ATMOSPHERIC STUDIES: Ice crystal shapes are assessed in the Earth’s atmosphere

Sept. 1, 2008
Atmospheric scientists have created an optical-scattering instrument designed to capture high-resolution spatial light-scattering patterns of ice crystals like those found in high-altitude cirrus clouds.
(Courtesy of University of Hertfordshire)
The scattering pattern of individual ice crystals (left) can be used to accurately identify the size and shape of a micron-size column and hexagonal platelet of ice (right). Such microscopic data, which was previously unattainable, can aid in understanding how clouds affect Earth’s climate.
The scattering pattern of individual ice crystals (left) can be used to accurately identify the size and shape of a micron-size column and hexagonal platelet of ice (right). Such microscopic data, which was previously unattainable, can aid in understanding how clouds affect Earth’s climate.

Atmospheric scientists have created an optical-scattering instrument designed to capture high-resolution spatial light-scattering patterns of ice crystals like those found in high-altitude cirrus clouds. The instrument assesses the shapes and sizes of atmospheric cloud particles down to the scale of a single micron. The observations may help reduce the uncertainty of the effects of ice crystals in the computer models used to predict climate change, according to a new study.1

One of the hundreds of factors affecting climate modeling is the nature of clouds. Cirrus clouds in particular can trap thermal radiation from Earth and warm the atmosphere, leading potentially to the production of more cirrus and more warming. However, they also reflect solar radiation back into space, effectively cooling the atmosphere. These processes are dependent on the sizes, shapes, and abundance of the ice crystals within the clouds, so a detailed knowledge of these crystals is essential to understand and model their influence on global climate. The primary means of studying atmospheric ice crystals has been to image them, but optical aberrations make this impractical for crystals less than about 25 µm in size.

Novel instrument

Researchers from the University of Hertfordshire (Hatfield, England), the University of Manchester (Manchester, England), and Colorado State University (Fort Collins, CO) have developed an optical-scattering instrument that can evaluate the size of the crystals to a much better optical resolution than current cloud-particle imaging probes. Professor Paul Kaye and colleagues direct air laden with ice particles through a tapered nozzle and across the beam of a 150 mW, 532 nm Nd:YAG laser from Crystalaser (Reno, NV). The laser beam is circularly polarized to minimize polarization-dependant variations in the scattering patterns.

A gated, intensified charge-coupled device (CCD) records the scattering patterns of individual crystals with single-photon sensitivity across 582 × 582 pixels, at a rate of 20 scattering patterns per second. The CCD collects these patterns over scattering angles from 6° to 25°, encompassing the 22° halo scattering that is typical of ice columns. The particle scattering patterns can then be interpreted using theoretical models or previously recorded patterns from known crystal shapes. From this data, the group is creating of a database of crystal shapes and sizes.

“The scattering technique is capable of revealing particle features down to about the wavelength of the light, in our case about 0.5 µm,” says Hertfordshire professor Paul Kaye. “The challenge is always to be able to take the scattering pattern and ‘decode’ it to get the shape of the particle that produced it.” The development of theoretical models for crystals of more-complex shapes beyond columns, platelets, or small bullet rosettes is still ongoing, says Kaye.

The team built two versions of the instrument: one for use in ground-based cloud-simulation chambers or in the fuselage of research aircraft; and one aerodynamic version that fits under the wing of an aircraft in flight. Senior research scientist Paul Demott of Colorado State University (Fort Collins, CO) is using the lab version of the instrument in his cloud-physics laboratory, and the wing-mounted version is currently undergoing wind-tunnel and other tests at the University of Hertfordshire.

“We hope the instruments will provide a means of establishing the shapes of the smallest cloud ice particles that are too small to be imaged using conventional cloud imaging probes,” says Kaye. “This data could then allow climate modelers to significantly reduce one area of uncertainty in their models: how ice clouds interact with light and heat radiation.”

REFERENCE

1. P.H. Kaye et al., Opt. Lett. 33(13) 1545 (2008).

About the Author

Valerie Coffey-Rosich | Contributing Editor

Valerie Coffey-Rosich is a freelance science and technology writer and editor and a contributing editor for Laser Focus World; she previously served as an Associate Technical Editor (2000-2003) and a Senior Technical Editor (2007-2008) for Laser Focus World.

Valerie holds a BS in physics from the University of Nevada, Reno, and an MA in astronomy from Boston University. She specializes in editing and writing about optics, photonics, astronomy, and physics in academic, reference, and business-to-business publications. In addition to Laser Focus World, her work has appeared online and in print for clients such as the American Institute of Physics, American Heritage Dictionary, BioPhotonics, Encyclopedia Britannica, EuroPhotonics, the Optical Society of America, Photonics Focus, Photonics Spectra, Sky & Telescope, and many others. She is based in Palm Springs, California. 

Sponsored Recommendations

Brain Computer Interface (BCI) electrode manufacturing

Jan. 31, 2025
Learn how an industry-leading Brain Computer Interface Electrode (BCI) manufacturer used precision laser micromachining to produce high-density neural microelectrode arrays.

Electro-Optic Sensor and System Performance Verification with Motion Systems

Jan. 31, 2025
To learn how to use motion control equipment for electro-optic sensor testing, click here to read our whitepaper!

How nanopositioning helped achieve fusion ignition

Jan. 31, 2025
In December 2022, the Lawrence Livermore National Laboratory's National Ignition Facility (NIF) achieved fusion ignition. Learn how Aerotech nanopositioning contributed to this...

Nanometer Scale Industrial Automation for Optical Device Manufacturing

Jan. 31, 2025
In optical device manufacturing, choosing automation technologies at the R&D level that are also suitable for production environments is critical to bringing new devices to market...

Voice your opinion!

To join the conversation, and become an exclusive member of Laser Focus World, create an account today!