small ice crystals (< 100 micron) and large aerosols (> 1 micron) remains a major challenge for both field and laboratory experiments.

·       Cloud seeding as a major source of INPs in some seasons and regions, not always accounted for.

·       Importance of contextualizing observations before integrating into models; need for cloud regime-oriented approaches that account for both micro- and macrophysics.

·       Instrumentation and measurement frequencies needed to understand model subgrid variability.

• Collaborative, Multi-Scale, Regime-Based Process System Studies:

  • Focus on specific cloud regimes (e.g., shallow cumulus, deep convection, polar clouds) to systematically test hypotheses about PIP and SIP.

  • Develop a multi-scale “process system” approach that links microscale physics to macroscale cloud evolution, using field campaign data for model evaluation.

• Instrument and Retrieval Method Development:

  • Improved quantification and characterization of small ice crystals, supermicron aerosol and size-resolved INPs.  Real-time INP instruments to assess rapid temporal variations.

  • High-time-resolution vertical velocity measurements.

• Targeted Laboratory Experiments to Refine Predictive Understanding of Specific Ice Processes:

  • Tabletop and wind tunnel experiments of individual SIP mechanisms.

  • SIP studies that take a multi-angle approach (combining field, lab, and models).

• Targeted Field Campaign Efforts:

  • Dedicated field campaign for intercomparison of remote sensing / retrieval methods.

  • Lagrangian field experiments following a cloud-containing air mass.

  • Aerosol – Ice Formation Closure Studies.

N/A

See “Action Items” below.

Publish ASR ice processes workshop report.