Clouds in the highest layers of the atmosphere show a different behavior than we imagine

Researchers at the Brookhaven National Laboratory in New York have been able to examine the structure of the upper layers of clouds with an unprecedented resolution by using a very precise lidar. These results show that the behavior of water droplets in the uppermost part of the clouds is much more complicated than the current models and can affect the prediction of precipitation and sunlight reflection.

If you have experience flying, you are probably familiar with the appearance of the top of the clouds; Usually white and fluffy with blue-gray parts. However, the physics of the upper layers of clouds puzzled scientists for years until now a clearer answer has been found.

At the Brookhaven National Laboratory facility in Long Island, New York, researchers have developed a new type of lidar, a laser-based remote sensing instrument. This lidar is able to record the very fine details of the cloud structure on a scale of about one centimeter; Resolution that is between 100 and 1000 times higher than traditional instruments. In a study recently published in the Journal of the National Academy of Sciences, Brookhaven’s team combined this lidar with cloud chamber experiments. This research is the first experimental description that specifies the difference between the water structures on top of the cloud and its inner parts; Structures that determine how clouds form, how precipitation is produced, and how they affect Earth’s energy balance.

According to the researchers, this new lidar provides exceptionally high-resolution images of cloud dynamics. Remarkably, the device can detect and count individual photons (massless light-carrying particles) emitted from the cloud after being hit by ultrafast laser pulses. A data sampling algorithm then converts these photon signals into a detailed profile of cloud structure. Fan Yang, lead author of the study and Brookhaven researcher, described the lidar as a “microscope for clouds.”

The research team took the device to the cloud chamber in Michigan; where clouds can be artificially produced under controlled conditions of temperature and humidity. This controlled environment allowed them to record the exact physics of how water droplets are distributed in different parts of a cloud. The results showed that the current models are not accurate enough in describing the physics of clouds. Lidar measurements showed that in the upper layer of the cloud, the distribution of water droplets is highly variable, while in the inner parts of the cloud this distribution is more uniform.

Researchers believe that this difference is related to the two processes of “mixing” and “deposition”. In the mixing process, the dry and clear air above the cloud is pulled down, causing a patchy distribution of droplets in the upper layer of the cloud. At the same time, deposition sorts droplets by size; Larger droplets fall into the cloud faster and smaller droplets stay aloft longer.

On the contrary, the inner and bulky part of the cloud is usually very turbulent, which makes the water droplets combine quickly and find a uniform distribution. But in the upper layer of the cloud, the intensity of turbulence is much less and only smaller droplets can remain suspended in that region. “Many atmospheric models ignore droplet deposition or assume all droplets of different sizes fall at the same speed,” Yang explains. “This simplification is acceptable in the inner part of the cloud, where the turbulence is strong, but in the upper layer of the cloud, where the turbulence is weaker, it reduces the accuracy of the model.”

According to the researchers, these findings have important implications for atmospheric science. For example, misrepresentation of the physics of the upper layer of clouds can introduce significant uncertainty into models’ predictions about the amount of sunlight reflected and the onset of precipitation. Researchers hope that this lidar will be used in the future to directly measure clouds in the real atmosphere and help improve current models. Of course, they admitted that the cloud chamber is still not a perfect copy of the real behavior of the clouds, but technological advances have made it possible to get closer to the real conditions in an unprecedented way.