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).
