Tiny aerosol particles may be able to strengthen tropical convective clouds, but scientists have debated the idea for decades. Aerosols influence the formation and growth of cloud droplets, which can affect condensation and the release of latent heat.
One proposed process, called condensational aerosol convective invigoration, depends on clouds developing very high water vapor supersaturation. When those conditions occur, additional aerosol particles can form new droplets. That can increase condensation, release more latent heat, and potentially accelerate rising air inside the cloud.
Why Earlier Studies May Have Missed It
Aircraft measurements have generally not detected the high levels of quasi-steady-state supersaturation required for this process. However, that does not mean such conditions never occur in the atmosphere.
Many previous measurements focused on cloud environments where extreme supersaturation was unlikely. These included relatively polluted clouds, shallow warm clouds, and clouds sampled below their deeper convective regions. At greater heights, droplet collisions, precipitation formation, and faster updrafts can reduce the total surface area of droplets, allowing supersaturation to build.
Aircraft Data From Tropical Clouds
A new study published in Advances in Atmospheric Sciences examined aircraft observations collected during NASA’s Cloud, Aerosol and Monsoon Processes Philippines Experiment. The campaign took place over the Philippines and nearby tropical oceans in 2019.
Researchers from China, the US, and Israel estimated quasi-steady-state supersaturation using measurements of updraft speeds and cloud droplet size distributions. The method captures the balance between water vapor produced as air rises and water vapor removed as it condenses onto droplets.
The results indicate that tropical convective clouds can reach supersaturation levels far higher than those recorded in previous aircraft studies using comparable methods.
Supersaturation increased as the aircraft sampled higher portions of the clouds, reaching about 10% at approximately −5°C. At that temperature, the updraft regions were still composed mainly of supercooled liquid droplets.
Estimated supersaturation continued rising at colder temperatures. However, ice began to form in those regions, making estimates based only on the liquid phase less certain.
Extreme Supersaturation in Deep Updrafts
A recently published companion study provides additional support for the findings. Researchers using data from the ESCAPE aircraft campaign over coastal Texas and Louisiana independently detected rare but extreme quasi-steady-state supersaturation of about 11% inside deep convective updrafts.
Together, the studies suggest that high water vapor supersaturation occurs in the cloud environments where condensational aerosol convective invigoration is most likely.
The largest reliable values were found in strong updrafts with relatively low droplet concentrations. When clouds contained more droplets, their combined surface area increased. More vapor then condensed onto those droplets, reducing the estimated supersaturation.
The Hidden Fuel Inside Tropical Clouds
The observations do not prove that aerosols caused the sampled clouds to become stronger. Instead, they demonstrate that the atmospheric conditions required for condensational aerosol invigoration can develop inside real tropical convective clouds.
High supersaturation acts as the “fuel” that additional fine or ultrafine aerosol particles could use to form more droplets. Those droplets could increase condensation, release additional latent heat, and potentially strengthen the cloud’s updrafts.
The central finding is not only that extreme supersaturation exists. Scientists must also examine the correct types of clouds to detect it.
“Previous studies looked at polluted or shallow clouds — types that don’t typically create the high-supersaturation conditions needed for condensational invigoration. So it’s no surprise they didn’t see that mechanism in action,” said Daniel Rosenfeld of The Hebrew University of Jerusalem and Wuhan University, who participated in both studies. “Our observations show: if you want to see this mechanism in action, you need to look at deep, clean clouds over the ocean.”
Testing the Cloud Mechanism Directly
Researchers now plan to test the proposed process more directly through dedicated aircraft campaigns. These studies would compare clean and polluted tropical convective clouds, with particular attention to powerful updraft regions. Scientists will also need to better distinguish between the liquid and ice phases inside the clouds.
“Ultimately, our goal is to improve the physical understanding and prediction of aerosol effects on deep convection, rainfall, lightning, and climate,” said Rosenfeld.