On the dome effect of Eppley pyrgeometers and pyranometers

Pyrgeometers and Pyranometers are fundamental instruments widely used for quantifying atmosphere-surface energetics in climate studies. The dome effect of these instruments can cause a measurement uncertainty larger than 10 W m(-2). Based on careful analysis, the dome factors of our two new pyrgeometers are found to lie in the range between 1.1 and 2.0. These values are far smaller than the value of 4.0 suggested by the World Meteorological Organization. The laboratory-determined dome factors fall within this range, if pyrgeometers approach equilibrium with the blackbody target during calibration cycles. From recent field campaigns, consistent results for the dome factors are also obtained by analyzing nighttime pyrgeometer measurements, which were regarded as approaching equilibrium state. Furthermore, we utilized an energy balance equation to describe the thermal dome effect of pyranometers that is commonly referred to as the nighttime negative outputs or the dark-offset. Lacking direct measurements of the dome and case temperatures of pyranometer, we used measurements from a pyrgeometer to derive and to account for the thermal dome effect of collocated pyranometers. This approximation revealed a reasonable agreement between calculations and measurements.