Breakout Summary Report


ARM/ASR User and PI Meeting

Convective Processes Working Group
27 October 2022
2:00 PM - 5:00 PM
Hugh Morrison and Adam Varble

Breakout Description

The Convective Processes Working Group focuses on 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 effects on cloud microphysics and precipitation; 2) transitions in cloud populations and mesoscale (medium-scale) organization of convection; and 3) interactions between cloud microphysics, aerosols, precipitation, and radiation. Research approaches involve both direct observations and retrievals of environmental and cloud properties, as well as process development and improvement of convective representations in models.

Main Discussion

This session featured seven talks followed by general discussion. The first talk was primarily by Adam Varble and gave an overview of the Convective Processes Working Group (CPWG), including the mission statement, research themes, summary of discussion at previous CPWG meetings, and recent relevant field campaigns (CACTI, COMBLE, TRACER, SAIL, BNF). Scott Collis discussed interaction with ARM via the ARM Translator Group, ARM Open-Source Resources, and other ARM data products relevant to the CPWG. Links to provide feedback to ARM’s future plans were also provided.
The science talks were:
• Jiwen Fan (remote): “Notable impact of wildfires in the western U.S. on weather hazards in the central U.S.”
• Henrique Barbosa: “GoAmazon 2014/15 observations of the shallow-to-deep convection transition in Amazonia”
• Zhe Feng: “Bridging observations and LES towards reducing uncertainties in convection-permitting models”
• Dan Kirshbaum: “Cumulus dilution: Correlation versus causation”
• John Peters (remote): “Toward a unified perspective on what controls entrainment in deep convection”
• Jake Mulholland: “Does vertical velocity influence entrainment in moist thermals”
• Hugh Morrison: “Control of the shallow-to-deep convective transition by environmental relative humidity and the horizontal scale of sub-cloud forcing”

Discussion centered mainly on cloud entrainment and dilution. There was also some discussion about the role of microphysics-dynamics interactions, use of LASSO-CACTI in CPWG-related research, and issues concerning model resolution and convergence moving to fine scales.

Key Findings

Key results from the talk and discussion were:
• Distinguishing between entrainment and dilution is important including distinctions in how these are calculated and defined. For example, dilution across the cloud is important for understanding overall cloud structure and vertical mass flux, while the properties of the least dilute parcel are important for the transition to deep convection and cloud top height.
• Much of the discussion was centered around the challenge in separating correlation versus causation in the factors driving cloud dilution, particularly the role of cloud vertical velocity.
• There was general agreement that a key aspect of cloud dilution is the impact of entraining dry environmental air, providing a direct connection of the properties of deep convection and its initiation to relative humidity of the free troposphere. CACTI observations further support this view, in that failed events for convection initiation in which there was CAPE and little convective inhibition generally occurred when the free troposphere was dry.
• The LASSO-CACTI data set is beginning to be mined for studies on convective dynamics, factors driving convection initiation, and entrainment/dilution in particular, with considerable promise as a tool to investigate these issues.
• The links of deep convective clouds with aerosols and microphysics continues to be a topic of debate, particularly the role of aerosols in “convective invigoration”. There is some consensus on the “warm invigoration” mechanism driven by higher supersaturation with respect to liquid in pristine compared to polluted clouds, but little consensus on “cold invigoration” related to ice processes.
• A number of studies on processes controlling deep convection in the Amazon have now been performed since GoAmazon, and some agreed that it would potentially be worthwhile for the authors of those studies to write a paper that assesses the cumulative conclusions of those studies including discrepancies and areas of agreement.
• With MCS tracking in new regional and global storm-resolving models, there is potential to isolate MCS contributions to persistent GCM biases in mean state and diurnal cycle.







Future Plans

There was brief discussion on a possible white paper and targeted future workshop on entrainment and dilution in deep convective clouds, potentially leading to a review article. However, there are no definite plans for this, and the main barrier is to find someone with the time to lead such an effort.

Action Items