Cirrus clouds, with their dramatic strokes of white against the blue sky, are more often associated with spectacular scenery than with weather either good or bad. But despite their featherlight looks, they do have an impact on climate.
It all has to do with the Earth radiative budget that balances what comes in and how much goes out. Cirrus clouds are too thin to be able to shade the Earth from the Sun’s radiation. On the other hand, they are very good at absorbing part of the infrared radiation reflected by the Earth’s surface. This means that cirrus clouds have a greenhouse effect and that their contribution needs to be better accounted for in climate models.
To interpret the influence of cirrus on climate, scientists assume that they are planar and homogeneous, with an even ice water content spread equally across the cloud. The assumption is useful because it reduces the amounts of calculations required for the analysis.
But is this hypothesis too simplistic?
Céline Cornet, an atmospheric scientist based at the University of Lille in France, and her colleagues decided to investigate what kind of errors can be introduced from their observations from space by assuming that cirrus clouds are homogeneous.
To do this the team used a model based on the statistical Monte-Carlo method to trace how photons are reflected by clouds with different ice water distributions.
“For the study,” explains Cornet, “we computed radiances for eleven cases of cirrus clouds and three wavelengths.” On a single core computer it would have taken about three years to compute the necessary information for each example for a single wavelength. “For all cases, this leads to more than hundred of years, which is obviously not possible,” she adds.
They concluded that uneven distributions of ice water in cirrus cloud cannot be neglected because they have an “impact on the retrieved parameters such as optical thickness, cloud emissivity and ice effective radius that are directly linked with the ice water content,” says Cornet.
The results were published in the journal Atmospheric Measurement Techniques.
Credit: Fir0002/Flagstaffotos (Wikimedia Commons)
Fauchez et al. 2015 Impacts of cloud heterogeneities on cirrus optical properties retrieved from space-based thermal infrared radiometry. Atmospheric Measurement Techniques doi:10.5194/amt-8-633-2015