by Julie Chao

The climate models that scientists use to understand and project climate change are improving constantly, with better representations of the oceans, ice, land surfaces and other factors in the atmosphere. While there is still some degree of uncertainty in all these components, the largest source of uncertainty in today’s climate models are clouds.

Clouds can both cool the planet, by acting as a shield against the sun, and warm the planet, by trapping heat. But why do clouds behave the way they do? And how will a warming planet affect the cloud cover?

Lawrence Berkeley National Laboratory scientist David Romps has made it his mission to answer these questions. “We don’t understand many basic things about clouds,” he says. “We don’t know why clouds rise at the speeds they do. We don’t know why they are the sizes they are. We lack a fundamental theory for what is a very peculiar case of fluid flow. There’s a lot of theory that remains to be done.”

The Earth’s response to changes in atmospheric CO₂ is studied using what are known as global climate models (GCMs), which run on supercomputers. Due to computational limitations, however, these GCMs are unable to explicitly model atmospheric phenomena less than 100 kilometers in size. Since convective clouds have sizes closer to 1 km, they cannot be resolved by a GCM. “So the GCM has to ask a submodel: what clouds do I have and what are they doing?” Romps says.

The submodel takes the temperature and humidity profile of a column of air and has to answer the question, “what’s happening right now?” Unfortunately, despite decades of research and development on these submodels, they remain far from perfect.

“If you ask meteorologists to do this, it would be a very challenging task,” Romps says. “There are some things you can infer, but it’s hard to know, for example, what the rain rate is. That’s one of the key things this model has to tell the GCM.”

This is a large-eddy simulation of a deep cumulonimbus (a tall rain cloud). The grey color shows the cloud, and the bluish color shows the precipitation.

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