The clouds are stranger at the top than we thought

If you’ve ever flown, you probably know what cloud tops look like: usually white and fluffy, with blue-gray undertones here and there. But the physics behind cloud tops has long intrigued scientists, until today.

At the Brookhaven National Laboratory facility on Long Island, New York, researchers have developed a new type of lidar, a laser remote sensing device. This lidar captures the finest details of cloud structures at a scale of about 0.4 inches (1 centimeter), making it 100 to 1,000 times clearer than traditional instruments. For a recent study published in Proceedings of the National Academy of SciencesBrookhaven and his collaborators combined this lidar with chamber experiments.

This is the first experimental description to differentiate between water structures at the top and interior of clouds, characteristics which, in turn, dictate how clouds “evolve, form precipitation, and affect the Earth’s energy budget,” the researchers explain in a paper. statement.

“A microscope for clouds”

According to the researchers, the new lidar provides “ultra-high resolution” images of cloud dynamics. Impressively, lidar detects and counts individual photons (massless light-carrying particles) emerging from a cloud struck by ultrafast laser pulses.

Next, a custom data sampling algorithm translates the photonic signals into a profile of the cloud structure. Lidar is “essentially a microscope for clouds,” Fan Yang, lead author of the study and a Brookhaven researcher, said in the release.

The team took their device to a cloud chamber in Michigan, where researchers could artificially generate clouds under the temperature and humidity conditions of their choosing. This allowed them to document the precise physics of how cloud droplets are distributed throughout a cloud.

They found that, surprisingly, existing models failed to describe cloud physics. Specifically, the lidar measurements revealed a large variation in the cloud’s droplet distribution at the top, while things were more uniform throughout the rest of the cloud.

Physics of turbulent clouds

The researchers think this could be due to two processes: entrainment and sedimentation. The entrainment draws the clear, dry air above the cloud downward, resulting in uneven distribution of droplets on the upper layer of the cloud. At the same time, sedimentation automatically sorts droplets based on their size, so that heavier droplets fall into clouds more quickly than lighter ones.

During this time, the bulky interior of clouds usually experiences strong turbulence, so the droplets immediately mix evenly. In comparison, cloud tops have much lower turbulence, so only relatively small droplets remain suspended in this region of the cloud.

“Many atmospheric models completely neglect droplet sedimentation or represent droplets of different sizes with only one falling velocity,” Yang explained. “This simplification is reasonable in the overall cloud region, where turbulence is strong, but it breaks down near the cloud top, where turbulence is weaker.”

Looking for the positive side

The new findings have significant implications for atmospheric science, the researchers say in the paper. For example, inaccurate representations of cloud-top physics can “introduce substantial uncertainty into model predictions about how clouds reflect sunlight and trigger precipitation,” Yang said.

Researchers hope that lidar can eventually be used to directly measure clouds in the real atmosphere, in addition to refining current models. After all, they admitted, a cloud chamber is not the perfect representation of real cloud dynamics, although technological advances have allowed researchers to get impressively close.