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Scanning the clouds

Photonics Spectra
Aug 2009
Anne L. Fischer, Senior Editor, anne.fischer@laurin.com

Computed tomography (CT) commonly is used to take x-ray images of the human body, but researchers recently employed the technique to scan clouds, creating three-dimensional maps from the data for use in climate change studies and weather forecasting.

The research is being conducted by scientists from the US Department of Energy (DoE) ’s Brookhaven and Argonne national laboratories in Upton, N.Y., and Illinois, respectively, and from the University of Colorado at Boulder.

Researchers traditionally use microwave radiometers to observe clouds, but because clouds form, take shape and dissipate at varying spatial scales – from submicrons to thousands of kilometers – in situ cloud probes cannot provide the necessary information at the required spatial scale or resolution.

GLctscan1.jpg
Three microwave radiometers at the US Department of Energy’s ACRF Southern Great Plains site and another two from the University of Colorado at Boulder were arranged in series to continuously scan clouds passing overhead. The data collected will be used to reconstruct 3-D cloud maps. Courtesy of the US Department of Energy’s Atmospheric Radiation Measurement Program.

Atmospheric scientist Dong Huang of Brookhaven, the lead investigator, explained that there are mainly two categories of cloud measurement techniques: in situ, where probes, either radiometric or hot-wire, are placed in clouds, and remote sensing. Although in situ observations are expensive and can sample only a small volume, they provide more direct measurements; remote sensing techniques are less direct and usually involve complicated mathematical problems.

For the CT project, five sensors placed one mile apart along a north-south line at the DoE’s Atmospheric Radiation Measurement Climate Research Facility (ACRF) in Ponca City, Okla., continuously scanned a flat vertical plane. The instruments included three commercial WVR 1100 radiometers from Radiometrics Corp. of Boulder and two polarimetric scanning radiometers developed by the University of Colorado. One sensor at each of the five locations scanned without human intervention, Huang said.

The scientists gathered data on the cloud’s thermal emission – tracked as the cloud changed – enabling them to piece together a map of the cloud’s structure. This is important, Huang said, because clouds previously were assumed to be homogeneous. “Now, with 3-D cloud maps, we can calculate the radiation reaching the Earth’s surface much more accurately.” He said the information will be useful in understanding precipitation processes, which will help improve climate and weather forecasting models.

The data gathering is simple and straightforward. The real challenge lies in constructing the cloud structure from radiometric measurements, Huang said. Very complex mathematical problems must be resolved to turn the data taken from a “limited view cloud tomography problem” into a 3-D map.

The scientists plan to validate the quality of the cloud and moisture reconstruction by comparing it with data currently being collected from another study, this one by Brookhaven scientists. In the latter, investigators are collecting long-term data on low-optical-depth clouds using an in situ instrument suite designed for routine observations of cloud properties. The suite includes cloud microphysics, radiation, aerosol and meteorology instruments. The probes related to the cloud tomography study are cloud liquid water and water vapor instruments from Gerber Scientific Inc. of South Windsor, Conn.

The CT scan project was conducted from May to July 2009, after which the data will be available to the public through the Atmospheric Radiation Measurement Program.


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