Breakout Summary Report


ARM/ASR User and PI Meeting

Cloud, Aerosol, and Boundary-Layer Process Studies of High-Latitude Marine Post-Frontal Clouds: an Observationally Constrained Model Intercomparison Effort
26 October 2022
4:15 PM - 6:15 PM
Bart Geerts, Mikhail Ovchinnikov, Lulin Xue, Paul DeMott

Breakout Description

Mixed-phase shallow clouds, which are ubiquitous in high latitudes, play a key role in the surface energy budget, arctic amplification, and the global climate system. Following the June 2021 ARM/ASR Joint User Facility/PI Meeting, we had a workshop in Sept ’21 to discuss mixed-phase high-latitude clouds, especially shallow convective clouds over open water, and the challenges related to model parameterizations of many cloud processes such as ice multiplication and boundary-layer processes. Comparisons and attributions were found to be difficult, which led to the idea of a thorough model intercomparison effort. Plans are in progress for this, and a white paper has been submitted for consideration at the PANGASS meeting. This session will focus on refining the framework of the intercomparison effort and how data recently collected over boreal and austral oceans (in particular from COMBLE) can be used to address this challenge. This session is intended to encourage this intercomparison effort and define critical observational needs, their uncertainties, and ways to address these uncertainties. Ultimately, this session is intended to improve our understanding and numerical representation in a gray-scale region between limited-area models (LAM) and large-eddy simulation (LES) models, where cloud microphysical, radiative, boundary-layer, and dynamical processes interact to produce post-frontal shallow convective clouds.

Main Discussion

Following a breakout on the CAO cloud regime and cloud feedbacks in a warming climate at the previous ARM/ASR Joint User Facility/PI Meeting in June 2021, and a 2-day virtual workshop in September 2021 that followed up on that breakout, this session focused on discussing progress in mixed-phase high-latitude cloud research and the challenges related to model parameterizations of many cloud processes, especially in shallow convective clouds over open water. At the virtual workshop in Sept ’21, it was decided to collaborate on a CAO Cloud Regime Model intercomparison effort. A white paper (Fridlind et al. 2022) about this effort was submitted to the June ’22 PAN-GASS meeting in Monterey, where the concept was well received and several more groups signed on. This session focused on refining the framework of the intercomparison effort and how data recently collected over boreal and austral oceans (in particular from COMBLE) can be used to address this challenge. The session opened with a brief introduction of its goals followed by four invited presentations that framed subsequent discussion. Tim Juliano (NCAR) introduced the 13 March 2020 COMBLE cold-air outbreak case, which is the main case study in the intercomparison effort. The following three talks addressed the same case. Florian Tornow (Columbia Univ/GISS) addressed aerosol-cloud-precipitation processes operating in COMBLE’s CAO regime in the North-East Atlantic and contrasted them with those during ACTIVATE. He presented simulations of the March 13 case using prognostic aerosol CCN and INP and proposed four additional cases spanning a range of meteorological conditions. Next, Israel Silber (PSU) discussed evaluation of LES simulations using DHARMA LES with interactive three-mode aerosol scheme with observations from CALIPSO overpass midway through the trajectory and with KAZR at the trajectory’s endpoint at Andenes. Finally, Abigail Williams (UCSD) presented upstream and downstream aerosol data for the case as well as ice nucleating particle temperature spectra from Zeppelin/Svalbard and Andenes sites. A broader spectrum of the COMBLE-related ongoing research was reflected in a series of short lightning talks by Zeqian (Hazel) Xia (OU), Paul DeMott (CSU), Zach Mages (SBU), Peng Wu (PNNL), Christian Lackner (U Wyoming), and Xue Zheng (LLNL). The last 45 min was dedicated to a discussion, which was moderated by Lulin Xue and framed by Tim Juliano, who started with the motivation for and design of the COMBLE CAO LES/SCM intercomparison initiative. The discussion addressed observational constraints for LES: are the data robust, accurate, reliable, meaningful for the model, and mutually complementary? Are the upstream data at Zeppelin sufficiently representative of the arctic free troposphere? We then discussed model design and LES configuration choices: the pros and cons of various setup options. Here, SLIDO was used to gain feedback from participants. Finally, we discussed plans for hierarchical system development: how to link this LES/SCM initiative to climate models, and how to use this initiative to better understand cloud feedbacks? 

Key Findings

To help guide the development of the model intercomparison setup and to further enhance participation of in-person and online audiences, a SLIDO questionnaire was conducted, questions and answers from which are given below:

Question 1: Rank the importance of observations in terms of constraining LES simulations. The outcome was as follows:

1. LWP along the trajectory based on satellite data

2. Cloud morphology based on satellite

3. 2D cloud phase at Andenes (KAZR, MPL, MWR)

4. LWP at Andenes (MWR)

5. 2D vertical velocity and TKE at Andenes (KAZR, MPL)

6. T and moisture profiles at Andenes (AERI)

Question 2: What other observations are useful? (open-ended answers, random order)

  • Reanalysis subsidence and aerosol

  • Free tropospheric aerosol/microphysics

  • Aerosol properties (DSD, composition)

  • Aerosol cloud ice/liquid number information

  • In situ microphysics, cloud microphysics from aircraft

  • In situ observations of INP, and coincidental radar/lidar observations

  • Ice PSD at cloud tops

  • TBS

  • RH

  • Lagrangian balloon with instruments

  • Cloud top height and temperature from geostationary

Question 3: Which large-scale forcing approach do you prefer?

  • Calculate fluxes within LES, based on skin temperature from the large-scale forcing data set (66%)

  • Nudging (additional “forcing” term in conservation equations) (48%)

  • Pressure gradient controlled flow (28%)

  • Surface fluxes prescribed by the large-scale forcing (17%)


The intercomparison initiative involves at least 10 modeling groups and several teams analyzing COMBLE and satellite data. Since there is no call for proposals on the topic of high-latitude processes under the 2023 ASR FAO, some groups may not be able to participate, or their participation may depend on alternative funding sources. 




The above Slido questions 1 and 3 are important for the COMBLE CAO LES/SCM Intercomparison initiative. More effort will be placed on LWP retrieval along the trajectory and cloud morphology analysis, based on satellite data, as well as the 2D cloud phase at Andenes. Also, the intercomparison initiative will use model-computed surface fluxes, not prescribed ones. 

Future Plans

A COMBLE Intercomparison Webinar was conducted on November 4, 2022. The details of the current model intercomparison plan were discussed with interested modeling groups, both national and international, including several that are not represented in the ASR program. Based on their input, slight changes were made to the model design, and a timetable was established 

Action Items

Based on the feedback from the breakout session and the recent webinar, the LES/SCM forcing setup  will be adjusted and is expected to be released by the end of November 2022, with first results from a liquid-only case requested by February 2023.