Aircraft observations reveal unusually high water-vapor supersaturation inside powerful tropical updrafts, providing the atmospheric conditions needed for tiny aerosol particles to potentially strengthen convection.
Tiny particles suspended in the atmosphere may be capable of giving deep tropical clouds an extra burst of energy โ but only under the right conditions.
For decades, scientists have debated whether very small aerosol particles can intensify convection by creating additional cloud droplets, increasing condensation and releasing more heat into rising air. The proposed mechanism is known as condensational aerosol convective invigoration.
The idea has remained controversial partly because aircraft observations rarely found the extremely high water-vapor supersaturation needed for it to work.
Now, measurements from deep tropical clouds suggest those conditions really do occur.
Researchers examining aircraft observations from the Philippines found supersaturation levels reaching roughly 10% inside deep convective clouds, far above values commonly reported in earlier measurements. The strongest conditions developed in vigorous, relatively clean updrafts containing comparatively few droplets.
The findings, published in Advances in Atmospheric Sciences, suggest that previous studies may not necessarily have disproved the aerosol-invigoration mechanism. Instead, scientists may often have been examining cloud environments where the necessary conditions were unlikely to develop.
Why supersaturation matters
Cloud droplets form when water vapor condenses onto tiny airborne particles known as cloud condensation nuclei.
Normally, once droplets begin forming, they rapidly remove excess water vapor from the surrounding air. This keeps supersaturation โ the amount by which atmospheric moisture exceeds the level required for condensation โ relatively low.
But strong convective clouds can behave differently.
When warm, moist air rises rapidly, cooling continuously produces additional water vapor supersaturation. If condensation onto existing droplets cannot remove that vapor quickly enough, supersaturation can rise.
This creates an opportunity for smaller aerosol particles that would normally remain inactive.
Under sufficiently high supersaturation, these tiny particles can suddenly become new cloud droplets. More droplets mean more surfaces on which vapor can condense, potentially increasing the amount of condensation occurring inside the updraft.
That condensation releases latent heat, which warms the rising air relative to its surroundings.
In theory, the added buoyancy can make the updraft rise faster, potentially strengthening the cloud.
This process is the central idea behind condensational aerosol convective invigoration.
A mechanism scientists struggled to observe
The challenge has been finding convincing evidence that clouds naturally reach the supersaturation levels needed to activate large numbers of additional fine or ultrafine aerosol particles.
Previous aircraft studies generally reported much smaller values.
That apparent absence raised doubts about whether condensational invigoration could operate strongly in real clouds.
But researchers now argue that many earlier measurements were made in environments poorly suited to producing extreme supersaturation.
Some observations focused on polluted clouds, where large numbers of existing droplets provide enormous combined surface area for condensation. Water vapor is therefore consumed quickly, preventing supersaturation from building very high.
Other measurements examined shallow warm clouds or sampled deeper storms below the parts of the cloud where the strongest updrafts and microphysical changes occur.
The new study targeted a different environment: deep tropical convection over relatively clean marine regions.
Aircraft flew through deep tropical clouds
The researchers analyzed measurements collected during NASA's Cloud, Aerosol and Monsoon Processes Philippines Experiment, or CAMPยฒEx.
The aircraft campaign operated over the Philippines and surrounding tropical ocean during 2019, gathering detailed measurements of aerosols, cloud droplets, atmospheric motion, and other properties.
Using observed updraft velocities together with cloud-droplet size distributions, the researchers estimated quasi-steady-state supersaturation.
The calculation represents a balance between two competing processes.
As air rises and cools, it generates supersaturation. At the same time, condensation onto cloud droplets removes excess water vapor.
When upward motion becomes particularly strong while the available droplet surface area remains relatively small, production of supersaturation can temporarily outpace its removal.
That is exactly the combination the researchers found in some of the deepest tropical updrafts.
Supersaturation climbed dramatically with height
The aircraft observations revealed supersaturation values reaching approximately 10% in portions of deep tropical convective clouds containing supercooled liquid water.
The study found that supersaturation generally increased with altitude as collision and coalescence altered the droplet population and the updrafts accelerated.
The highest values were associated particularly with the โ10ยฐC to โ20ยฐC temperature range, according to the published study.
This was important because droplets can remain liquid even at temperatures below freezing.
As droplets collide and combine into larger drops, their total surface area can decrease. Larger drops may also begin falling out as precipitation.
At the same time, stronger updrafts continue rapidly cooling the rising air.
Together, these processes can create a situation in which water vapor is being driven toward supersaturation faster than the remaining droplets can absorb it.
The result can be unusually large supersaturation.
Fewer droplets can mean more available vapor
The observations also revealed an important relationship between droplet concentration and supersaturation.
When clouds contained many small droplets, supersaturation tended to be lower.
That makes physical sense: thousands of additional droplets create a much larger combined surface area on which water vapor can condense.
When droplet concentrations were lower, there was less surface area available to remove vapor from the air.
Supersaturation could therefore rise much higher.
The largest values appeared in environments combining strong upward motion, relatively low droplet concentrations, and cold supercooled conditions.
Those conditions provide exactly the kind of environment in which adding extra fine aerosol particles could potentially have the greatest effect.
