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Atmospheric Sciences & Global Change
Research Highlights

January 2007

Data from Saharan Dust Storm Reveal Model Deficiencies

Simultaneous surface and satellite measurements lead to first assessment of radiative impacts of a dust storm

Results: With European collaborators, researchers at Pacific Northwest National Laboratory report results from a study of a major dust storm that occurred in Niamey, Niger, in March 2006. Their paper, which appeared in Geophysical Research Letters, documents the first use of both satellite and surface measurements to examine the total absorbed and reflected energy—or radiative energy balance—of a dust storm. When compared with the satellite and ground-based observations, computer models did a good job of reproducing the radiative energy balance during the dust storm, but they underestimated the solar absorption within the atmosphere. This is critical information used in computer models that simulate both global and regional climate.

Why it matters: Scientists use a "soda straw" approach to represent the vertical layers of Earth's atmosphere from the ground to the top of the atmosphere. This soda straw approach is referred to as an atmospheric column. The energy balance of an atmospheric column is a key component in computer models that simulate both regional and global weather and climate. Though Saharan dust storms have been observed from space, their impact on the Earth's radiation balance through absorbing solar energy is poorly understood because of limited surface observations in areas affected by such storms.

Methods: Many of the researchers are members of the science team for the Department of Energy's Atmospheric Radiation Measurement (ARM) Program. They took advantage of the ARM Mobile Facility (AMF), a portable atmospheric research facility composed of a large suite of sophisticated instruments, sensors, and radars for obtaining measurements cloud and atmospheric properties. As part of an ongoing international study of the West African monsoons, the AMF was deployed in Niamey from January through December 2006.

Aerosol optical thickness chart
Aerosol optical thickness (aot) is a measure of how much a specific layer in the atmosphere reduces solar energy. In U.S. cities, typical values of this measure are less than a few tenths; the March storm in Niamey had optical depths in excess of 3. This dust decreased the solar radiation reaching the surface to about 15% of its usual value.

For the study of the dust storm in March, data obtained from the AMF were combined with continuous 15-minute observations from the Geostationary Earth Radiation Budget and Spinning Enhanced Visible and Infrared Imager instruments flown onboard the Meteosat-8 geo-stationary satellite platform. The researchers used measurements of atmospheric temperature and humidity profiles, and ground-based retrievals of aerosol optical properties—such as particle thickness and reflectivity—as input to radiation models to assess their ability to simulate the impact of the dust on solar radiation balance.

Next steps: Using a broader data set from the AMF deployment in Niamey, PNNL researchers will build on this study to (1) perform long-term calculations of warming and cooling effects of Saharan dust; (2) examine the warming and cooling effects of aerosols in climate models; and (3) compare surface-derived optical properties of dust with aircraft measurements from the Dust and Biomass Experiment conducted in Niamey between January and February 2006.

Total Sky Imager
Images taken by the AMF's Total Sky Imager instrument show the sky above Niamey on a normal day (left) and during the dust storm (right). The thick black bar is a tracking device to block the direct beam of the sun from the camera.

Research team: Tom Ackerman, Jim Barnard, Evgueni Kassianov, and Sally McFarlane, all PNNL; Anthony Slingo, Richard Allan, and Gary Robinson, University of Reading, United Kingdom; Mark Miller, Brookhaven National Laboratory; John Harries and Jacqueline Russell, Imperial College, United Kingdom; Steven Dewitte: Royal Meteorological Institute of Belgium, Belgium.

Source: Slingo, A, TP Ackerman, RP Allan, EI Kassianov, SA McFarlane, GJ Robinson, JC Barnard, MA Miller, JE Harries, JE Russell, S Dewitte, 2006. Observations of the impact of a major Saharan dust storm on the atmospheric radiation balance, Geophys. Res. Lett,. 33, L24817, 10.1029/2006GL027869.

Sponsors: U.S. Department of Energy's Office of Biological and Environmental Research, and the Natural Environment Research Council of the United Kingdom.

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