Guest Instrument Captures Sky Images at Southern Great Plains Site

 
Published: 14 December 2011

An image sensor inside the HDR-ASIS enclosure measures solar radiation across the sky dome in the visible spectral region (400-700 nm).
Between May and October 2011, the ARM Southern Great Plains site hosted a guest instrument for the U.S. Geological Survey (USGS) called the High Dynamic Range All-Sky Imaging System, or HDR-ASIS. This instrument combines an upward-looking (or zenith-pointing) temperature-controlled digital camera with HDR imaging techniques to capture hemispherical images of the sky dome. HDR imaging enables observation of an extremely wide range of sky brightness, far beyond what is possible with a standard camera. This design supports studies of cloud- and aerosol-induced variability of solar energy distributed in the sky. Researchers from USGS are particularly interested in how such variability influences photosynthesis and the capacity of Earth’s ecosystems to absorb carbon dioxide.

The HDR-ASIS differs from ARM’s total sky imager (TSI) in that it records the full range of sky radiance observed in red, green and blue spectral bands without radiometric saturation and without a shadow-band device to block the solar disk (view a full day of images in this short video clip. Data from the TSI will be used for comparison with the HDR-ASIS. Time-series data products from the HDR-ASIS will be available after April 2012, following initial data processing, quality control, and analysis.

The HDR-ASIS converts simple color composite images (top row) into physical measurements of solar energy at various intensities (bottom row).
Scientists will use data collected during the 5-month campaign to evaluate the performance of the new custom-designed instrument for potential use in applications requiring image-based monitoring of sky conditions and solar irradiance, such as parameterization and validation of radiative transfer models and related studies of radiation-cloud-aerosol interactions. Results from the field campaign will be used to guide future improvements in the instrument design.