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Flights reveal intriguing information about ice particles in clouds

February 06, 2006

In the clouds above Darwin, Australia, pilots guided by a team of international climate scientists are now one week into a series of carefully orchestrated flights to obtain key in situ data about tropical clouds. Preliminary results obtained from instrumentation on the Proteus -a space-age aircraft equipped with a suite of highly sophisticated sensors - reveal superior images of ice crystals in high-altitude tropical cirrus clouds.

"These images, combined with data from other aircraft probes, will provide us with a complete data set of detailed information about ice clouds, particularly the numbers of small ice crystals-a parameter that is poorly known and of considerable importance for understanding how clouds affect radiation and climate," said Dr. Greg McFarquhar, one of many U.S. scientists involved in the effort and funded by the Department of Energy's Atmospheric Radiation Measurement (ARM) Program.

The images were taken by the Cloud Particle Imager, an instrument developed by SPEC Inc. that provides very high resolution images of ice crystals. They were obtained as the Proteus aircraft was climbing through a thin layer of aged cirrus clouds, collecting data to help scientists determine how the properties of ice clouds, including particle size and shape, vary with temperature and altitude. These factors influence the longevity of the cloud, and therefore the amount of radiative energy both reaching and escaping the earth.

The Proteus, funded by the ARM Program, is one of five instrumented research aircraft taking part in the Tropical Warm Pool International Cloud Experiment, or TWP-ICE. Jointly led by the ARM Program and the Australian Bureau of Meteorology, the objectives of the experiment are to collect comprehensive cloud and atmospheric property measurements of both cirrus clouds and deep convective (thunderstorm) clouds-which are the source for much of the cirrus observed in the tropics-in an area approximately 200 km in diameter, centered on Darwin.

Other aircraft participating in the experiment include the Twin Otter, also funded by the ARM Program; the Dimona, sponsored by the Australian Bureau of Meteorology; and the Egrett and Dornier, sponsored by the Natural Environmental Research Council in the United Kingdom. These aircraft fly at altitudes ranging from 50 meters to 17 kilometers, and contain sensitive instruments for measuring various cloud properties, aerosol properties, temperature and humidity.

"Flying the aircraft on simultaneous missions at various elevations is a critical part of the experiment. We're extremely pleased that in just our third mission, we had all five aircraft up at the same time," said Dr. Jim Mather, the ARM Program's lead scientist for the experiment. "We were also able to fly several of the aircraft over our ground-based instrumentation. These aircraft data will be very useful for improving the cloud properties derived from the ground-based measurements."

The ARM Program operates a permanent climate research facility in Darwin. In place since April 2002, this site collects continuous measurements of tropical cloud and atmospheric properties. Scientists involved in the experiment installed a comparable set of instruments onboard a self-contained research vessel, provided by Australia's Commonwealth Scientific Industrial Research Organisation. During the experiment, the ship is located in the Timor Sea, about sixty miles west of Darwin. Measurements taken by the aircraft will be compared to the measurements from these two sites.

The ship also serves as one of five surface sites for launching weather balloons every three hours to obtain a continuous record of atmospheric observations throughout the course of the experiment. One of four surface flux stations, for measuring the exchange of energy between the surface and atmosphere is also located on the ship. These flux measurements are important for understanding the development of convective storms. The combination of surface -based and aircraft data will provide scientists with a detailed look at cirrus structure in the tropics. The experiment concludes on February 13.

DOE/Pacific Northwest National Laboratory