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Atmospher Sci & Global Chg
Research Highlights

September 2011

Tally a Twofer in Cloud Characterization

Measuring both atmospheric particles and sky cover in one fell swoop

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The Multi-Filter Rotating Shadowband Radiometer (left) and Pyranometer (right) are located at the Department of Energy’s ARM Climate Research Facility in Oklahoma. The Total Sky Imager (see video) provided sky images to support their research. Enlarge Image

Results: Two for the price of one. That's the bargain scored by scientists at Pacific Northwest National Laboratory when they convinced a single-use instrument to pull double duty. They converted a ground-based instrument designed for measuring interactions of sunlight and atmospheric particles to also measure the amount of sky covered by clouds, called "fractional sky cover." Their results were validated using a well-established independent method to measure sky cover and published in Atmospheric Measurement Techniques.

Why it matters: Guessing the amount of sky covered by clouds is a familiar pastime. Partly cloudy, overcast, clear sky: these are descriptions captured by the human eye that scientists describe as fractional sky cover. But scientists working to understand the complex warming and cooling effects of direct and reflected sunlight need a more exact approach because measuring cloud fraction, or how much of the sky is covered by clouds, is an important piece of that story. Researchers for this study extended the capabilities of one instrument to measure important characteristics of clouds. And who doesn't prize a two-for-one bargain?

"Sky cover has the largest impact on the amount of sunlight transmitted to the ground level. More effective measurements of the fractional sky cover, including different cloud types, will help scientists understand the warming and cooling effect of clouds and aerosols for Earth," said Dr. Evgueni Kassianov, atmospheric scientist at PNNL.

Methods: Researchers from PNNL investigated using a more effective method for measuring fractional sky cover of different cloud types, including cumuli. The ground-based Multi-Filter Rotating Shadowband Radiometer, or MFRSR, is an instrument used for measuring interactions of sunlight and atmospheric particles for cloudless sky conditions. They extended the capabilities of this instrument to measure sky cover, an important parameter of clouds, and they did so under a range of cloud conditions. They validated the new method with a ground-based pyranometer, a well-established instrument for measuring sky cover.

Previous measurements captured sky cover for thick and dark clouds. The new approach measures sky cover during any type of cloud condition, from optically thick, to thin and shallow cumulus cloud conditions. Now, MFRSRs located at stations worldwide can offer researchers integrated data on cloud and aerosol properties, and water vapor.

The two instruments, the MFRSR and pyranometer, were co-located at the Department of Energy's Atmospheric Radiation Measurement Climate Research Facility at the Southern Great Plains site during the summer of 2007. The research used data over a 13-day period with cloudy skies.

What's next: Researchers plan to repeat the success of this study and apply it to additional cloud and aerosol conditions at other ground-based sampling locations around the world.

Acknowledgments: This work was funded by the Department of Energy's Office of Biological and Environmental Research as part of the Atmospheric Radiation Measurement (ARM) Climate Research Facility, and Atmospheric Systems Research program.

References: Kassianov E, Barnard JC, Berg LK, Flynn C, and Long CN. "Sky Cover from MFRSR Observations." 2011. Atmospheric Measurement Techniques, 4, 1463-1470. DOI:10.5194/amt-4-1463-2011.

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The Total Sky Imager

The Total Sky Imager records a hemispherical view of the sky cloud conditions, from sunrise to sundown, to compare time-stamped sky conditions with aerosol particle sampling and other atmospheric measurements. Researchers used this data in their work to extend the capability of the MFRSR.