Deep tropical clouds can reach far higher water-vapor supersaturation than previously documented, new aircraft observations show — providing the long-sought 'fuel' for a proposed mechanism by which tiny aerosol particles could make storm clouds grow stronger.
The study, published in Advances in Atmospheric Sciences, analyzed data from NASA's Cloud, Aerosol and Monsoon Processes Philippines Experiment (CAMP2Ex), which flew over the Philippines and nearby tropical oceans in 2019. An international team from China, the US and Israel inferred supersaturation — a state in which air holds more water vapor than equilibrium allows — from measured updraft speeds and cloud-droplet size distributions. Values climbed with altitude, reaching about 10% near −5°C in vigorous updrafts dominated by supercooled liquid droplets. A companion study from the ESCAPE campaign over coastal Texas and Louisiana independently found rare extremes of about 11% in deep convective updrafts.
The mechanism, called condensational aerosol convective invigoration, works like this: in a highly supersaturated cloud, adding aerosol particles nucleates extra droplets, boosting condensation and releasing latent heat that strengthens rising air currents. Past aircraft measurements generally failed to find high supersaturation because they sampled polluted or shallow clouds — precisely the environments where the effect is unlikely.
'Our observations show: if you want to see this mechanism in action, you need to look at deep, clean clouds over the ocean,' said Daniel Rosenfeld of The Hebrew University of Jerusalem and Wuhan University, who participated in both studies. The team's next step is to compare clean and polluted tropical convective clouds directly during future campaigns.
