Cloudy correction: spectral change needed

Looking at clouds from all sides

Atmospheric and climate models may have overlooked the fact that exactly how clouds appear to reduce the amount of sunlight available for warming the surface of the earth depends on the wavelength being measured across the spectrum from infrared to ultraviolet. The finding could now help researchers improve climate models by factoring in the effects of cloud cover more precisely.

Understanding changes in the Earth's reflectivity, its albedo, is critical to developing robust models of climate and climate change. After all, the way in which energy from the sun is reflected back into space rather than being absorbed by the oceans and lands can force warming or cooling. Atmospheric scientists have attempted to discern what characteristics of those great reflectors in the sky, the clouds, are at play in modulating the amount of sunlight that is available to warm the Earth, but until now they had overlooked a key point: the effect of clouds depends on the wavelength of sunlight being measured. The unexpected finding by a team in the US Department of Energy's Pacific Northwest National Laboratory (PNNL) is reported in the current issue of the journal Geophysical Research Letters.

"The amount of the sun's energy that reaches the Earth's surface is the main driver of the Earth's temperature," explains PNNL's Evgueni Kassianov. "However, clouds are one of the least understood aspects of climate change." He points out that clouds can block sunlight, but light can also ricochet from one cloud into another cloud's shadow, and then may strike the earth, and therefore increase the amount of solar energy reaching the Earth's surface depending on weather conditions. This latter point is essentially the reason why beachgoers can get sunburn even on overcast days and reveals and important component of the contribution of clouds to the Earth's overall energy balance.

Obviously, climate scientists have known for some time that clouds can have a significant effect on cooling and warming of the earth's surface. They cool the Earth by reflecting sunlight back into space, but they also act as an insulating layer by bouncing some sunlight back on to the surface. In general, clouds have a net cooling effect, but understanding the fine detail will improve climate models.

The team explains that exactly what is happening with clouds was not entirely clear. Fair-weather clouds are big and fluffy and bounce a lot of light around through scattering, increasing the apparent brightness of the sky. But, 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.

Until now, researchers have simply measured the net cloud effect by measuring a broad spectrum of sunlight that reaches the Earth's surface across the spectrum from infrared wavelengths into the ultraviolet. However, this overlooks an important point. Clouds tend to be white because the large water droplets within them scatter light of all colours almost equally in the visible spectrum. By contrast, aerosol particles within clouds and in the open sky scatter different wavelengths of light unequally, as is evidenced by blue skies and smoggy sunsets. Measurements that cover the whole gamut of wavelengths, in other words, fail to differentiate between the effects of different colours could be masking important details.

Blue skies thinking

Kassianov and his team have now abandoned the broadband measurement approach and have investigated the net cloud effect at four different wavelengths corresponding to violet, green, orange and red and at two infrared wavelengths. Using a spectral radiometer at the DOE's Atmospheric Radiation Measurement Climate Research Facility situated on the southern Great Plains in Oklahoma, the team was able to establish the brightness of the sky on a cloudless day. The spectroscopic measurements obtained with the radiometer correlate well with quantities and properties of atmospheric aerosols, which then gives a value for clear blue sky brightness.

Comparing measured values for cloudy sky to the calculated values for a clear, blue sky, the researchers found that, on average, fluffy fair-weather clouds cool the Earth's surface by several percent on a summer's day. However, although clouds lead to overall cooling, direct and scattered sunlight have opposite effects. The direct component accounts for the shade provided by clouds and leads to cooling, while scattering of light between and under clouds, which makes the sky brighter, leads to warming.

On Oklahoma's long summer's days, the scattered-light effect measured by the researchers reached quite high levels. For example, the shadow of a cloud passing over the radiometer led to the measured value of cloudy sky brightness in excess of the calculated clear sky value by up to 30 percent. Kassianov attributes such a large difference to scattered sunlight being "caught on tape" by the radiometer. "Sunlight scattered by three-dimensional, irregular clouds is responsible for the observed large difference," he says. "The one-dimensional cloud simulations currently used in large-scale climate models don't capture this diffuse light."

"If you want to study how aerosols and clouds interact," adds Kassianov, "you need to look in the region of the spectrum where aerosol effects are significant." The team found that, with direct light, the cooling caused by clouds was weakest at the violet end of the spectrum and strongest in the infrared. With scattered light, warming caused by clouds was also weakest at violet and strongest in the infrared. Overall, the least cooling and warming occurred at violet, and the most cooling and warming occurred at infrared.

Aerosols

Large droplets in clouds scatter sunlight almost uniformly across all wavelengths, which would suggests that clouds themselves cannot be the underlying cause as to why different wavelengths contribute differently to the net cloud effect. Aerosols particles are more than 100 times smaller than cloud droplets and scatter wavelengths differently. These results suggest that aerosols, which not only cause haziness but also contribute to cloud formation are responsible for the wavelength differences. This is an important point that must be incorporated into the cloudy aspects of climate models. "These results and the corresponding approach do not imply that climate models are currently incorrect," Kassianov told SpectroscopyNOW, "They can be beneficial to evaluate performance of the models (e.g., cloud-aerosol-radiation interactions) more thoroughly."

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.