Another aircraft campaign found the same extreme behavior
A separate study provides independent evidence that extremely high supersaturation can occur inside deep convection.
Researchers examining measurements from the ESCAPE aircraft campaign over coastal Texas and Louisiana analyzed 219 updraft-core segments.
Most of the measurements showed much more modest supersaturation, but rare extreme cases emerged.
In one powerful Louisiana updraft, estimated quasi-steady-state supersaturation approached 11%.
That measurement occurred inside a strong updraft and was accompanied by a relatively low concentration of droplets โ conditions resembling those expected to produce unusually high supersaturation.
The ESCAPE analysis also found that supersaturation above 1% was more likely under stronger updraft conditions and at subfreezing temperatures.
Together, the Philippines and U.S. aircraft observations strengthen the case that very high supersaturation can occur naturally inside certain deep convective clouds.
Why clean clouds may be the best place to look
The results help explain why previous attempts to detect the mechanism may have produced conflicting conclusions.
A heavily polluted cloud already contains large numbers of aerosol particles and droplets.
That abundance of droplets rapidly consumes supersaturated vapor.
Paradoxically, therefore, the cloud environments containing the most aerosol pollution may not be the easiest places to detect a mechanism requiring exceptionally high supersaturation.
Deep, cleaner marine clouds can provide a more favorable setting.
As coalescence reduces droplet numbers while strong updrafts accelerate, supersaturation can rise dramatically.
Once that happens, even very small aerosol particles entering the cloud could potentially activate into new droplets.
Daniel Rosenfeld, who participated in both studies, argues that researchers therefore need to target the right cloud environments when testing the mechanism.
The implication is clear: failing to observe extreme supersaturation in shallow or highly polluted clouds does not necessarily mean it cannot occur elsewhere.
Tiny aerosols could tap into hidden atmospheric fuel
High supersaturation can be thought of as a reservoir of unused condensation potential.
If extra fine or ultrafine aerosols enter that environment, some may become activated as new droplets.
Those droplets would provide additional surfaces for condensation.
More condensation could release more latent heat.
That additional heat could increase buoyancy and potentially strengthen the upward flow feeding the cloud.
In this sense, high supersaturation provides the โfuelโ needed for the aerosol-invigoration mechanism.
But the researchers emphasize an important limitation.
The aircraft observations show that the necessary atmospheric conditions exist. They do not yet prove that aerosols actually caused the observed clouds to become stronger.
Demonstrating that causal connection requires comparing otherwise similar clouds under different aerosol conditions while carefully measuring their microphysics and dynamics.
What this could mean for rainfall and storms
Deep convective clouds play a major role in Earth's weather and climate.
They transport enormous quantities of heat and moisture through the atmosphere and are responsible for much of the heavy rainfall in tropical regions.
Changes in their updraft strength can influence cloud development, precipitation formation, ice production, lightning, and storm organization.
If tiny aerosol particles can measurably alter these processes, the implications could extend from local weather prediction to climate modeling.
Scientists have long struggled to represent aerosol-cloud interactions accurately because the underlying processes occur on microscopic scales while influencing clouds spanning many kilometers.
Understanding exactly when aerosols strengthen convection โ and when they do not โ could help reduce some of that uncertainty.
The next step is to test the mechanism directly
Researchers now want to move beyond simply identifying extreme supersaturation.
Future aircraft campaigns could deliberately compare clean and polluted deep tropical clouds, particularly inside their strongest updraft regions.
Such observations could reveal whether adding aerosol particles under high-supersaturation conditions actually increases droplet formation, condensation, latent-heat release, and upward acceleration.
Scientists will also need to improve measurements in mixed-phase regions where supercooled liquid droplets and ice particles coexist.
Once ice becomes abundant, estimating supersaturation using only liquid-water measurements becomes considerably more difficult.
Ultimately, the aim is to determine how aerosols influence the development of deep convection and to incorporate those processes more accurately into models of rainfall, severe weather, lightning, and climate.
For now, the aircraft observations have resolved one major piece of the puzzle.
The extreme supersaturation required by condensational aerosol convective invigoration is not merely a theoretical possibility.
Under the right conditions โ particularly inside deep, vigorous, relatively clean convective clouds โ the atmosphere appears capable of producing it.
Journal references
Zengxin Pan, Daniel Rosenfeld, Lin Zang, Feiyue Mao, and Jiwen Fan. โAircraft-observed High Supersaturation Indicates the Potential of Aerosol Convective Invigoration Effect.โ Advances in Atmospheric Sciences, 2026. The DOI resolves to the exact Springer article.
https://doi.org/10.1007/s00376-026-5894-y
Saurabh Patil, Greg M. McFarquhar, Yongjie Huang, Greg Roberts, Mengistu Wolde, Leonid Nichman, Cuong Nguyen, Keyvan Ranjbar, Natalia Bliankinshtein, Amanda Richter, Pavlos Kollias, and Daniel Rosenfeld. โQuasi-Steady State Supersaturation: Do High Values Derived From ESCAPE Represent Real High Supersaturations and the Potential for Condensational Invigoration?โ Journal of Geophysical Research: Atmospheres, 2026, 131, e2025JD045547. The DOI matches the exact AGU/Wiley article.