Breakout Summary Report

ARM/ASR User and PI Meeting

19 - 23 March 2018

Session Title: Use of field measurements to constrain ice nucleation models
Session Date: 20 March 2018
Session Time: 1:30 PM - 3:30 PM
Number of Attendees: 43
Summary Authors: Xiaohong Liu, Paul DeMott, Ann Fridlind, Daniel Knopf, Nicole Riemer

Breakout Description

Many measurements of ice nucleating particle (INP) spectra obtained by DOE ASR and used by modelers to date were designed to perform ice crystal–INP closure studies, establishing to what degree observed INP can explain observed ice. For stratiform mixed-phase clouds, for instance, such measurements were consistently made at cloud-top temperature, intentionally generating a data set with maximum sample size over a narrow dynamic range of environmental conditions. However, constraining ice nucleation schemes used in climate models is better served by testing a wide dynamic range. This breakout will be dedicated to two related discussion topics: (i) the design of long-term and IOP measurements optimized to constrain climate model schemes where they are most uncertain, and (ii) low-overhead preliminary intercomparison activities that could be accomplished with existing data sets.

Main Discussion

The breakout session was devoted to two sequential discussion areas:
(1) INP field campaign design
• aerosol-ice formation closure concept: limited analog to aerosol-CCN closure
• required measurements: PSD, composition, mixing+phase state, projected area, INP spectra (mode-specific)
• general questions: where and how? past lessons? points of model deficiency?
(2) INP model evaluation with field observations (pre-campaign)
• motivations, challenges, existing examples
• potential objectives of a pre-campaign study organized around the closure concept: establish global model spread, biases, relevance, methods, campaign priorities

Key Findings

What existing field campaign data sets are available for pre-campaign pilot study (comparing climate model aerosol and INP to observations within closure framework)?
• FIN03 at Storm Peak (multiple INP methods, large aerosol suite)
• various SGP (3-week CFDC deployment in spring, other 3-month deployment, particle sampling on substrate)
• MARCUS over the Southern Ocean
• AMF3 deployment at Oliktok Point
• MAGIC shipboard
Miscellaneous information
• there is now an APS for supermicron aerosol PSD at SGP (characterization of efficiency?)
• SGP has long-term particle composition data (recently reprocessed)
• independent open-path lidar measurement of aerosol SA may require multi-wavelenth lidar or single-wavelength + radiometric retrieval
• while lab and field measurements must both be advanced, there is a recogized limitation of laboratory models to represent common ambient nucleating properties

Issues

What is missing from FIN3 relative to a closure campaign?
• size-segregated INP measurements (i.e., impactor)
• single-particle analysis other than mass spec (i.e., microscopic methods)
• continuous high-volume filter composition measurements (do have every 3rd day from nearby Mt. Zirkel IMPROVE site at similar altitude)
• HSRL/lidar open-path-type measurement of total aerosol surface area

Needs

What is missing from presented list of required measurements (above)?
• size resolution of INP spectra
• biological/microbial/macromolecule measurements (spectral?)
• CFDC aerosol concentrator (10–100X particle enhancement) and INP capture on filters
• mineralogy composition

Decisions

No objections raised that aerosol-ice formation closure concept is worthwhile to pursue (never explicitly attempted to our knowledge). A looser interpretation of the concept is that key supporting measurements are designed to support substantially more robust model INP evaluation than is possible with existing data sets. FIN03 is the best data set to start with for pre-campaign climate model-observation comparison of IFN within context of size-distributed total surface area and composition (loose test of closure approach).

Future Plans

• Fridlind and Liu to collaborate with observationalists (DeMott, Hallar) on comparison of CAM and ModelE results extracted at Storm Peak with FIN3 observations (aerosol PSD, composition, INP spectra)
• further discussion of field campaign proposal, likely at SGP, not an instrument comparison as in FIN3, but a closure-driven field campaign

Action Items

• Fridlind and Liu to start FIN03-based analysis of CAM and ModelE
• establish experimental uncertainties of required measurements (e.g., supermicron surface area)
• contact Laura Riihimaki regarding future development of lidar-based total surface area retrieval
• contact Jim Mather re IMPROVE data near SGP
• is it possible to combine MPL and sun photometer to obtain total aerosol SA, as done by Patric Seifert/TROPOS a few years ago?