Scientists have for decades tried to learn more about the complex and mysterious chain of events by which tiny droplets in clouds grow large enough to begin falling toward the ground. A better understanding of this process, known as the “rain formation block,” is essential to improving weather and climate computer model simulations and ultimately generating better forecasts.
Now a research team led by scientists at the US National Science Foundation’s National Center for Atmospheric Research (NSF NCAR) has discovered that turbulent air movements in clouds play a key role in the growth of droplets and the initiation of rain.
The researchers applied advanced computer modeling to detailed observations of points in cumulus clouds that were taken during a NASA field campaign. This enabled them to trace the effects of turbulence on the embryonic droplets that eventually coalesce into raindrops.
“This research shows that turbulent effects on droplet coalescence are critical for the evolution of droplet sizes and the initiation of rain,” said NSF NCAR scientist Kamal Kant Chandrakar, lead author. “Turbulence in cumulus clouds substantially accelerates precipitation and leads to much larger amounts of rain.”
Chandrakar and his colleagues found that rain formed about 20 minutes earlier in computer simulations with turbulence than in computer simulations without turbulence. The mass of rainwater was more than seven times higher in simulations that included turbulence.
The study was published in the journal Proceedings of the National Academy of Sciences. It was funded by NASA, the US Department of Energy and NSF.
From small drops of water in the rain
The process of rain begins when tiny water droplets in clouds condense around microscopic particles of dust, salt or other materials, which are called cloud condensation nuclei (CCN). As the millions of droplets collide with each other, they coalesce into larger droplets that eventually become heavy enough to fall from the cloud.
The formation of raindrops can vary under different conditions, such as the distribution of different cloud droplet sizes, as well as other factors such as turbulent motions and cloud particle properties.
Accurately representing this process in computer models of weather events and the climate system is vital to improving the reliability of these models. The coalescence of water droplets is important not only for accurately predicting precipitation, but also for better understanding the evolution of clouds and the extent to which they reflect heat back into space, thereby affecting temperatures.
To tease out the onset of precipitation, Chandrakar and his colleagues turned to observations of drop size distributions taken by research aircraft that flew into cumulus congestus clouds during a 2019 NASA field campaign, the Philippines Cloud Processes Experiment. , Aerosol and Monsoon (CAMP2Ex).
Using a specialized computer model, the research team developed a series of high-resolution simulations to recreate the cloud conditions that were observed during the campaign and to see how the droplets merged with different turbulent flows.
The simulations showed the major role of turbulence in both the timing and extent of precipitation. They also showed that the presence of large CCNs, which has been the focus of several theories of rain formation, cannot account for the observed droplet sizes and evolution. In simulations with large CCN and little turbulence, droplet coalescence occurred more slowly and generated less rain.
“Rain development is essential to clouds, weather and the entire climate system,” Chandrakar said. “Better understanding of this process could point the way to significant improvements in our computer models and ultimately in weather forecasting and climate predictions that help protect society.”
About the article
Title: “Are the effects of turbulence on droplet collision a key to understanding the observed formation of rain in clouds?”
Authors: Kamal Kant Chandrakar, Hugh Morrison, Wojciech W. Grabowski and Paul Lawson
Publication: Proceedings of the National Academy of Sciences
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This material is based on work supported by the NSF National Center for Atmospheric Research, a major facility sponsored by the US National Science Foundation and managed by the University Corporation for Atmospheric Research. Any opinions, findings and conclusions or recommendations expressed in this material do not necessarily reflect the views of NSF.
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