On the Dome Effect of Eppley Pyrgeometers and Pyranometers

Pyrgeometers and Pyranometers are fundamental instruments widely used for quantifying atmosphereit is not trivial to determinethe domefactor [e.g.,Philipona et al., 1995]. We foundthat one may infer a larger value of surface energetics in climate studies. The dome effect of the dome factor if the PIR does not reach equilibrium with these instruments can cause a measurement uncertainty the target during calibration. The non-equilibrium effect largerthan 10 W m-2. Basedon carefulanalysis,the dome can also be seen by comparing daytime and nighttime field factors of our two new pyrgeometersare found to lie in the measurements. range between 1.1 and 2.0. These values are far smaller Lacking precise theoretical analysis, the thermal dome than the value of 4.0 suggestedby the World Meteorologi- effect of a PSP, which can cause an underestimation of the cal Organization. The laboratory-determined dome factors downwelling irradianceexceeding 10 W m-2 [Bushet al., fall within this range, if pyrgeometersapproach equilibrium 2000],has beenignoredor crudelycorrectedfor decades.In with the blackbody target during calibration cycles. From order to use the PSP to determine the role of clear sky solar recent field campaigns, consistent results for the dome fac- radiativefluxes[e.g., Valeroand Bush,1999],aerosoldirect tors are also obtained by analyzing nighttime pyrgeometer radiative forcing [e.g., Charlsonand Heintzenberg,1995], measurements, which were regarded as approaching equi- and the cloudabsorptionanomaly[e.g.,Stephensand Tsay, librium state. Furthermore, we utilized an energy balance 1990; Pilewskieand Valero, 1995] in climate studies, the equation to describe the thermal dome effect of pyranome- dome effect must be considered. To support the experimenters that is commonly referred to as the nighttime negative tal resultsby Bush et al. [2000],we providea theoretical outputs or the dark-offset. Lacking direct measurements of explanation of the thermal dome effect of the PSP. In addithe dome and case temperatures of pyranometer, we used tion, the nighttime negativeoutputs of a PSP (dark-offset) measurementsfrom a pyrgeometer to derive and to account in field experiments are demonstrated to be the result of for the thermal dome effect of collocated pyranometers. This thermal dome effect. approximation revealed a reasonableagreement between calculations and measurements.