On the dome effect of Eppley pyrgeometers and pyranometers

Pyrgeometers and Pyranometers are fundamental instruments widely used for q uantifying atmosphere-surface energetics in climate studies. The dome effec t 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 pyrgeome ters are found to lie in the range between 1.1 and 2.0. These values are fa r smaller than the value of 4.0 suggested by the World Meteorological Organ ization. The laboratory-determined dome factors fall within this range, if pyrgeometers approach equilibrium with the blackbody target during calibrat ion cycles. From recent field campaigns, consistent results for the dome fa ctors are also obtained by analyzing nighttime pyrgeometer measurements, wh ich were regarded as approaching equilibrium state. Furthermore, we utilize d an energy balance equation to describe the thermal dome effect of pyranom eters 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 a ccount for the thermal dome effect of collocated pyranometers. This approxi mation revealed a reasonable agreement between calculations and measurement s.