Cloud shapes and formations affect global warming, but we still don’t understand them

Over the Atlantic Ocean, puffy white clouds race across the sky hit by invisible trade winds. They are not particularly large, impressive or extensive, “says Dr Sandrine Bony, climatologist and director of research at the French National Center for Scientific Research.” But they are the most omnipresent clouds on Earth. ”

Clouds are one of the biggest question marks in global climate models and a wild card in predicting what will happen to the climate as temperatures rise. They play a vital role in the amount of solar radiation that arrives and remains trapped in our atmosphere. The more clouds there are, the more radiation bounces off their tops and is reflected back into space; it also means that if there are more clouds, the radiation reflected from the Earth is trapped. Historically, researchers have struggled to understand the properties of clouds, how they currently behave and how they will react to rising temperatures caused by climate change.

It’s a question of scale, explains Dr. Bony. From the microscopic interactions of atoms to atmospheric currents that act for thousands of kilometers, many forces affect the way clouds form, their composition and behavior.

The cotton wool-like clouds in the Atlantic, which Dr. Bony and her colleagues study, are a good example. “A small change in their properties has a huge impact on the global radiative balance (the balance between how much energy from the sun gets into the Earth’s atmosphere and how much it escapes),” he said. Because these clouds of good weather (known as cumulus clouds) are so common, a small change has “huge” statistical weight in the global climate.

“It’s the bigger question, there is no bigger question,” said Professor Bjorn Stevens, director of the Max Planck Institute for Meteorology in Germany and co-leader of Dr. Bony of the EUREC4A project which set out to investigate these. fluffy white clouds. “For 50 years, people have been making climate projections, but they have all had a false representation of the clouds.” These projections, he says, suffered from an inadequate understanding of the factors that determine how cloudy the weather will be and were not adequately represented in the models.

Field experiment

The EUREC4A project, which started as a modest field experiment to measure air movement and cloud cover, has attracted numerous partners and expanded their scope. Eventually, it included five manned and six remotely piloted research aircraft, four ocean-going research vessels, a flotilla of drifters and gliders, a series of satellites and measurements from the Barbados Cloud Observatory.

“The experiment has grown in complexity and scope to address a number of other fascinating questions,” said prof. Stevens, like the amount and ease with which clouds rain and how the eddies in the ocean and the clouds above affect each other. The team is currently writing the results and hopes their measurements will provide the answers to these questions. “We will establish a basic truth for a new set of climate models,” he said.

For Dr. Bony, the next step goes beyond understanding the properties of clouds and the area they cover.

“Now we are finding that it is not only the total area, but also the way the clouds are distributed and organized,” he said. The patterns they form could also influence how they block or absorb radiation, and this information could have implications for the role of clouds in climate change.

Dr Jan Härter, a specialist in atmospheric complexity at the Leibniz Center for Tropical Marine Research, the Jacobs University of Bremen, Germany and the Niels Bohr Institute in Denmark, is investigating this issue in his INTERACTION project. “Many types of clouds show organizing characteristics, but storm clouds (in the tropics) show self-organization,” he said. INTERACTION examines how thunderstorms group together, using simulation and developing basic models for their behavior.

Self-organization

Clouds can organize for many reasons, such as when they are over an urban area that tends to be warmer than the countryside due to all the concrete and asphalt. Self-organization occurs when clouds form and cluster even though the underlying conditions and the sunlight above them are uniform.

Storm clouds, known as cumulonimbus clouds (which comes from the Latin cumulus “heaped” and nimbus “rainstorm”), are tall vertical clouds that often bring rain. These clouds are the dominant type of cloud in the tropics and are also the key to understanding global radiation balance. “They are located at the latitude where most of the heat comes to Earth, and the solar radiation is much stronger there,” said Dr. Härter. These tower-like clouds affect the amount of sunlight entering the atmosphere, which has direct implications for warming.

“The question is how much these tall clouds change in the cluster when, for example, the temperature changes,” he said. However, like most problems involving clouds, this is a difficult question to answer.

INTERACTION is approaching the issue from two different perspectives: one is to run simulations, which require a large amount of processing time, and another is to develop “toy” models that explain the fundamental time-cloud interactions.

“Toy models are very basic simulations that talk about the fundamental interactions between storm clouds. For example, Dr. Härter and his colleagues are trying to understand how these clouds” talk “to each other and self-organize by breaking down these complex physical interactions. in their basic components.

When there is a thunderstorm, most of the rain falls to the ground, but some of it evaporates into the air under the cloud. This air, having incorporated the freezing humidity, becomes a cold pool, “explains Härter.” This evaporation is crucial to communicate signals from one cloud to another. ”

If there are hundreds and thousands of clouds in a large area, the cold pools below collide, pushing air into the colder parts of the atmosphere and sowing new storm clouds.

One of their “toy” models portrays how these cold pools interact and this cycle – of cold pools colliding and generating new clouds – can last for generations (one lasts about six hours) of clouds, encoding memories of clouds and storms. pass into the present cloud of actuality. Cold pools can continue to affect cloud generation for weeks.

These basic models are needed, says Dr. Härter, to remove some of the unknowns for simulating cloud behavior, such as how these cold pools interact. The team’s simulations already incorporate parameters such as wind speed, humidity, temperature and cloud composition, which are the different ratios of water, ice, and an icy mixture called graupel.

Echoing Dr. Bony and Prof. Stevens of EUREC4A, Dr. Härter said, “We don’t know how clouds work, especially these storm clouds that occur at scales that are difficult or impossible to resolve with current models. climatic “.

Simulation

To take into account the vastness of clouds and their driving forces, an accurate simulation would have to include disparate variables from the motion of atoms and the energy they dissipate (nanometers) to the Earth’s rotation and global winds on a scale of around 10,000 km. “The best we can do for, say, a week of simulations is to solve (the 100-meter scale) for an area of ​​one kilometer by one kilometer, or so,” he said. “And this is a great simulation.”

The ultimate goal of the project is to have a model for cloud organization that captures the interactions between past and present storm clouds and feeds this information to the next generation of climate models. The next step is to start field work and insert new measurements into their models.

“We need to have a clearer understanding of the different cloud system feedbacks to make a strong statement on climate change here,” said Dr. Härter. “The models have different ways of representing high and low clouds, and this is something that cannot be solved without closer observation data.”

And to prepare for a warm climate and predict how the world’s insulating cloud layer will change, we first need to understand how it works now.

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