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October 2007

Ice in clouds

Front Range field campaign begins next month

cloud

During ICE-L, researchers will study ice formation processes in lenticlar clouds (such as the one pictured here) along the Front Range.

Much of Earth’s precipitation begins its journey from the atmosphere to the ground in the form of ice. The process by which ice crystals (which become rain, snow, and hail) initially form in clouds is a complicated one that scientists have been studying for more than 50 years without coming to consensus.

A team of researchers hopes to shine more light on the issue next month during a series of research flights along the Front Range, using NCAR’s C-130 and a privately-owned Lear Jet. The field campaign, called Ice in Clouds Experiment–Layer (ICE-L), will improve scientists’ understanding of ice formation, with applications for cloud models, precipitation forecasts, and climate change research.

“We’re basically looking at how the ice nucleation process occurs, and whether we can predict it based on measurements of ice nuclei,” explains Andy Heymsfield (ESSL/MMM), who’s leading the project along with Paul Field (ESSL/MMM; UK Met Office), David Rogers (EOL), and Jeff Stith (EOL). In total, about 35 co-investigators are participating, drawn from NCAR, universities, government agencies, and private companies.

The initial formation of ice is closely tied to the physical and chemical properties of aerosol particles. (Aerosols are tiny particles suspended in the air from sources such as dust, volcanoes, wildfires, vegetation, sea spray, and fossil fuel burning.) Among the wide variety of aerosol particles in the atmosphere, there are a few that catalyze ice formation. These are called ice nuclei. Depending on temperature and humidity, ice nuclei may make up only one out of a million particles. They are difficult to identify, but critical for forming ice in clouds and, ultimately, precipitation.

At sufficiently cold temperatures (–40ºF/C), pure liquid water freezes spontaneously in a process called homogeneous nucleation. At warmer temperatures, ice nuclei form ice from vapor or supercooled liquid water (chilled below freezing point without having turned solid). This process is known as heterogenous nucleation.

kip eagan

Kip Eagan (EOL) prepares a cloud radar for installation on the C-130. The radar has the ability to simultaneously look both up and down into clouds.

The ICE-L project will make airborne observations of both types of nucleation, with the C-130 focusing on the heterogenous process. The researchers hope to show that direct measurements of ice nuclei and ice nucleation can be used to predict the number of ice particles that form in a cloud. The research is especially important because scientists’ lack of understanding of the processes of ice initiation in clouds leads to uncertainties in modeling precipitation amounts and distribution, as well as in predicting climate change.

“In any reasonably good-to-excellent cloud model, you need to predict the concentration of ice crystals as a function of cloud conditions,” Andy says.

A number of recent advances in instrumentation make this an ideal time for ICE-L. For example, it’s been difficult for scientists to measure the number of ice particles in clouds using airborne instruments because large ice particles can shatter on the edges of the probes, producing high concentrations of smaller particles.

“Until now, we’ve been unable to accurately measure ice concentrations in clouds to within probably about a factor of ten,” Andy says.

Faster electro-optics and innovative designs have improved this situation. The researchers will use new instruments that reduce the likelihood of shattering ice particles and allow them to sample true concentrations of smaller ice crystals. They’ll employ three different probes (the small ice detector, cloud droplet probe, and 2D stereo) to sample ice crystals. Using aerosol mass spectrometers, they’ll measure particle chemical composition. A special cloud chamber on the aircraft will allow the sampling of ice nuclei, and a specialized inlet will evaporate cloud ice particles to allow the residual aerosol particles they contain to be tested for their ice-forming abilities.

The team will also use two instruments from the University of Wyoming, a cloud radar and cloud lidar. The radar has the ability to look both up and down into clouds simultaneously while giving information on the growth of ice particles and the vertical winds above and below a cloud, while the lidar functions as a very sensitive indicator of the first ice forming in wave clouds.

“The unprecedented part of this project is that we’re going to have the Wyoming lidar and radar—that’s just incredible,” Andy says.

A major goal of the project is to determine the effectiveness of new instruments at identifying critical ice formation regions that have previously been out of scientists’ reach. “This will be a starting point for testing instruments,” Andy says. “If it works, the instruments can be taken to other places.”

The researchers plan to make about three five-hour flights per week for four weeks, starting in November. They’ll focus mainly on lenticular clouds forming over the Front Range (which is conveniently one of the best places in the world for the formation of these clouds). Lenticular clouds provide an almost laboratory-like setting for studying ice formation processes. Air in these nearly-stationary clouds ascends in mountain-induced waves, making the growth time of particles the transit time through the cloud. Researchers aboard aircraft can repeat sampling the clouds at different levels to study influences of time and temperature on ice production processes.

“We’ll also be looking at ice formation in upslope clouds, which are common during this time of year and contribute to precipitation along the Front Range,” Jeff Stith says.

In addition, the project’s proximity to Denver will allow the team to study the effects of urban aerosols on ice nucleation processes, as well as the impact of any Asian dust outbreaks that happen to occur during ICE-L’s time frame.

Although some initial discoveries are expected during the field project, the analysis of data and publication of the results will occur over a period of several years following the study.

On the Web

More about ICE-L


In this issue...

Ice in clouds

Surprise finding in the desert

Tackling disasters in an energy-restricted Boulder

Jeffco bears fruit

Short Takes

Getting their paws wet

Delphi Question

Random profile: Chris Golubieski

Just One Look


 

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