The Science of Clouds — Why They Matter, and Why There May be Fewer of Them

A cloud may look like just a billowing mass of air, but cloud dynamics in fact involve complicated physics. One of the most important factors in cloud dynamics, for example, is entrainment, which is when convecting clouds take environmental air and fold it into themselves as they are rising—a turbulent exchange of air between clouds and their environment. The more they do this, the less buoyant the cloud becomes and the less likely it is to rain. This process is difficult to study and build a model for because of the phase changes and complicating influence of latent heat being released.

The same cloud, without precipitation. David Romps uses these simulations to learn more about the turbulent entrainment process.

What’s worse, the entrainment rate may be the single most influential factor on climate models. “The entrainment rate is this parameter representing this really esoteric process, and it’s the parameter to which the climate is most sensitive — that’s an awful state of affairs!” Romps says. “We don’t understand entrainment.”

How would an improved understanding and model of clouds affect the global climate model? “It would rain in the right places,” Romps says. “The large-scale winds would look better because the release of latent heat drives a lot of those winds, and climate sensitivity would be better constrained because not only is the base state highly dependent on convective parameterization but the model predictions for future climate change are also very sensitive to that as well.”

Romps has also tested theories on how future climate change will affect clouds. One of the biggest predictions is that higher temperatures will result in, roughly, fewer clouds, or more precisely, a smaller convective mass flux. Why? If the earth is warmed by 1 degree Celsius, then the amount of water vapor in the atmosphere increases by 7 percent. Meanwhile climate models say that the amount of rain in the future will increase by 1 to 3 percent.

“If you think of a cloud as rising up and wringing out the water from a column of air, then you know something from those two numbers,” Romps explains. “The column of air holds 7 percent more water. But clouds need to release only 1 to 3 percent more water in a warmer world. That means there’s going to be between 4 to 6 percent fewer convecting clouds.”

Tests that he ran using a high-resolution model confirmed this theory. Romps’ research is funded in part by Berkeley Lab’s Laboratory Directed Research and Development funding program.

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The Science of Clouds — Why They Matter, and Why There May be Fewer of Them

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