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Making clouds in the lab

April 26, 2006 By Ned Stafford This article courtesy of Nature News.

Artificial cloud-maker could yield clues about climate change.

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An office block with a huge silver silo perched on top of it seems an unlikely place to produce clouds. But nestled within just such a building at the Leibniz Institute for Tropospheric Research in Leipzig, Germany, lies some cutting-edge equipment: an 8-metre long, pencil-thin steel tube that can hold man-made clouds.

The facility, called the Leipzig Aerosol Cloud Interaction Simulator (LACIS), was launched this Tuesday after seven years of planning and construction, at a cost of nearly 3 million euros (US$3.7 million). "This is a dream that we cloud researchers have had for 20 years, to bring clouds into the lab," says institute director Jost Heintzenberg.

The simulator should allow researchers to investigate in depth the processes by which cloud droplets form on bits of particulate matter in the air, called aerosols. With a better understanding of such processes, researchers hope to elucidate how cloud cover affects climate (and vice versa), and how these effects may change in the future thanks to mankind's activities.

"The linkage between aerosols and clouds is one of the key uncertainties in our ability to determine climate," says Raymond Shaw, an atmospheric physicist at Michigan Technological University in Houghton. "Understanding this process — the interaction between aerosols and clouds, and so how and when and why water droplets are formed — is crucial. That is one reason why LACIS is so special."

Going with the flow

There are other cloud simulators in laboratories scattered around the world. But Heintzenberg says these are static systems that don't allow for air-flow or changing climate conditions within the simulator. LACIS has the advantage of simulating the development of cloud droplets in a dynamic fashion.

Air containing aerosol particles flows through the 1.5-cm-wide tube, passing through different conditions simulating a range of climates. The temperature can be fixed between -50 and 20 °C with a precision of 0.01 °C, and humidity is set with a tiny margin of error of 0.1%. Optical spectrometers, positioned at different sections of the tube, count the number and size of water droplets passing by.

The narrowness of the channel helps the researchers to measure just one particle or droplet with the spectrometer at a time. This means researchers can study in detail how soot, salt or biomass particles spur cloud growth at different temperatures and humidity. From that, says Heintzenberg, researchers can calculate factors such as how big such clouds would grow under the same conditions outside, and how much they would reflect sunlight — a crucial part of climate studies.

Adding the clouds is easy. "It's something everybody does when walking down the street in winter," Heintzenberg says. "You breathe out clouds." The hard part, says Heintzenberg, is controlling the conditions inside such a long tube.

Inside out

Laboratory-based cloud research has been seriously neglected in the past, says Shaw. Instead, research has largely focused on airplane-based observations and computer modelling. A duo of satellites called CloudSat and CALIPSO, due to launch this week (but so far delayed, in part due to too-cloudy conditions), will help fill in some of the gaps in our understanding of clouds. But only lab-based projects like LACIS allow experiments to be carried out under controlled, reproducible conditions.

"Put simply, there is no other facility in the world with the capability to study aerosol-cloud interactions in quite the way LACIS does," says Shaw.

A smaller, 1-metre-long prototype of LACIS has proven that the facility does the job well; research using this prototype has already been published.

Shaw says he intends to apply for funding to do research in the LACIS lab. He expects other cloud researchers to flock to it too.

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