Breakout Summary Report

Breakout Description

Agenda, “Primary and secondary ice production and impacts on mixed-phase and ice clouds” breakout session

Wednesday, August 9, 2:00 – 4:00 pm EDT, Breakout Session 5

Conveners: Xiaohong Liu, Ann Fridlind, Daniel Knopf, Nicole Riemer

2-3:10pm: Short presentations on advances in understanding and modeling ice formation:

2-2:10pm: Gourihar Kulkarni: Potential vertical BBSOA and Organics INP concentrations from SGP and Amazon regions

2:10-2:20pm: Naruki Hiranuma: (Virtual) Role of Surface Pressure Variability on the Ground-Level Ice-Nucleating Particle (INP) Abundance on the North Slope of Alaska

2:20-2:30pm: Vaughan Phillips: Multiple SIP mechanisms: organisation by cloud-type and by cloud age

2:30-2:40pm: Minghui Diao: (Virtual) Hemispheric Comparisons of Ice and Mixed-Phase Cloud Properties based on In-situ Observations and DOE E3SMv1 Model

2:40-2:50pm: Ann Fridlind: Context for a potential AIDA-based cirrus cloud parcel model intercomparison

2:50-3pm: Sylwester Arabas: Immersion freezing in particle-based aerosol-cloud microphysics: a probabilistic perspective on singular and time-dependent models

3-3:10pm: Jessie Creamean: Establishment of long-term DOE ARM INP measurements in space and time

3:10-3:50: Discussions on issues, questions, and new ideas to motivate future campaigns and group activities such as:

What are the current bottlenecks in enhancing our predictability of ice nucleation activity?

• What are the contributions of terrestrial and maritime biogenic aerosols to ambient INPs?


• What are the best venues in the field and laboratory (e.g., in cloud chambers) to conduct an aerosol-ice formation closure campaign? How can these venues serve as a testbed to advance our fundamental understanding of ice formation in clouds?


• 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 through interactions with other microphysical/dynamical processes?

3:50-4pm: Final wrapup

Main Discussion

The breakout session was organized into two portions. In the first portion, we had seven talks followed by active discussion. The remaining time was devoted to discussing big-picture bottlenecks that inhibit us in enhancing our predictability of ice nucleation activity.

Gourihar Kulkarnie started off showing a combined laboratory and field study to assess the vertical distribution of biomass burning SOA and organic INPs from ARM SGP and Amazon regions. Surrogate organic compounds (guaiacyl acetate, α-pinene) examined as INPs in the laboratory are applied to vertically and composition-resolved aerosol particles. This allows for a first-order estimation of potentially present INPs stemming from the organic aerosol fraction. It was acknowledged that the particles are chemically more complex than the surrogate compounds and phase state may also differ.

Naruki Hiranuma reported on long-term INP measurements using the PINE chamber at ARM NSA and SGP sites. Naruki showed INP number concentrations and activated fractions for varying seasons over a period of one year. The long-term data set allowed his group to correlate INP number concentrations with surface pressure (e.g., Aleutian Low - Beaufort High), observing seasonal variations due to different air masses reaching the measurement site. Naruki suggested to correlate other continental features to INP numbers, like surface roughness/turbulence.

Vaughan Phillips presented an overview of multiple secondary ice production (SIP) mechanisms and his group efforts to implement and simulate convective cloud systems. He reported on new experimental results on snow flakes shattering. Also, raindrop-on-ice shattering was pointed out to be a dominant SIP mechanism. Vaughan showed how cloud type and age relate to different SIP processes. For example, ice-ice collisions can prevail in the first 30 mins of cloud evolution. Overall, each SIP mechanism has its own characteristic time scale.

Minghui Diao presented north-to-south comparisons of ice and mixed-phase cloud properties based on DOE MARCUS and COMBLE observations and DOE's E3SMv1 Model. In situ field campaign measurements were compared to E3SM results. The supercooled liquid fraction in northern hemisphere is greater than the southern hemisphere. It appears that E3SM underestimates ice formation in the northern hemisphere.

Ann Fridlind put forward the case for a potential AIDA-based cirrus cloud parcel model intercomparison. The reason for this is to better understand cirrus formation under gravity wave conditions, such as encountered during the SPARTICUS campaign. How do gravity waves and wind shear impact heterogeneous ice nucleation under synoptic cirrus conditions? Too few observations are at hand.

Sylwester Arabas introduced the concept of super-particle modeling to study immersion freezing. Sylwester demonstrated the stark difference in ice crystal number concentration when using particle-based aerosol-cloud microphysic in a super-particle model that implemented a singular and time-dependent freezing description. The model results highlight the issue that under different cloud cooling rates, a time-dependent immersion freezing description can yield significantly greater ice crystal number concentrations compared to a singular approach. Discussion involved how generally valid this sensitivity study is and if INPs are treated prognostically.

Jessie Creamean, who serves as an ARM INP mentor, showcased long-term DOE ARM INP measurements. This included how to access the data, the nature of the different data sets, and some data, including complementary treatments including heat or hydrogen peroxide treatment to distinguish inorganic from organic INPs. Jessie also conducted a live poll on INP measurements asking where INP measurements should be conducted to gain feedback from the community. 

In the shorter-than-planned discussion portion of the session, issues, remaining questions, and challenges were discussed, which included:

What are the best venues in the field and laboratory (e.g., in cloud chambers) to conduct an aerosol-ice formation closure campaign? How can these venues serve as a testbed to advance our fundamental understanding of ice formation in clouds?

What are the impacts of ice formation processes on mixed-phase and ice cloud properties through interactions with other microphysical/dynamical processes?

Key Findings

• Constraining SIP processes is still a challenging task. Convincing field measurements are rare. Laboratory measurements are scarce.  A dedicated field campaign for the SIP importance and ice formation closure is needed.


• Make use of opportunities when field INP and aerosol measurements are available to attempt closure to gauge parameterizations.


• The debate of ice nucleation being described as a time-independent or time-dependent process, that started in the 1950s, is still an open question. This has to be addressed by focused studies.  


• Characterization of aerosol particles that serve as  INPs is key. This includes composition characterization of refractory aerosol with appropriate  size resolution. If dust is involved, one would need to determine the mineralogy. Call to make use of EMSL facilities.


• Polling by audience showed interest in sampling INP in: free troposphere, Brazil, Arctic, and Southern Ocean.

Issues

N/A

Needs

Despite having a very different breakout session, the needs resemble the ones from  last year’s breakout session. ASR and ARM communities are working on those issues and this work in progress is reflected in some of the needs:

• aerosol-INP closure studies: ground, airborne, vertical (TBS) are needed to provide real-world evaluations


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


• SIP: 
  
  
  
  • instrument intercomparison for ice crystal number measurements in clouds would help to reduce the uncertainty associated with small ice crystals
  
  
  • Best approaches to identify SIP in field measurements
  
  
  • Model: assess primary ice nucleation rate (uncertainties) with subsequent implications for SIP.
  
  
  
  


• INPs in cloud systems have to be modeled prognostically.


• Careful lab and field studies that specifically address the time-dependency and possibly stochastic nature of freezing.


• More observations of upper tropospheric gravity waves are needed.


• Freezing mechanisms under gravity wave conditions need to be studied.


• Cirrus ice crystal properties have to be assessed.


• INPs in the free troposphere have  to be characterized.

Decisions

N/A

Future Plans

N/A

Action Items

N/A