Satellites seeing clearly despite clouds
Two researchers at Pacific Northwest National Laboratory (PNNL) have developed a mathematical tweak which dramatically improves air pollution detection on cloudy days. They've found a way to reduce cloud-induced glare when satellites measure blue skies on cloudy days, by as much as ten-fold in some cases. 'Because clouds represent one of the largest areas of uncertainty, eventually this could lead to improved climate models.' The method is remarkably simple. Researchers discovered that combining measurements of reflected light at several wavelengths can eliminate the cloud effects, making air pollution measurements much more accurate. These results need to be confirmed, but the method could soon be used to data being collected by NASA's Earth Observing System in skies near clouds. But read more...
Here are some results obtained by the researchers. "Using the ratio method (right) clears up the satellite view (left) of a partly cloudy sky." (Credit: Evgueni Kassianov and Mikhail Ovtchinnikov, PNNL) Here is a link to a larger version of this illustration.
This new approach, which will improve air pollution detection on cloudy days, has been developed by Evgueni Kassianov and Mikhail Ovtchinnikov, two PNNL scientists focused on climate physics.
In another news release, "Seeing clearly despite the clouds" (April 17, 2008), PNNL describes the problem. "Blue skies might seem empty, but they are full of naked-to-the-eye particles called aerosols, which are made up of water and bits of matter. These aerosols reflect sunlight. The more aerosols, the more sunlight is reflected back to the satellite. But on cloudy days, clouds bounce sunlight all around and make nearby aerosols seem brighter than they really are. Previous research has shown that clouds can brighten aerosols even up to three kilometers (almost two miles) away."
The idea of Kassianov and Ovtchinnikov was based on the fact that clouds largely reflect the same amount of light regardless of its wavelength. And it worked. "Using powerful computers at the Environmental Molecular Sciences Laboratory, a U.S. Department of Energy national scientific user facility at PNNL, the researchers constructed two images of a patch of cloudy sky using complex computational methods that can subtract the estimated cloud-induced glare. One method was based on total reflected light, and the other was based on ratios of how much light was reflected at two different wavelengths. The ratio image provided a view of the cloudy sky with much better contrast than the reflected light method, indicating that ratios better delineate clear sky from clouds."
This research work has been published in the Geophysical Research Letters under the name "On reflectance ratios and aerosol optical depth retrieval in the presence of cumulus clouds" (Volume 35, Article L06807, March 28, 2008). Here is a link to the abstract. "The traditional conversion of satellite-observed reflectances to the aerosol optical depth (AOD) is highly conjectural in the vicinity of clouds due to the 3D cloud-induces enhancement of the apparent reflectance. This study uses 3D Monte Carlo radiative transfer calculations and simulated cloud and aerosol fields to illustrate that for clear pixels the reflectance ratios for two pairs of wavelengths (660, 470 nm and 870, 470 nm) are less sensitive to the 3D cloud effects than the reflectances themselves. We develop a new algorithm for converting these two ratios to three spectral values of AOD and show that it is accurate to within 10% for the majority of clear pixels in our model-inverse problem."
So what's next? "The team plans more testing to further validate the method, by comparing the results with additional measurements taken during two ARM [Atmospheric Radiation Measurement] Program field campaigns in 2007. If the results hold up, the new approach may be applied to data being collected by the NASA's Earth Observing System, a web of satellites for long-term global observations of land, atmosphere, and oceans."
Sources: PNNL Research Highlights, April 2008; and various websites
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