Washington, May 21 : Satellites taking atmospheric measurements would now be able to see the skies clearer than before, by reducing cloud-induced glare when they measure blue skies on cloudy days.
Found by researchers at the US Department of Energy's Pacific Northwest National Laboratory, the approach involves using an indirect measurement of reflected light to estimate the measurements of cloud-bordered blue skies to within about 10 percent of what other instruments indicate.
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. he 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.
"When researchers try to apply satellite technology originally developed for clear skies to partly cloudy conditions, they find additional light reflected from clouds," said PNNL atmospheric scientist Evgueni Kassianov. "We can't use the same technology we use for clear skies for complex cloudy skies," he added.
Previous research has shown that clouds can brighten aerosols even up to three kilometers (almost two miles) away.
To address the problem, Kassianov and his fellow PNNL researcher Mikhail Ovtchinnikov took advantage of the fact that clouds largely reflect the same amount of light regardless of the wavelength of light.
Aerosols, on the other hand, reflect sunlight at different wavelengths to differing degrees.
So, the two tested whether using ratios of sunlight reflectance at different wavelengths might allow the extra reflectance from clouds to drop out of their atmospheric images.
The team created a database that related wavelength ratios, particles sizes, number of particles and aerosol optical depth. From this database, two ratios would allow them to determine their aerosol properties of interest.
The two then determined the accuracy of the ratio method.
They selected data from a typical summer day in the southern Great Plains, gathered via the DOE's Atmospheric Radiation Measurement Climate Research Facility (ACRF) in Oklahoma.
Using the ratio method, they retrieved the aerosol optical depth at three wavelengths (470, 660 and 870 nanometers) and compared this to the original data.
The ratio method estimated aerosol optical depth under partly cloudy conditions with an error of only about 10 percent.
The result might lead to more accurate estimates of the amount of sunlight penetrating the atmosphere.
Also, because clouds represent one of the largest areas of uncertainty, eventually this could lead to improved climate models.
According to Kassianov, if the results hold up with additional testing, this approach could be applied to data being collected by NASA's Earth Observing System in skies near clouds.