PNNL

The Science

The “mixed-” or “cold-phase invigoration” of deep convection operates via enhanced aerosol particle concentration that increases the number of cloud droplets, which suppresses formation of rain via liquid-only processes and increases the amount of water that is frozen. This enhances latent heating and buoyancy, which in turn increase updraft speed and storm strength. This theory has been extensively used to explain correlations between aerosol and storm properties but has also been shown in recent studies to have critical flaws in its underlying assumptions. More realistic assumptions of how fast liquid freezes and precipitates yield weakly positive or negative effects on updraft buoyancy, while including mixing between the storm updraft and nearby environment enhances negative effects. Recent studies have turned to a second potential invigoration mechanism via enhanced condensational heating caused by lowering an updraft’s supersaturation when the number of droplets is enlarged by increasing the number of aerosol particles. This mechanism is feasible, but its magnitude and effects in the real world are unknown because supersaturations in deep convective updrafts are not well understood.

More complex atmospheric models have also been used to study invigoration processes, but persistent storm property biases and cloud process representation uncertainties undermine confidence in their output, which varies substantially depending on the model setup and case being simulated. Thus, generalizable and quantifiable sensitivities of convective updraft speed to aerosols do not exist. Even if they did, models require comparison with observations to be considered trustworthy. Therefore, many studies have used correlations between aerosol and cloud properties to support theoretical and modeling arguments for aerosol invigoration of deep convection. However, the foundational observational studies upon which many subsequent studies have relied used questionable and sometimes faulty methods without sufficient consideration of alternative explanations.

Based on critical review of key theoretical, modeling, and observational studies, this paper concludes that net invigoration of deep convective updrafts via condensational or mixed phase pathways is at best highly questionable. Based on this finding, steps are provided that support a path forward for scientific progress in this area.

The Impact

The review’s first recommendation for the research community is to determine a common definition for aerosol invigoration of storms. Many studies conflate changes in the updraft speed that defines a storm’s convective intensity with changes in observable properties, such as precipitation, lightning, and radar reflectivity, that do not necessarily require a change in updraft speed. Secondly, estimates of aerosol effect magnitudes across various atmospheric and cloud conditions are needed to design observational and modeling approaches that have a reasonable chance at isolating them. Lastly, much more research is required to constrain a plethora of uncertain cloud processes and their multiscale interactions with the ambient environment to isolate and quantify aerosol effects. Requisite measurements and modeling needed to develop such constraints still need to be identified. Observational targets related specifically to aerosol invigoration of storms mostly rely on aircraft measurements of updraft speed, hydrometeor size distributions, and thermodynamic properties from which key modulating properties, such as quasi-steady supersaturation, condensate loading, phase partitioning, and dilution, can be retrieved.

Summary

Hypothesized aerosol invigoration of deep convection has received significant attention in the research community because of its modulation by anthropogenic emissions that affect aerosol properties. Foundational theoretical, modeling, and observational invigoration studies have methodological shortcomings that render their conclusions and interpretations by many subsequent studies highly questionable. Methodological recommendations are made to limit these shortcomings in future studies in addition to suggestions for observational targets and research that could advance knowledge of aerosol–deep convection interactions.

The following approaches for observational studies are recommended:

1. Continue improving cloud condensation nuclei, convective updraft, and atmospheric state retrievals and consider impacts from deficiencies of proxies used in analyses.
2. Isolate single convective cloud types (e.g., liquid vs. mixed phase) and assess the representativeness of aerosol, cloud, and meteorological sampling times.
3. Avoid post-hoc or subjective selections of sampling times and regions that fit a preconceived narrative.
4. Control atmospheric state parameters known to modulate the convective strength proxy with multivariate analyses that account for covariabilities between all predictor variables.
5. Apply appropriate significance testing that accounts for dependent sampling and non-parametric distributions.
6. Avoid adopting explanations for aerosol-cloud relationships from previous studies without evidence that such explanations are more likely than possible alternatives.

The following points are additionally recommended for modeling studies:

1. Continue improving model representation of updraft dynamics and cloud microphysical processes.
2. Expand usage of large eddy simulations to limit under-resolved deep convective updraft biases.
3. Avoid strong conclusions based on a single simulation; use initial/boundary condition ensembles, simulations across different convective regimes, and model intercomparisons to assess the robustness of results.
4. Consider the limitations of chosen boundary conditions, time integration, domain size, and physics parameterizations in applying findings to the real world.
5. Use objective and representative sampling methods to evaluate model output.
6. Provide observational context to assess confidence in model-derived sensitivities.

The hope is that this study promotes more scientific debate and challenges the community to more frequently think “outside of the box” with scrutiny of influential, high-visibility studies and their interpretations in subsequent studies.

Funding

This study is supported by the Department of Energy, Office of Science, Biological and Environmental Research program as part of the Atmospheric System Research program area through the Integrated Cloud, Land-Surface, and Aerosol System Study scientific focus area.