Questions about Climate - the Answers are in the Clouds

Like any patient, the Earth requires a series of diagnostic tests to gauge the overall health of the planet. Unlike a person, the Earth does not have to don an embarrassing white smock for these procedures. Instead, scientists are training noninvasive sensors, similar to those used in a medical setting, on the clouds in the sky in order to improve our understanding of weather and climate conditions on the planet.

Quite simply, clouds are the visible collection of water vapor and ice that forms and dissipates high in the atmosphere. "[And these] clouds play a critical role in Earth's weather and climate," said Dong Huang from Brookhaven National Laboratory.

These ephemeral features affect how incoming solar radiation is absorbed by the planet or reflected back into space. Unfortunately, scientists only have a rudimentary understanding of the dynamic conditions inside the cloud.

"Poor understanding of clouds has long limited scientists' ability to make accurate predictions about weather and climate change" said Huang.

The study of clouds is plagued by two complications – time and scale. The lifetime of an average cloud ranges from mere minutes to a few hours and a cloud can cover an area that spans hundreds of kilometers across the sky. In the past, scientists were limited to taking readings with probes that encompassed a volume of less than a few cubic centimeters. The probes only offered snap shots of the dynamic environment inside the cloud at one location.

Photo Credit: Dan Rusk

A shelf cloud moving across the sky at the Southern Great Plains (SGP) site. Visible in the foreground is the Atmospheric Radiation Measurement (ARM) Climate Research Facility's (ACRF) instrumentation.

"Using these methods, it would take hundreds of years to take readings from an entire cloud," said Warren Wiscombe, chief scientists at the Atmospheric Radiation Measurement (ARM) Climate Research Facility (ACRF) in Ponaca City, Okla.

A team of ARM scientists led by Huang are evaluating a technique similar to a CAT or CT scan, a non-invasive medical technique that has been used for the past few decades to diagnose disease in the human body. The group deployed five sensors along a straight five-mile stretch of farmland in Billings, OK.

"All of the sensors will be scanning the same cloud at the same time" said Huang. The scans are combined to produce three-dimensional images that scientists use to produce maps of conditions inside the cloud.

"Using [these] three-dimensional ‘maps' [it will be possible to view] the distribution of clouds, atmospheric moisture, and to infer other characteristics in the atmosphere, including solar and infrared radiation, over an area as wide as 10 kilometers (6 miles)," said Huang.

Data will be collected independently by another research group using probes attached to an atmospheric aircraft will be used to ground truth the data collected during Huang's study.

Huang's study is a short-term analysis to determine the capability of sensors to accurately generate 3-D images of clouds.

While Huang and his team pursue proof of concept for this new approach, the Department of Energy is currently working to deploy new sensors using a currently available, off-the-shelf technology at all ARM facilities, both fixed and mobile, around the world. These new instruments will generate 3-D images of clouds and also characterize precipitation.

"This new technology will allow atmospheric scientists to scan the sky from horizon to horizon spanning 20 kilometers in one scan" said Wanda Ferrell, ARM Climate Research Facility Program Director.

The instruments operate continuously and the data are made available for use by scientists worldwide through the ARM Climate Research Facility (ACRF). This user facility has enormous potential to advance scientific knowledge in a wide range of scientific disciplines.

Photo credit: Dong Huang

Cloud tomography measurements of cloud thermal emission taken from multiple directions. The plot shows a cross-sectional snapshot of the liquid water structure of a stratocumulus cloud simulated by a cloud model.

The new ACRF measurements will be used to improve global climate models - powerful computer programs that use a variety of parameters to simulate the magnitude of regional and global fluctuations in climate.

In the past, global climate models and high-resolution cloud models have been hampered by inadequate measurements of clouds and aerosols. As the secrets of the clouds are revealed, scientists will begin funneling this information into these models. The updated cloud information will reduce the uncertainty from the model simulations and provide a better diagnosis of planetary health

ARM facilities are located around the world, with permanent facilities in Barrow, Alaska; Billings, Oklahoma; Darwin, Australia; Manus Island, Papua New Guinea; and Nauru Island, and mobile facilities have been deployed in Point Reyes, California (2005); Heselbach, Germany(2007); Niamey, Niger (2006); Shouxian, China (2008) and Graciosa, Azores (2009).

This work is supported by the Department of Energy (DOE) Office of Science, Office of Biological Environmental Research. DOE invests in science and solving critical issues impacting people's daily lives and the nation's future. For more information, visit http://www.arm.gov/.

References
Huang, D., Y. Liu, and W. Wiscombe (2008), Determination of cloud liquid water distribution using 3D cloud tomography, J. Geophys. Res., 113, D13201, doi:10.1029/2007JD009133.

Huang, D., Y. Liu, and W. Wiscombe (2008), Cloud tomography: Role of constraints and a new algorithm, J. Geophys. Res., 113, D23203, doi:10.1029/2008JD009952.

Huang, D., A. Gasiewski, and W. Wiscombe (2009), Retrieval of cloud liquid water distributions from a single scanning microwave radiometer aboard a moving platform. Part I: field trial results from the Wakasa Bay experiment, Atmospheric Chemistry and Physics Discussions, 9, 12027-12064.

Huang, D., A. Gasiewski, and W. Wiscombe (2009), Retrieval of cloud liquid water distributions from a single scanning microwave radiometer abroad a mobile platform. Part II: observation system simulation experiments, Atmospheric Chemistry and Physics Discussions, 9, 12065-12099.

Warner, J., J.F. Drake, and P.R. Krehbiel (1985), Determination of cloud liquid water distribution by inversion of radiometric data, J. Atmos. Sci. Ocean. Tech., 2, 293-303.

This article was written by Stacy W. Kish