Breakout Summary Report

ARM/ASR User and PI Meeting

13 - 17 March 2017

Session Title: Cloud Phase
Session Date: 13 March 2017
Session Time: 4:00 PM - 6:00 PM
Number of Attendees: 45
Summary Authors: Gijs de Boer

Breakout Description

Agenda:
4:00-5:10 (Research Highlights)
- 4:00-4:10: Introductory remarks
- 4:10-4:25: “Ice nucleation and the microphysical properties of mixed-phase clouds” (R. Shaw)
- 4:25-4:40: “Improving the Simulations of Mixed-Phase cloud partitioning in CAM” (X. Liu)
- 4:40-4:55: “Dynamical responses of mixed layer cloud to seeder-feeder effects” (Y. Chen)
- 4:55-5:10 “Contrasting ice nucleation characteristics in stratiform mixed phase clouds over the north and south poles” (D. Zhang)
5:10-5:50 (Ongoing Activity – Comparing in-situ, ground-based and satellite perspectives on cloud phase, coordinated by M. Maahn)
5:50-6:00 Group discussion – Relevance of cloud phase within the new ASR WG framework

Main Discussion

Before getting started, the two “new” PIs present (C. Naud and N. Wood) were asked to introduce themselves to the group and state their interest in the Cloud Phase focus group. After these introductions, we moved into a variety of brief presentations that were selected to broaden awareness of current studies focused on development of understanding of cloud phase and its relevance in the Earth system. Descriptions of these presentations are included in the "Key findings" box. For the second half, Maximilian Maahn (Oliktok Site Science Team) provided an overview of the current ongoing group activity, aimed at comparing measurements of cloud phase from a variety of perspectives, including in situ (airborne), surface-based remote sensing, and satellite-based remote sensing. The idea for such an evaluation was hatched during the Cloud Phase breakout session at last year’s PI meeting. To ensure familiarity with this effort within the group, Max first introduced this topic and the purpose of this work to the group. From there, he showed some initial results for a comparison based on the ARM ISDAC campaign completed several years ago at/around Barrow, as well as a longer-term study completed through comparison with the European CloudNET measurements from the JOYCE and Ny Alesund sites. Results from this study are presented in the "key findings" box.

Key Findings

- Raymond Shaw (MTU) provided an overview of ongoing efforts to understand the lifecycle of ice and liquid in the mixed-phase cloud environment. This work was motivated in part by the development and maintenance of lake-surface-driven convective mixed-phase clouds resulting from cold air outbreaks over the Great Lakes and the related snow events. He demonstrated a variety of power law relationships connecting ice water content and ice crystal concentration to different nucleation regions in the cloud. Further, he provided insight into modeling studies used to evaluate the life cycle of individual ice crystals within a cloud structure, accounting for recycling of ice crystals. Finally, Raymond provided an exciting first look at results from the cloud chamber at Michigan Tech and mixed-phase cloud experiments that are being conducted in this chamber.
- Xiaohong Liu (U. Wyoming) gave an overview of efforts to evaluate cloud phase in CAM, as well as results from a sensitivity study comparing different parameterization combinations and their representation of cloud phase. Most directly, simulations for which the ice nucleation parameterization was updated to replace the traditional Meyers scheme with one built upon classical nucleation theory (Wang and Liu) provided improved results when compared to satellite-based estimates of cloud phase. The other simulations did not adequately produce liquid water at colder temperatures. However, all simulations were shown to have lower supercooled liquid water fractions when compared to surface-based retrievals at Barrow.
- Yaosheng Chen (PSU) provided information on ongoing modeling work to understand the impacts of ice-induced holes on dynamics and the liquid portion of Arctic stratiform clouds. This work was motivated by observations from the North Slope of Alaska site. The modeling results, obtained using the LES version of the RAMS model, showed a clear dynamical response to the latent heating of the atmosphere resulting from the cloud seeding. This dynamical response was shown to result in further modification of the cloud structure, providing insight into the consequences of phase partitioning within such a cloud layer.
- Damao Zhang (U. Wyoming) gave an overview of work to compare stratiform cloud phase at NSA to that observed during AWARE. This work demonstrated that stratiform cloud frequency of occurrence had similar seasonal distributions at the two sites. However, the AWARE cases were shown to occur at colder temperatures than those occurring at NSA. In addition, CloudSAT reflectivities for these cloud were shown to generally be lower around McMurdo than around NSA, particular at warmer temperatures. Interestingly, this evaluation looked very different from space than from the surface using the ARM measurements.
- Maahn presentation: Comparisons between surface- and space-based estimates of cloud phase as a function of cloud top temperature from the CloudNET comparison was quite good. However, similar comparisons for the ISDAC data showed wide disagreement between perspectives. These results sparked substantial discussion about the best ways to move this study forward, including comments regarding the definition of clouds, thresholds, sensitivity, and the potential need to integrate a modeling product and appropriate forward models to make sense of the differences. Additionally, there was some discussion on other potential data sets that could be integrated, and suggestions ranged from ENA, to Oliktok Point, and to measurements from AWARE. These sorts of comparisons could be facilitated thanks in part to processing of the CloudSat/CALIPSO perspective completed by the University of Wyoming (Wang/Zhang).

Issues

Lack of cloud phase evaluation at mid-latitude (i.e., ENA) sites.

Needs

To wrap up the breakout session, we had a brief group discussion on the future of the Cloud Phase focus group and how it fits into the new ASR working group structure. As a part of this, attendees were polled as to which focus groups they associated with. Somewhat surprisingly, six of the attendees felt strongly connected to the Warm Boundary Layer Processes working group. Similarly, six attendees felt strongly connected to the Convective Processes working group, and four attendees were connected to the Aerosol Processes working group. Not surprisingly, most (around 30-35) attendees felt strongly linked to the High-Latitude Processes working group. It was encouraging to see engagement from all the focus groups, and interesting that a specific recommendation was made to evaluate the frequency of occurrence of ice-containing clouds at lower-latitude sites, such as the ARM Eastern North Atlantic Site.

Decisions

N/A.

Future Plans

Continue current exercise on derivation of cloud phase from different observational perspectives.

Action Items

Evaluate additional sites/IOPs for evaluating cloud phase measurement.
Include modeling component to assess the impact of subsampling and scale.