SCIENCE

Accounting for wavelength effects will likely improve climate models

Download Original Image Cloudy dissonance: These cumulus clouds above Oklahoma both shade the earth and make shadows brighter. The larger ones have a distinct cauliflower shape, providing even more opportunities for light to bounce off of them.

Atmospheric scientists trying to pin down how clouds curb the amount of sunlight available to warm the earth have found that it depends on the wavelength of sunlight being measured. This unexpected result will help researchers improve how they portray clouds in climate models.

Additionally, the researchers found that sunlight scattered by clouds — the reason why beachgoers can get sunburned on overcast days — is an important component of cloud contributions to the earth's energy balance. Capturing such contributions will increase the accuracy of climate models, the team from the Department of Energy's Pacific Northwest National Laboratory reported in Geophysical Research Letters earlier this month.

"The amount of the sun's energy that reaches the earth's surface is the main driver of the earth's temperature. Clouds are one of the least understood aspects of climate change. They can block the sun, but light can also bounce off one cloud into another cloud's shadow and increase the solar energy hitting earth," said PNNL atmospheric scientist Evgueni Kassianov.

White clouds

Clouds both cool down and warm up the earth's surface. They cool the earth by reflecting some sunlight up into outer space, and they warm it by bouncing some sunlight down to the surface. Overall, most clouds have a net cooling effect, but atmospheric scientists need to accurately measure when they cool and warm to produce better climate models that incorporate clouds faithfully.

But it's a hard number to get. Fair-weather clouds are big puffy white objects that bounce a lot of light around. They can make the sky around them look brighter when they're there, but they float about and reform constantly. Cloud droplets and aerosol particles in the sky — tiny bits of dirt and water in the air that cause haziness — scatter light in three dimensions, even into cloud shadows.

To determine the net cloud effect, researchers need two numbers. First they need to measure the total amount of sunlight in a cloudy sky. Then they need to determine how bright that sky would be without the clouds, imagining that same sky to be blue and cloudless, when aerosols are in charge of a sky's brightness. The difference between those numbers is the net cloud effect.

Rainbow energy

Researchers have traditionally estimated the net cloud effect by measuring a broad spectrum of sunlight that makes it to the earth's surface, from ultraviolet to infrared. But clouds are white — that's because the large water droplets within them scatter light of all colors almost equally in the visible spectrum, the part of the electromagnetic spectrum that includes the colors of the rainbow.

On the other hand, aerosols — both within clouds and in the open sky — bounce different-colored light unequally. Broadband measurements that fail to distinguish color differences might be covering up important details, the researchers thought.

Instead of taking one broadband measurement that covers everything from ultraviolet to infrared, Kassianov and crew wanted to determine how individual wavelengths contribute to the net cloud effect. To do so, the team used an instrument that can measure brightness at four different wavelengths of color — violet, green, orange, red — and two of infrared.

In addition, this instrument, a spectral radiometer at DOE's