Preliminary Results from the ARM/ASR/BSRN Evaluating the World Infrared Standard Group Campaign

Description

Accurate atmospheric longwave radiation measurements are required for understanding radiative impacts on the Earth’s energy budget. The standard to which broadband infrared irradiance radiometers (pyrgeometers) are compared is called the World Infrared Standard Group (WISG), maintained in Davos, Switzerland, at the World Radiation Center. WISG consists of four pyrgeometers that were calibrated using Rolf Philipona’s Absolute Sky-scanning Radiometer (ASR) (Appl. Opts., 40, 2376, 2001). ARM has recently adopted this as their standard and all Eppley PIR pyrgeometers in ARM are currently being calibrated by transfer calibrations from WISG. Subsequently, Julian Gröbner (Metrologia, 49, S105 (2012)) developed the Infrared Integrating Sphere (IRIS) radiometer and Ibrahim Reda (J. Atmos. Terr. Phys., 77, 132, 2012) developed the Absolute Cavity Pyrgeometer (ACP) that make absolute measurements of broadband infrared irradiance using different instruments. These two instruments have been compared to each other and to the WISG (Gröbner et al., JGR, 119, 7083, 2014). The two instruments agree within about 1 Wm-2 with each other, but not with the WISG, which exhibits a 2-5 Wm-2 low bias depending on the column water vapor loading. Consequently, a case for changing the current WISG has been made by Gröbner and Reda, however, this is based on a total of only nine hours of comparison data. Note that comparisons must be made under clear stable conditions, which are infrequent in Davos. The "Evaluating the World Infrared Standard Group" campaign was conducted for two separate 2-week periods at the SGP Central Facility from where the IRIS and ACP, along with a number of pyrgeometers directly traceable to WISG, made measurements under nighttime clear-skies. Late October through early December is ideal for such a comparison based on climatological conditions at the SGP site. Often the sky is clear and the water vapor column is both below and above the 1 cm level that is crucial for distinguishing the water vapor dependence noted in Gröbner et al. (JGR, 119, 7083, 2014). The campaign goal is to determine whether the next improvement in infrared radiometry uncertainty can be made with confidence based on comparisons of two independently developed absolute instruments and the current WISG, along with other high quality downwelling longwave irradiance products such as those determined using AERI data, a long-term goal of the Baseline Surface Radiation Network (BSRN) and ARM.