Broadband shortwave calibration results for East Asian Regional Experiment 2005

Abstract

In order to accurately estimate radiative forcing from observations, it is necessary to make accurate measurements and to maintain the calibration of the instruments used. The ABC‐Asia project involves long‐term measurements of solar irradiance. For EAREX 2005, we investigated calibration methods and procedures for pyranometers and pyrheliometers as the first phase of the project. Several methods for calibrating pyranometers and pyrheliometers were tried. It was most troublesome to take account of the deviation from a cosine law response in side‐by‐side intercomparisons of pyranometers. A method using a collimation tube was one of the easiest. In addition, a new method to determine both the sensitivity and the deviation from a cosine law response was developed. In this method, the direct component is extracted from the global irradiance measurement and utilized. The dispersion of sensitivity among these methods was ±1%. There is also a possibility of year‐to‐year variations in sensitivity. It is therefore recommended that each pyranometer be recalibrated every 2 or 3 a. The sensitivity of the pyrheliometer was determined by comparing it with an absolute cavity radiometer calibrated at the World Radiation Center (Davos, Switzerland). The dispersion of sensitivity for pyrheliometers was less than 0.2%. For the pyrheliometers the determined values were almost the same as those provided by the radiometer manufacturer.