Can tiny aerosol particles make tropical convective clouds grow stronger? For decades, scientists have debated this question because aerosols can change how cloud droplets form, grow, and release latent heat. One proposed pathway, known as condensational aerosol convective invigoration, requires clouds to contain high water-vapor supersaturation. Under such conditions, adding aerosol particles can create many new droplets, enhance condensation, release additional latent heat, and potentially strengthen convective updrafts.
Until now, aircraft-based estimates of quasi-steady-state supersaturation have generally not documented such high supersaturation values. But this does not necessarily mean that high supersaturation is absent in nature. Earlier measurements were mostly made in cloud regimes where high values are not expected: relatively polluted clouds, shallow warm clouds, or clouds sampled below the deeper convective levels where coalescence, precipitation development, and accelerating updrafts can reduce the droplet surface area and allow supersaturation to rise.
A new study published in Advances in Atmospheric Sciences used aircraft observations from NASA's Cloud, Aerosol and Monsoon Processes Philippines Experiment, conducted over the Philippines and surrounding tropical oceans in 2019. An international scientist team from China, US and Israel inferred quasi-steady-state supersaturation from measured updraft speeds and cloud-droplet size distributions. This method reflects the physical balance between vapor production by rising air and vapor removal by condensation onto cloud droplets.
Results show that tropical convective clouds can reach much higher supersaturation than previously documented by comparable aircraft-based approaches. The inferred quasi-steady-state supersaturation increased with height and reached about 10% at approximately −5°C, where the updraft clouds were still dominated by supercooled liquid droplets. The inferred values continued to increase at colder temperatures, but there the onset of ice formation made the liquid-phase SQSS estimates increasingly uncertain.
The timeliness of this result is underscored by a newly published companion study from the ESCAPE aircraft campaign over coastal Texas and Louisiana, which independently found rare but extreme quasi-steady-state supersaturations reaching about 11% in deep convective updrafts, further supporting the view that high vapor supersaturation occurs in the specific cloud regimes where condensational aerosol convective invigoration is expected. Additionally, the largest reliable values were associated with strong updrafts and low droplet concentrations. Conversely, when droplet concentrations increased, the total cloud-droplet surface area increased and the inferred supersaturation decreased, consistent with enhanced condensation onto more numerous droplets.
This does not by itself prove that aerosols caused stronger clouds in the observed cases. Rather, it establishes something more fundamental: the atmospheric conditions required for condensational aerosol invigoration can exist in real tropical convective clouds. High supersaturation provides the "fuel" that added fine or ultrafine aerosol particles could use to nucleate additional droplets, increase condensation, and release more latent heat.
The key message is therefore not simply that high supersaturation exists, but that one must look in the right clouds to find 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."
The next step is to test the mechanism more directly with dedicated aircraft campaigns that sample clean and polluted tropical convective clouds, especially vigorous updraft regions, while better constraining the liquid and ice phases.
"Ultimately, our goal is to improve the physical understanding and prediction of aerosol effects on deep convection, rainfall, lightning, and climate." said Rosenfeld.
