In October, researchers funded by the National Science Foundation completed a six-month redeployment of the Analytical Spectral Devices (ASD) shortwave spectroradiometer at the ARM North Slope of Alaska site in Barrow. Each minute, the ASD instrument automatically records spectra in the wavelength range 350-2200 nm, which encompasses most of the sun’s energy. Measurements from the infrared portion contain information about cloud microphysical properties, such as optical depth, cloud phase, and effective particle size. Previously deployed in April 2008 for the Indirect and Semi-Direct Aerosol Campaign (ISDAC), the latest set of measurements carry through the summer, providing many more pure liquid water cloud cases for refining the algorithms used in measuring cloud microphysical properties with surface-based instruments.

The ASD is essentially a shortwave version of a baseline ARM instrument, the Atmospheric Emitted Radiance Interferometer, in that it provides fundamental surface radiation measurements at similar time resolution. Using a customized radiative transfer model, analysis of data from the ISDAC campaign indicated that most clouds sampled during April-May 2008 were complicated mixed-phase clouds. By extending the sampling period into summer, researchers anticipated more frequent single-phase (liquid) clouds. The additional data will allow the researchers to better validate and refine their data analysis methods, while also documenting the influence of cloud phase transitions on the surface energy budget.

The ASD instrument was deployed on the roof of the NSA guest facility, exactly as it was during 2008, and it operated automatically around the clock. Operations staff at the Barrow site checked the instrument daily to run a data backup and check the instrument status in case a restart was needed. The final product will be a set of radiometrically calibrated irradiance spectra, recorded at 1-minute intervals throughout the daylight period.