7.3.9 Calibration for direct solar radiation

   Pyrheliometric scales, which form the basis of solar radiation observation, formerly included the Ångström pyrheliometric scale, which was based on the Ångström pyrheliometer and used in Europe, and the Smithsonian pyrheliometric scale, which was based on water-flow and silver-disk pyrheliometers and used in U.S.A. However, comparisons between these two older scales revealed systematic bias caused by structural differences of the standard systems. For this reason, an integrated, international pyrheliometric scale (IPS-1956) was established in 1956. In 1980, WMO adopted the World Radiometric Reference (WRR), which were maintained by the World Standard Group (WSG) (Aerological Observatory, 1996). Japan had long used the Smithsonian pyrheliometric scale since the beginning of solar radiation observation, but adopted the international pyrheliometric scale (IPS-1956) in 1957 and the World Radiometric Reference (WRR) in 1981.

  To obtain globally homogenous, high precision solar radiation data through the maintenance of standards for salar radiation observations, a framework for the calibration of pyrheliometers was established in 1971 under the WMO World Radiation Center (WRC) at the Physical Meteorological Observatory Davos (PMOD) in Switzerland, in which the WRR is defined based on the WSG maintained by the WRC. The WRR is transmitted to the national or regional standards through international comparison every 5 years, and accuracy in solar radiation observations can thus be maintained in a globally homogeneous manner.

  JMA was appointed as the Regional Radiation Center (RRC) for WMO Regional Association-II (Asia) at its fourth session in 1965. Since 1970, JMA has been participating in the intercomparison with the WRR held at the WRC every 5 years, and committed to the maintenance of the regional standard as well as to intercomparison with other countries in the region to contribute to assurance of accuracy in observations of solar radiation in the region.

  The JMA also serves as the national radiation center and calibrates pyrheliometers used in Japan. To assure accuracy of the pyrheliometers used for observation in JMA are subjected to testing once every 5 years consisting of intercomparison with the national standard and linearity check of outputs that depend on temperature changes.