Calibration for Ultraviolet Observation

Japanese National Standard Calibration

   JMA calibrates the national standard Brewer spectrophotometer for ultraviolet observation based on spectral irradiance data from standard lamps certified by the National Institute of Standards and Technology (NIST). The Aerological Observatory maintains the national standard instrument based on periodic measurement of spectral irradiance via the NIST lamps. To ensure that UV observation values are in harmony with international observation scales under the WMO framework, the Japanese national standard is intercompared with a Canadian standard group of three Brewer spectrophotometers owned by Environment and Climate Change Canada (ECCC), which developed the Brewer spectrophotometer and maintains advanced technological capability in regard to its calibration. Table 1 shows all previous intercomparisons with Canadian standards owned by ECCC, the latest of which was carried out in Toronto in 2018.

Operational Instruments Calibration

   The Brewer spectrophotometer used for operational observation at Tsukuba is calibrated approximately once every two years on such occasions as intercomparison with the Japanese national standard instrument to help ensure the precision of related observation instruments.

   The Brewer spectrophotometer for Syowa in Antarctica is calibrated against the national standard at the Aerological Observatory and sent to the station every three years to be used in observation there. Once an instrument has been used for operational observation for over three years, another calibrated unit is sent to Syowa for comparison.

   Detailed weekly inspections are also performed using external standard tungsten-halogen lamps to monitor ongoing changes in wavelength-dependent sensitivity.

   The broadband UV-B radiometer, which measures the total amount of global UV-B at Tsukuba, is corrected using the monthly mean values from Brewer spectrophotometer in terms of changes in instrument sensitivity.