Breakout Summary Report

ARM/ASR User and PI Meeting

Breakout Description

Aerosol effects on deep convective clouds have received considerable attention over the last 20 years. However, several recent studies have called into question the magnitude and sign of these effects, in addition to showing strong correlations between aerosol loadings and meteorological conditions. A multitude of poorly understood, complex processes across a range of spatiotemporal scales has challenged the scientific community’s ability to pinpoint the precise sign and magnitude of aerosol effects on deep convective cloud properties, including updraft velocities, precipitation, and radiative effects. Recent ARM-sponsored field campaigns have specifically targeted this complex problem (e.g., CACTI and TRACER). These data provide some of the best portrayals of the aerosol-cloud-precipitation parameter space for deep convective clouds to date. Motivated by the recent literature and field campaigns, this breakout session aims to: 1) review arguments for and against aerosol “invigoration” of deep convective clouds based on theory, modeling, and observations; 2) discuss how the community can unite around new ARM-sponsored field campaign data sets, statistical techniques, and modeling setups to improve understanding of key processes and better quantify the magnitudes of aerosol impacts on deep convective clouds.

Main Discussion

The session featured eight talks followed by a general discussion. An introductory presentation was given by Dié Wang with assistance from Adele Igel and Hugh Morrison. This introduction provided an overview of the current state of aerosol-convection interaction studies and the challenges in investigating these processes using both observations and numerical models. Most of the presentations in the session focused on proposing or validating potential aerosol invigoration mechanisms in deep convective clouds using various sources of observations such as ground-based and satellite and different modeling frameworks including idealized and realistic model setups. The scientific presentations were as follows: 

1. Influence of environmental moisture on aerosol indirect effects in Houston convection  Saleeby, S. van den Heever, M. Jensen, D. Wang, and S. Giangrande


2. Observational Analysis of Aerosol Impacts on Deep Convective Cell Evolution Using CSAPR2 Cell Tracking Data  Oue


3. Invigoration or Enervation of Convective Clouds by Aerosols?  Igel, S. van den Heever


4. Entrainment Makes Pollution More Likely to Weaken Deep Convective Updrafts than Invigorate them Peters


5. Dependence of Supersaturation Fluctuation and Warm-Phase (Condensational) Invigoration on Aerosol Size Distribution Fan


6. Do Ultrafine Cloud Condensation Nuclei Invigorate Deep Convection? Grabowski, H. Morrison


7. Aerosols Unable to Intensify Storms via Warm-Phase Invigoration Romps


8. A Critical Evaluation of the Evidence for Aerosol Invigoration of Deep Convection Varble

The discussion focused on whether aerosols invigorate deep convection and the specific conditions under which this phenomenon might occur. Another aspect of the conversation centered on the potential for warm-phase (or condensational) invigoration within similar or dissimilar environmental contexts. Notably, the absence of direct supersaturation measurements within clouds was underscored as a limiting factor in validating potential invigoration pathways. While there exist supersaturation estimations from various field campaigns, the need for more direct measurements was emphasized. Observational and modeling evidence was presented for relatively high supersaturation aloft, but the importance of this was questioned because of the sharp reduction in water vapor content with height (reduction with temperature).

Additionally, discussions encompassed the appropriate methodologies for extracting the environmental conditions that trigger the formation of deep convective clouds when employing observational data sets to investigate this subject. 

Key Findings

1. The question of whether aerosols indeed invigorate deep convection remains unresolved, necessitating careful model configurations and comprehensive evidence gleaned from long-term observational data. Presently, we lack direct measurements of both updraft velocity and supersaturation within the deep convective cores. Consequently, imposing direct observational constraints poses considerable challenges. Nonetheless, certain recent advancements provide promising avenues. For instance, the adapted radar cell tracking data from TRACER mark encouraging initial steps in addressing these challenges.


2. When studying aerosol-convection interactions, the role of entrainment or the three-way interactions between aerosol, convection, and environment (humidity) cannot be ignored. Interesting results shown by S. Saleeby and J. Peters emphasized the importance of considering the environmental impacts. 


3. Reducing uncertainty in estimates of aerosol-induced invigoration from numerical models requires continued model improvement to reduce biases in simulated dynamical and microphysical convective properties. Models currently have substantial biases that may render estimates of invigoration questionable, and different models produce a large spread in results.


4. Careful consideration of the model setup is needed to put simulation results for invigoration in context, including spread from natural variability and “randomness” of convection.

Issues

There was limited time for discussion because some of the talks ran over time. For future breakout sessions, particularly on this topic in which having sufficient discussion time is a key, we recommend fewer talks ed to a fixed number (not taking last- minute requests).

Needs

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Decisions

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Future Plans

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Action Items

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