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• Droplets in clouds are not uniformly distributed everywhere, but are clustering in regions of around one meter
• Such regions of high particle clustering may be the key for rain formation in shallow clouds
• These measurements are made possible by the unique CloudKite measurement platform, investigating the distribution of microscopic particles within clouds at unprecedented resolution
The initiation of rain in clouds without ice remains one of the largest unsolved mysteries in atmospheric science. Researchers form the of the Max Planck Institute for Dynamics and Self-Organization (MPI-DS) now uncovered previously invisible structures within shallow cumulus clouds in their study published in PNAS. Using the unique Max Planck CloudKite platform, the team discovered sub-meter regions where cloud droplets cluster together. Such areas provide ideal conditions for the collision of droplets, which in turn has a high chance to trigger rainfall.
Revealing reasons for rainfall
Shallow clouds globally constitute a large proportion of all clouds covering the oceans and land. They do not contain any ice crystals that could serve as seeds for rainfall. Instead, particularly shallow cumulus clouds are solely composed of tiny liquid water droplets. While they are able to produce rain within minutes, it remains unclear what causes the formation of larger rain droplets to ultimately predict rainfall. Before rain initiation, the droplets face a bottleneck as they have to collide with others to form a larger drop that eventually falls. The scientists thus investigated how this initial bottleneck can be overcome.
“We assed the structure of a warm cloud at high spatial resolution,” says Mohsen Bagheri, group leader at MPI-DS and last author of the study. “It is like a 3D-microscope in the clouds investigating particle size and distribution,” he continues. When using planes to measure particles, their high speeds allow for only a few measurements taken at greater intervals. In contrast, drones are limited in flight time and conditions and may additionally create turbulence, altering the cloud composition. Using a helikite balloon, MPI-DS scientists around group leader Mohsen Bagheri and director Eberhard Bodenschatz therefore developed a unique measuring platform to study the cloud structure with unprecedented detail.
The CloudKite: a unique airborne laboratory
At the heart of this observatory are two custom-built optical imaging systems using powerful lasers and high-speed cameras.
One of them is capable of reconstructing the three-dimensional positions and sizes of individual cloud droplets at a rate of 75 times per second, thus being one of the fastest airborne holographic instruments ever developed. The other measures the turbulence within clouds, being the first airborne particle image velocimetry system. Both instruments operate autonomously, allowing them to simultaneously capture cloud microphysics and turbulence from micrometers to kilometers. The CloudKite platform drifts through clouds at only about 10 meters per second, significantly slower than research aircraft.
“Together with the instrument’s high imaging rate, this enables measurements every 12 centimeters – around 250 times more frequently than previous airborne observations,” says Eberhard Bodenschatz. “The CloudKite thus opens a new observational window into clouds,” he summarizes.
Hidden hotspots where rain may begin
Using the data from the CloudKite, researchers reconstructed the internal anatomy of a 55-metre section of a shallow cumulus cloud. Unlike being evenly distributed, droplets formed highly localized “hotspots” only about a meter across or even less. Within these regions, droplets are much closer together than elsewhere in the cloud, which significantly increases the chances of collisions and formation of larger drops.
“Because droplets cluster there, collisions become much more likely. These localized hotspots may therefore represent the places where rain starts in shallow cumulus clouds.” says Birte Thiede, first author of the study.
These observations challenge the long-standing assumption that droplet clustering is weak and evenly distributed throughout clouds. Instead, clouds possess a hidden internal structure that had remained invisible until now.
Towards better climate predictions
The team is currently investigating how turbulence is linked with these localized clustering hotspots. Future field campaigns with the CloudKite observatory are planned in Amazonia, the Baltic Sea, and northern Finland.
As warm clouds are responsible for much of the Earth’s rainfall, particularly in the tropics, they play a central role in regulating the Earth’s energy budget.
Their lifetime and ability to reflect sunlight back into space depends critically on how efficiently cloud droplets grow into raindrops and is one of the largest uncertainties in climate projections. “Revealing the hidden structure of warn clouds will lead to better descriptions of rain formation and more accurate weather forecasts,” Mohsen Bagheri concludes.
https://www.pnas.org/doi/10.1073/pnas.2602976123
https://www.ds.mpg.de/4136151/260806_anatomy_clouds
Shallow clouds show regions of high particle clustering that may be the key for rain formation
Merkmale dieser Pressemitteilung:
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Geowissenschaften, Meer / Klima, Physik / Astronomie
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Shallow clouds show regions of high particle clustering that may be the key for rain formation
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