Fine aerosol particles can intensify tropical convective clouds, but this topic has remained a subject of scientific debate for decades. Aerosols influence the formation and growth of cloud droplets, which can affect condensation processes and the release of latent heat.

Aerosol Enhancement Mechanism

One of the processes under consideration—condensational aerosol enhancement—depends on whether high levels of water vapor supersaturation occur within clouds. Under such conditions, additional aerosol particles can promote the formation of new droplets, increasing condensation, releasing more latent heat, and potentially accelerating updrafts within the cloud.

Measurements and Observations

Previous aircraft-based measurements generally did not detect the high levels of quasi-steady supersaturation required for this process. Most studies focused on clouds with a low likelihood of extreme supersaturation, such as relatively polluted or shallow clouds, as well as clouds sampled below deep convective regions. At higher altitudes, droplet collisions, precipitation formation, and rapid updrafts can reduce the total droplet surface area, allowing supersaturation to accumulate.

New Data

In a study published in Advances in Atmospheric Sciences, researchers analyzed data collected from aircraft in tropical clouds during NASA’s Cloud, Aerosol and Monsoon Processes Philippines Experiment in 2019 over the Philippines and adjacent tropical oceans. Scientists from China, the USA, and Israel assessed quasi-steady supersaturation using measurements of updraft velocities and cloud droplet size distributions. This method accounts for the balance between water vapor produced by rising air and vapor removed by condensation onto droplets.

Research Findings

The results showed that tropical convective clouds can reach supersaturation levels significantly higher than those recorded in previous studies. Supersaturation increased with altitude within the clouds, reaching about 10% at temperatures near –5°C, when the updraft regions consisted mainly of supercooled liquid droplets. At lower temperatures, ice began to form, which reduced the accuracy of estimates based solely on the liquid phase.

A recently published companion study, based on data from the ESCAPE aircraft campaign over the Texas and Louisiana coasts, independently identified rare but extreme quasi-steady supersaturation values—around 11% inside deep convective updrafts.

Significance of Supersaturation and Conditions for the Effect

Both studies indicate that high water vapor supersaturation arises in cloud environments where condensational aerosol enhancement is most likely to occur. The highest supersaturation values were found in strong updrafts with relatively low droplet concentrations. As the number of droplets increased, their total surface area grew, causing more vapor to condense onto them and reducing the supersaturation level.

The observations do not confirm that aerosols intensified the clouds studied. Instead, they show that the atmospheric conditions necessary for condensational aerosol enhancement can indeed develop within real tropical convective clouds.

High supersaturation acts as “fuel” that additional fine or ultrafine aerosol particles can use to form new droplets. These droplets can boost condensation, release extra latent heat, and potentially strengthen updrafts within the cloud.

Future Research Prospects

The key takeaway is not only the existence of extreme supersaturation, but also the need to study the right types of clouds to detect it. Previous research often focused on polluted or shallow clouds, which typically do not create the high-supersaturation conditions required for condensational enhancement. As a result, such mechanisms were not observed in those environments.

Future plans include conducting dedicated aircraft campaigns to more directly test the proposed process. These studies will compare clean and polluted tropical convective clouds, with special attention to powerful updraft regions. Researchers also aim to more accurately distinguish between liquid and ice phases within clouds.

The main goal of further work is to improve the physical understanding and prediction of how aerosols affect deep convection, precipitation, lightning, and climate.