Characterization of Thermal Effects in Pyranometers: A Data Correction Algorithm for Improved Measurement of Surface Insolation

Abstract

Pyranometers are reliable, economical radiometers commonly used to measure solar irradiances at the surface in a long-term, monitoring mode. This paper presents a discussion of the response of these instruments to varying environmental conditions, including the magnitude and variability of the irradiance being measured. It is found that different conditions, commonly occurring in field experiments, affect the thermal balance and temperature gradients within the instrument in a variety of ways. Such an effect results in variable offset systematic errors whose origin and magnitude are investigated in laboratory and field experiments. It is shown that these offset errors are proportional to the difference between the fourth power of the dome and detector temperatures, following closely the Stefan?Boltzmann radiation law. Results of field experiments are presented for daytime and nighttime operation over a variety of atmospheric conditions ranging from clear to heavy overcast and rain. All measurements took place from May through October 1998 in La Jolla, California, at the Scripps Institution of Oceanography. Laboratory experiments are used to quantify the magnitude of the thermal offset errors under controlled conditions and to calibrate them as a function of thermal gradients between the dome and the detector. The quality of the data resulting from pyranometer measurements can be improved in a significant way by proper knowledge of the thermal parameters affecting the operation of the thermopile system. To that end, a data correction algorithm that requires an extensive thermal calibration procedure and a simple modification of the instrument is proposed. Such an algorithm needs to be applied to the power calibration procedure as well as to the retrieval of data acquired during normal field operations. The experimental results presented in this paper could potentially affect analyses based on surface insolation measurements performed using pyranometers, in particular those related to the measurement of diffuse radiation fields.