IRSIINTERCOMPAR

IRSI Inter-Comparison Study

27 August 2007 - 23 September 2007

Lead Scientist: Victor Morris

Observatory: SGP (Southern Great Plains)

The principle objective of this campaign was to compare measurements of cloud fraction from different types of commercially available infrared sky imagers (IRSI) and to compare the daytime values with an operational Total Sky Imager (TSI). In 2004, the Radiative Processes working group requested that an infrared sky imaging system be added to our measurement suite to provide a nighttime cloud fraction product. A Blue Sky Imaging Ltd. (BSI) All-Sky Thermal Infrared Camera (ASTIC) was purchased and deployed at the SGP Guest Instrument Facility in October 2005. The characteristics of the data were compared to the ARM TSI located at the site. The daytime images compare favorably with those from the TSI but the derived cloud fraction data significantly underestimate the TSI values when the sky is more than 50% cloudy. It is unknown whether the poor cloud fraction data is a product of the BSI software or the calibration of the imager. Alternatives to the BSI instrument were investigated, including the Heitronics Nubiscan, Atmos Cloud Infrared Radiometer, and Solmirus All Sky Infrared Visible Analyzer. The respective vendors of these three instruments agreed to a simultaneous demonstration at the SGP site in the late summer of 2007 allowing an extended (about 1 month) inter-comparison with the ASTIC and TSI so that reliability and performance of the systems could be verified.