Sunphotometer Supports SPARTICUS Campaign and Satellite Validation Studies

Published: 15 May 2010

A telescope in the SAM instrument tracks the sun and its surrounding “aureole”—the whitish area right around the sun. These measurements provide an indication of the size and thickness of particles that scatter the sun’s light, which is important for understanding how the atmosphere affects incoming solar radiation.

To supplement airborne cirrus particle size measurements obtained during SPARTICUS, a Sun and Aureole Measurement (SAM) imaging sunphotometer is collecting ground-based radiance measurements at the ARM Southern Great Plains (SGP) site’s Central Facility. Measurements of the solar disk and aureole radiance profile at a wavelength of 670 nm are ideally suited to investigate the larger droplets found in water and the smaller ice crystals found in cirrus clouds. Researchers leading the SAM Support for SPARTICUS campaign plan to apply a diffraction technique to the SAM profile measurements that, in previous studies, has yielded particle size distribution results in the 5 to 50 micron range. This is the size range of the “small” ice crystals of special interest in the SPARTICUS campaign.

With the extension of the SPARTICUS campaign into June, comparisons between the SAM and airborne measurements have not yet begun. However, comparisons of SAM measurements with MODIS satellite retrievals and with ARM Raman lidar and Cimel sun photometer measurements were reported in a poster at the first ASR Science Team Meeting in Bethesda, Maryland, in March. The data show excellent agreement between SAM and the Cimel photometer, and reasonable agreement with the Raman lidar given their different lines-of-sight. Satellite-retrieved parameters resulted in considerably dimmer solar disk radiances than measured by SAM. The aureole profiles also differed qualitatively, confirming the value of ongoing investigation into different treatments of ice crystal surface optical properties.

Once the SPARTICUS campaign ends, the SAM instrument will remain in place at the Central Facility and begin obtaining data for another campaign, Validation of MODIS Cirrus Algorithm Improvements.

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The ARM Climate Research Facility is a DOE Office of Science user facility. The ARM Facility is operated by nine DOE national laboratories, including .