IPI

Scientific hypothesis: An absolute instrument does not exist for broadband infrared radiation measurements (as exists for the broadband shortwave, namely the absolute cavity radiometer). Two groups -- one at the World Radiation Center in Switzerland and one at the Australian Bureau of Meteorology -- have built scanning instruments that alternately measure radiance from the sky and calibrate against a black body forming the basis for a instrument that could be used as a standard for the terrestrial infrared. These instruments were brought to the Southern Great Plains site to make continuous scans of sky irradiance that will be integrated to produce a measure of infrared surface irradiance. These integrated values were compared to pyrgeometer measurements as a means to calibrate and to compare these results with those from calibrations based on using blackbody sources for calibration of the pyrgeometers.

Approach to test hypothesis: When they arrived, the pyrgeometers were to have had a calibration that would represent the originating laboratory's best estimate of the calibration based on their own laboratory's calibration or one that they had received from the vendor or one that was obtained from a third source. These scanning Swiss and Australian instruments were to require some time, perhaps several minutes, to sample all parts of the sky with enough resolution to have a reasonable representation of the radiance distribution. Assuming that there is little change in the sky radiance distribution during the time it takes to make a full scan, one could integrate these results to estimate the surface irradiance and compare to that measured by the pyrgeometers. Of course, the scanning instruments can be compared to each other to determine their consistency as well. Further, the pyrgeometers were to be calibrated on site in the classical way using a blackbody cavity built for the ARM program by Eppley Laboratory, Inc. Thus, comparisons would be able to be made among all of these techniques in an attempt to better understand broadband infrared measurements.

Activity Summary

The Longwave Working Group hosted an International Pyrgeometer Intercomparison among Baseline Surface Radiation Network (BSRN) members September 20 - October 1, 1999. The comparison actually was referred to as the International Pyrgeometer/Absolute Sky-scanning Radiometer Comparison, a bit harder to say, but a more accurate title. Twenty-one instruments of four different types from 10countries participated, including Switzerland, Germany, Japan, Israel, Nigeria, Australia, Canada, Saudi Arabia, Holland and the United States.

The four calibration trailer Sci-Tec trackers plus one brought from NREL were used to shade 15 pyrgeometers. Another three were used unshaded, and two chopped, pyroelectric-based pyrgeometers were used unshaded to measure downwelling infrared sky irradiance simultaneously.

Before the field deployment at the SGP site, many of the instruments were calibrated in a standard procedure by reference to a blackbody cavity. This particular cavity at CMDL was judged one of the five best in a recent round-robin comparison of 11 different laboratory calibrators. In the field the instruments showed a spread of +/- 5 W per sq-m. This is probably because the sky as opposed to a solid cavity is not actually a blackbody, but is a modified blackbody with windows through the atmosphere, leading to subtle differences in response.

The sky-scanner in the title was built to measure radiance at four elevations and eight azimuth directions plus the zenith. During its scan of a stable sky, it frequently looks internally at a temperature-controlled blackbody to maintain its reference. The radiance integrated over all the hemisphere weighted by the cosine of the zenith angle is the irradiance that should be measured by the pyrgeometers.

Timeline

Campaign Data Sets