Radiometer Calibration Trends

Calibrations of Atmospheric Radiation Measurement (ARM) broadband radiometers occur on an annual schedule, made necessary by an expected drift of instrument sensitivity and the possibility of other physical or environmental factors affecting sensitivity. The Southern Great Plains (SGP), Tropical Western Pacific (TWP), and North Slope of Alaska (NSA) field measurement sites use the Eppley Laboratory, Inc. models precision spectral pyranometer (PSP), 8-48, and normal incident pyrheliometer (NIP) for broadband shortwave measurements. Previous studies characterizing PSP sensitivity show that changes are always negative (becoming less sensitive) and are a function of temperature and exposure to sunlight, possibly the ultraviolet portion of the solar spectrum (Wilcox et al. 2001a). This exposure likely causes physical change in the material on the instrument’s sensing surface affecting its ability to absorb solar energy. However, in this study we show that the NIP model radiometers do not always follow that trend, and in field use can increase in sensitivity. The ARM Radiometer Calibration Trailer at the SGP site has been calibrating radiometers since 1997 (Wilcox et al. 1999, 2001b). The networks have calibration spare radiometers at about 50 % of the field deployment number to allow for uninterrupted measurements during the two annual calibration events a