Papers in Meteorology and Geophysics Vol. 27, No. 1, pp. 33-39. March 1976 On the Calibration of the Radiometer for Longwave Radiation (II) - The Case of Pyrgeometer -

Abstract

The conditions for a radiation source of a calibration device for pyrgeometers, which serve to measure the hemispherical (downward or upward) radiation flux, are discussed in some detail. A new calibration device for pyrgeometers meeting the conditions is Proposed. In this calibration device the water surface with numerous tiny water jets is employed as a radiation source. Some results of the calibration of the conventional pyrgeometers done by the present device are shown.
The influence of wind field on the sensi t i vity of the conventional pyrgeometer, which is one of the important factors influencing measurement accuracy proper to the pyrgeometer, is examined by the present device. The sensitivity of the pyrgeometer was found to decrease with the increase of wind speed, and this decrease is larger in the "ventilation-type" pyrgeometer (Gier and Dunkle-type pyrgeometer) than in the "wind shield-type" pyrgeometer (Funk-type and Eko-type pyrgeometers). The sensitivities of pyrgeometers in a wind of 5m/sec decrease by about 20% and 11% from a windless case, respectively in the cases of wind perpendicular to the ventilation and of wind opposite to the ventilation for the Gier and Dunkle-type pyrgeometer, by about 7% for Funk-type pyrgeometer and by about 8% for the Eko-type pyrgeometer. Error in hemispherical radiation flux measured by the conventional pyrgeometers are discussed.