Breakout Summary Report
ARM/ASR User and PI Meeting
10 - 13 June 2019
11 June 2019
1:30 PM - 3:30 PM
60
Xiaohong Liu, Paul DeMott, Daniel Knopf, Nicole Riemer
Breakout Description
The purpose of this breakout session was to (1) review progress on ice nucleation from ARM/ASR-funded observations to modeling studies, and to (2) identify and discuss existing and potential avenues for the use of ARM/ASR data sets or future field campaigns to advance understanding and constraints on model prediction of ice nucleation. The breakout session was organized into two portions. The first consisted of eight short talks that provided a review of how existing and potential future ARM/ASR measurements can be used to make specific advances in understanding and modeling ice nucleation in climate models. The second portion was devoted to discussions on main issues/remaining questions/new ideas to motivate future campaigns/group activities.Main Discussion
The session included eight diverse talks covering the recent progress on ice nucleation from campaign, laboratory, and remote-sensing observations, to modeling studies.
Daniel Knopf introduced the ARM SGP Aerosol-ice formation closure study (AIC-Pilot). Naruki Hiranuma introduced his DOE early career study and some first data from the new PINE instrument (semi-automated, mini-expansion cloud chamber). Gourihar Kulkani discussed the use of a revised version of the PNNL continuous flow ice nucleation chamber to derive the sensitivity of frozen fraction for different aerosol types to cooling rate, and for comparison of data to empirical and theoretical parameterizations of the time dependence of INP activation. Will Cantrell explored the INP concentrations required for glaciation of clouds to occur in the MTU Pi chamber. Swarup China presented experiments and data on ice nucleation by glassy soil organic particles. Susannah Burrows spoke on what drives immersion-mode freezing in mixed-phase clouds (what compositions matter). Nicholas Kedzuf showed retrieval of ice number concentrations of pristine ice and aggregates using polarimetric radar observations, which displays moderate secondary ice production (SIP) on the basis of comparison to aerosol-constrained INP parameterizations. Yi Zhao discussed SIP in the context of a climate model.
The discussion portion of the breakout centered on three topics:
- What is the contribution of supermicron-sized particles to ambient INPs? Could particle hygroscopicity of supermicron-sized particles be used as a modeling predictive parameter for immersion freezing?
- What is the role/importance of deposition ice nucleation to total INPs? At which temperature and humidity range of activation does deposition ice nucleation proceed?
- Where and under what conditions does ice multiplication occur? How to address it by lab, field (observational), and modeling studies? What needs to be done to improve predictability?
Key Findings
There are more questions than answers for the three discussed topics.
- Supermicron-Sized Particles: Attendees supported the need for more aerosol measurements at supermicron sizes that include dust and biological particles. Attendees wondered about the abundance of large particles aloft from these surface sampling sites and the lifetime of very-large-size particles (10-20 µm). Convection may be one effective transport mechanism. The challenge in sampling these sizes with aircraft inlets was pointed out. Tethered balloon system (TBS) filter collections will be useful for addressing this issue. Lidar may represent a way to detect these typically non-spherical particles aloft.
- Deposition Ice Nucleation: current studies suggested that a more important role for deposition ice nucleation may be in the cirrus regime rather than mixed-phase clouds. Could this become a focus for study when the new DOE jet is operational (e.g., altostratus, cirrus)? Polar regions represent another natural region for study the deposition nucleation.
- Ice Multiplication: Hallett-Mossop mechanism seems insufficient to explain observations. Associated with freezing of drizzle and larger drops and not simply classical graupel-driven H-M. But why do the larger drops freeze under low INP concentrations? Behavior seems restricted to warm temperatures. Has the absence of SIP been demonstrated in cirrus? What DOE-specific studies besides ACAPEX exist for ambient cloud investigations of ice multiplication (SIP)? Can remote retrievals add to database of case studies of SIP? These questions motivated the need for specific new campaigns and laboratory studies.