Breakout Summary Report

ARM/ASR User and PI Meeting

Session Title: Convective Processes Working Group
Session Date: 26 June 2020
Session Time: 11:00 AM - 1:00 PM
Number of Attendees: 213
Summary Authors: Adam Varble and Hugh Morrison

Breakout Description

The convective processes working group focuses on improving understanding and model
representation of convective (heat-transferring) cloud processes and properties, including cloud cover,
precipitation, life cycle, dynamics, and microphysics, over a range of spatial scales. Research areas include:
1) convective vertical velocity (upward heat transfer) and interactions with cloud microphysics and
precipitation; 2) shallow to deep cloud transitions and organization of convective clouds on larger scales;
and 3) interactions between cloud microphysics, aerosols, precipitation, and radiation.
Participants: Please contact Adam Varble (adam.varble@pnnl.gov) and Hugh (morrison@ucar.edu) if you
have suggestions for discussion topics.

Main Discussion

The session was divided into two main parts. In the first, there were short (~5 min) presentations on recent ARM campaigns/activities relevant to the CPWG. After a short introduction, these were given by Adam Varble on CACTI, Bill Gustafson on LASSO, Mike Jensen on TRACER, Scott Giangrande on SEUS, Luiz Machado on ATTO (non-ARM observing site in the Amazon), and Mike Jensen on CPMSG. There were some questions and discussion using the “chat” feature in Zoom related to these talks, mainly centered around clarification questions on instrumentation.
The second part of the session was focused on ~10 min science talks followed by discussion. John Peters gave a talk on entrainment, convective updraft structure, and role of vertical wind shear in the environment. Courtney Schumacher presented work on mesoscale convective organization focusing on cases from GO-AMAZON. The main purpose of these talks was to facilitate discussion on the main science questions relevant to the CPWG. The discussion that followed was organized around three main science themes: 1) convective draft dynamics and entrainment, 2) mesoscale convective organization, and 3) convective microphysics.
Most discussion centered on topic #1, particularly on the role of entrainment and convective draft structure on the shallow-to-deep convective transition (convective initiation). This focus maps well onto recent and future ARM activities including LASSO, which is focused on convective initiation and early lifecycle of CACTI cases, as well as recent and planned observations from CACTI, TRACER, and SEUS. Discussion centered particularly on how to define updraft entities (i.e., calculate their radius), which is not straightforward even from model large eddy simulation data. This is important since there is ample evidence for the importance of updraft radius in influencing entrainment and draft dynamics. There was also discussion on what instrumentation and platforms might best address these questions, including sonde/profiler data to capture near-storm environments, storm penetrating aircraft, and usage of scanning radars (e.g. XSAPR). There was discussion of how these observational data map into CPMSG in the context of these science questions. The current recommendations in CPMSG are very extensive and more prioritization will likely be needed. It was also noted that important science questions regarding basic draft dynamics and convective microphysics could be explored in tropical oceanic convective systems with weaker updrafts that can be penetrated by aircraft, more easily interpretable radar data, and fewer complications from a heterogeneous surface and baroclinicity.

Key Findings

The vast majority of the science discussion centered on questions related to updraft dynamics, entrainment, and mesoscale organization. This seems to reflect a shift in recent ASR PI science toward these topics away from convective microphysics, which is also reflected in the submitted poster abstracts. The reasons for this shift are unclear but additional clarity on how ARM and ASR activities map onto one another would be useful for the working group.
The following suggestions had multi-person support:
● There is a lot of interest in Doppler spectra, which is now compressible to the point that it should not present archival issues and thus should be saved whenever possible.
● Near-cloud meteorological information is currently inadequate for examining environmental controls on convective clouds. A more distributed profiling network would be very beneficial.
● ARM may be in a unique position to further tackle stratiform-anvil-radiation issues related to mesoscale organized convective systems.
● There was the most interest across observational and modeling communities to work much more on updraft size, strength, and entrainment, which are critical to shallow-to-deep transition, vertical transport, and cumulus parameterizations. This could serve as a possible workshop topic given a lot of research and progress currently happening in this area. Related to this, there were several subtopics discussed:
○ The roles of many environmental properties such as stability, relative humidity, vertical wind shear, boundary layer depth, and mesoscale modification of these properties on convective updraft size, entrainment, and strength are still mostly open questions even though this is at the heart of cumulus parameterization.
○ There is a critical need to formulate new radar, aircraft, and UAV observational strategies to target updraft size and strength. There are tradeoffs to various strategies but precise pros and cons are not well defined and most approaches have come up short of what is usually promised. Retrievals are needed that resolve km-scale thermals and their evolution at ~minute time scales across many, variable cases and cloud life cycle stages.
○ Tropical, oceanic shallow through deep convection was highlighted as being more ideal to target than continental convection like that targeted in recent and upcoming field deployments in that (i) aircraft can often penetrate deep convective updrafts because of little lightning, graupel, or hail, (ii) radar retrievals are easier to interpret because of little hail and relatively clean conditions that provide observable echoes prior to ice formation, (iii) environmental conditions affecting clouds are relatively simpler, and (iv) large sample sizes can be obtained. In the US, locations that were mentioned were Florida, Puerto Rico, and Guam even though Florida and Puerto Rico are arguably subtropical.

Issues

The virtual format of the session made discussions very difficult to manage. There were many parallel conversations occurring without much of an ability to distill the discussion down to key points. We should consider whether improvements can be made if any future meetings will be held virtually.
Our hope was to prioritize CPMSG recommendations and critical science that can benefit from recent and planned ARM measurements through discussions. We were partially successful in that convective updraft structure and entrainment clearly resonated the most with attendees, perhaps because this also reflects ongoing ASR-funded research. However, we were unable to get attendees to provide feedback in terms of prioritization. Instead, attendees just added to a long list of wants, which has been a problem for ARM to handle up until now in that many things get partially done or not particularly well done because of limited resources. The current CPMSG recommendations are too extensive and are therefore likely to have limited benefit without prioritization and focus on the most critical steps.

Needs

N/A

Decisions

See “Future Plans”.

Future Plans

Based on the listed issues and a lack of topically focused groups since the reorganization of working groups, there is some support for a topically focused workshop from which concrete suggestions for progress can be distilled. A workshop is admittedly a lot of work, so one idea is to try to make sure that something like a review paper comes out of the workshop. The most likely topic for such a workshop would be convective updraft dynamics and entrainment given widespread community interest including within ASR in this topic currently.
A fall virtual working group meeting could be beneficial but is also a lot of work, so the pros and cons should be carefully considered before making formal plans.

Action Items

See “Future Plans”.