Mapping the Frozen Sky: Study looks at clouds from both sides now

Scientists have combined simultaneous observations from satellite sensors and ground-based radar to construct a detailed, three-dimensional map of a high-altitude cirrus cloud. Such thin, wispy clouds waft above as much as 30 percent of Earth’s surface, and the new analytical technique may help researchers quantify the clouds’ precise effect on Earth’s climate.

SKY-HIGH ICE. High-altitude cirrus clouds, made mainly of ice particles, overlie up to 30 percent of Earth’s surface. The particles’ size and shape affect weather and climate.

Cirrus clouds, which typically appear at high altitudes and are predominantly made of ice crystals, can occur at any latitude in any season, says Kuo-Nan Liou, an atmospheric scientist at the University of California, Los Angeles. The clouds cool Earth’s surface slightly by reflecting some sunlight back into space, but they also warm the lower atmosphere by trapping some of the planet’s outbound infrared radiation. The exact influence of the global fleet of cirrus clouds depends upon the average size and distribution of the clouds’ ice particles, among other factors.

That’s where the new method can help, says Liou.

Spaceborne sensors can measure the degree to which a cloud blocks various wavelengths of light passing through it. However, satellites can’t assess other properties of a cloud, such as its vertical thickness or characteristics of its constituent particles. Cirrus researchers recently developed a way to fill in those blanks with data from Doppler radar and other ground-based instruments. By combining the space-down and ground-up observations, scientists can get a more complete picture of how cirrus clouds vary in both the horizontal and vertical directions.

That, in turn, will help researchers model how cirrus clouds scatter radiation and thereby affect temperature and other atmospheric conditions.

The analytical technique developed by Liou and his team provides results that match simultaneous measurements obtained by aircraft flying through clouds, but it’s easier and less expensive. The researchers report their findings in an upcoming issue of Geophysical Research Letters.

A cirrus cloud’s ice particles come in several shapes, says Liou. About 50 percent of the dust-size particles are hexagonal prisms capped on each end by pointed pyramids, about 30 percent are hexagonal tubes, and the rest are flat hexagonal plates.

Although the flat plates make up the smallest fraction of particles in a cirrus cloud, if they align in similar directions they can have a strong influence on how much radiation the cloud blocks, says Liou. If light strikes a thin particle on its edge, the ice crystal doesn’t stop much of the radiation. If the plate is turned to face the sun, however, much more of the radiation can be reflected back into space.

Recent ground-based observations suggest that under some conditions, these flat crystals indeed align. Specifically, when conditions inside the cloud are calm, plate-like particles oscillate like seesaws through an angle of about 12 centered on the horizontal, says Luc Bissonnette of the Defence Research Establishment Valcartier in Val-Bélair, Quebec.

In their experiment, Bissonette and his team beamed an infrared laser up into a wintertime, low-level cirrus cloud. As they moved the beam from directly overhead to points near the horizon, they measured the amount of radiation scattered back to the ground. This group also reports its findings in an upcoming Geophysical Research Letters.

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