Clouds have a number of important functions. They act as reflectors through which water droplets in the cloud reflect radiation back to Earth, contributing to the greenhouse effect. Credit: University of Gothenburg
When clouds encounter clear skies, cloud droplets evaporate, mixing with dry air. A new study involving researchers from the University of Gothenburg has managed to capture what’s happening in a model. Ultimately, this could lead to more accurate climate modeling in the future.
Clouds in the sky have a big impact on our climate. Not only do they produce precipitation and provide shade from the sun, but they also act as large reflectors that prevent the radiation of Earth’s heat, commonly known as the greenhouse effect.
“Although clouds have been studied for a long time, they are one of the largest sources of uncertainty in climate models,” says Bernhard Mehlig, professor of complex systems at the University of Gothenburg. “That’s because many factors determine how clouds affect radiation. And turbulence in the atmosphere means everything is in constant motion. That makes things even more complicated.”
Focusing on the Cloud Edge
An article in Physical Examination Letters presents a new statistical model that describes how the number of water droplets, their size, and water vapor interact at the edge of turbulent clouds. The distribution of water droplets is important because it affects how clouds reflect radiation.
“The model describes how droplets shrink and grow at the cloud edge when turbulence mixes with drier air,” adds Johan Fries, a former physics doctoral student and co-author of the study.
The researchers identified the most important parameters and built their model accordingly. In short, the model takes into account the laws of thermodynamics and the turbulent movement of droplets. The model matches well with previous numerical computer simulations and explains their results.
The importance of evaporation
“But we are still far from the finish line,” continues Professor Mehlig. “Our model is currently able to describe what happens in a cubic meter of cloud. Let’s say that fifteen years ago there was only a cubic centimeter, so we are making progress.”
When policymakers discuss climate change, much emphasis is placed on IPCC climate models. However, according to the IPCC, the microphysical properties of clouds are among the least understood factors in climate science.
“Moreover, droplet evaporation is an important process, not only in the context of atmospheric clouds, but also in the field of infectious medicine. The tiny droplets produced when we sneeze can contain viral particles. If these droplets evaporate, the virus particles can remain in the air and infect others.”
Professor Mehlig also co-authored another study describing how solid particles, such as ice crystals, move in clouds.
“Ice crystals and water droplets influence each other. But we don’t yet know how.”
More information:
J. Fries et al, Lagrangian supersaturation fluctuations at the cloud periphery, Physical Examination Letters (2023). DOI: 10.1103/PhysRevLett.131.254201
Provided by the University of Gothenburg
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