New measurements reveal the possible origine of rainfall in one of the most dominant types of clouds
- Inhomogeneous clouds: Droplets in clouds are not uniformly distributed everywhere, but are clustering in regions of around one meter.
- Rainmaker: Regions of high particle clustering may be the origine of rain in shallow clouds.
- Precipitation forecasts: The improved understanding, how rain is formed, can help to improve weather forecasts.
Shallow clouds show regions of high particle clustering that may be the key for rain formation
© MPI for Dynamics and Self-Organization
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 now uncovered previously invisible structures within shallow cumulus clouds in their study published in PNAS. Using a helikite, 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.
An important type of clouds in the global climate system
Shallow warm clouds globally constitute a large proportion of all clouds covering the oceans and land. Warm clouds are the source of much of the Earth’s rainfall, particularly in the tropics, and they play a central role in regulating the Earth’s energy budget, as they reflect sunlight back into space. Their lifetime and ability to shield the earth from sunlight depends critically on how efficiently cloud droplets grow into raindrops and is one of the largest uncertainties in climate projections.
Warm clouds 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.
Clusters of cloud droplets facilitate the formation of rain drops
“We assessed the structure of a warm cloud at high spatial resolution,” says Mohsen Bagheri, group leader at the Max Planck Institute for Dynamics and Self-Organization. Using the data the 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 for the 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. “Revealing the hidden structure of warm clouds will lead to better descriptions of rain formation and more accurate weather forecasts,” says Mohsen Bagheri.
The Max Planck CloudKite measurement platform allows to measure the distribution of microscopic particles within clouds at unprecedented resolution.
© MPI for Dynamics and Self-Organization
The CloudKite: a unique airborne laboratory
For their measurements the team used an airborne platform, which scientists around Mohsen Bagheri and Eberhard Bodenschatz, director at the Max Planck Institute for Dynamics and Self-Organization developed on the basis of a helikite balloon. The so-called Max Planck Cloud makes it possible to study the cloud structure with unprecedented detail and without pertubations as they are caused by drones for example. “It is like a 3D-microscope in the clouds investigating particle size and distribution,” says Mohsen Bagheri.
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. 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.”
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.
