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Measuring clouds with digital holography

HOLODEC 2 installed on Gulfstream V aircraft.

HOLODEC 2 (visible on the right, with yellow stripe) installed on the Gulfstream V for testing.

A team of NCAR researchers, led by postdoctoral researcher Jacob Fugal, is developing and testing a specialized instrument that uses digital holography to measure tiny cloud droplets. The instrument, Holographic Detector for Clouds 2 (HOLODEC 2), will be used on the Gulfstream V.

Holography is a process by which 3-D images can be stored and reproduced using laser light and projected onto a 2-D surface. Scientists have applied holography to the study of cloud particles for several decades using photographic plates, a time-consuming process that involves scanning film in a lab to search for particles.

HOLODEC 2 uses digital technology, making the process of applying holography to cloud particle measurements much more efficient. Fugal has developed an algorithm that processes data from the instrument on the Lincoln supercomputer at the National Center for Supercomputing Applications, part of the NSF-sponsored TeraGrid, a network of supercomputers that comprise the nation’s most advanced infrastructure for open scientific research.

Another important feature of HOLODEC 2 is that, in addition to resolving particles as small as 4-8 micrometers, it samples a 10-cubic-centimeter region around a cloud droplet, unlike other cloud particle instruments. This will help scientists study the environment that surrounds cloud droplets, for while scientists have measurements of how clouds behave on large scales, they know less about processes on the scale of cloud droplets, which is where clouds begin.

“Cloud models are improved the more they accurately include the behavior of clouds on very small scales,” Fugal says. “Global climate models can also be improved when we understand cloud behavior on these small scales. That's where HOLODEC 2 will play a key role."

The research team ran HOLODEC 2 successfully on test flights in August and is currently processing ice crystal data. More tests are planned for September.

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