Hidden anatomy of clouds reveals how rain may begin without ice crystals

Scientists at the Max Planck Institute for Dynamics and Self-Organization have observed previously invisible structures inside shallow cumulus clouds that may explain how rain begins without ice crystals. The study, published in the Proceedings of the National Academy of Sciences, used a custom-built platform called the CloudKite to measure droplet positions and turbulence at submeter resolution.

Shallow cumulus clouds cover large areas of oceans and land and consist only of tiny liquid droplets. Unlike deeper clouds, they lack ice crystals that can act as seeds for raindrops, yet they can produce rain within minutes. The process by which droplets collide and grow large enough to fall has remained a major uncertainty in atmospheric science.

The CloudKite is a helikite balloon that drifts through clouds at roughly 10 meters per second, far slower than research aircraft. It carries two autonomous optical systems: a holographic imager that reconstructs the three-dimensional positions and sizes of individual droplets 75 times per second, and the first airborne particle image velocimetry system for measuring turbulence. Together they capture cloud microphysics and dynamics from micrometers to kilometers.

Data from a 55-meter transect of a shallow cumulus cloud showed that droplets are not evenly distributed. Instead, they form highly localized clusters, or hotspots, about a meter across or smaller. Within these regions, droplets are significantly closer together, making collisions and the formation of larger drops much more likely.

The findings challenge the long-standing assumption that droplet clustering in warm clouds is weak and uniform. First author Birte Thiede said these hotspots may represent the places where rain starts in shallow cumulus clouds. The team is now investigating how turbulence generates these clusters.

Because warm clouds are responsible for much of Earth's rainfall, especially in the tropics, their lifetime and reflectivity affect the planet's energy budget. Improving the representation of rain formation in these clouds could reduce one of the largest uncertainties in climate projections and lead to more accurate weather forecasts, said group leader Mohsen Bagheri.

Future field campaigns with the CloudKite are planned in Amazonia, the Baltic Sea, and northern Finland to further explore the link between turbulence and droplet clustering.