THE BASE-LINE SURFACE RADIATION NETWORK PYRGEOMETER ROUND-ROBIN CALIBRATION EXPERIMENT

With the aim of improving the consistency of terrestrial and atmospher ic longwave radiation measurements within the Baseline Surface Radiati on Network, five: Eppley Precision Infrared Radiometer (PIR) pyrgeomet ers and one modified Mereorological Research Flight (MRF) pyrgeometer were individually calibrated by 11 specialist laboratories. The round- robin experiment was conducted in a ''blind'' sense in that the partic ipants had no knowledge of the results of others until the wholl: seri es of calibrations had ended. The responsivities C(mu V/W m(-2)) deter mined by 6 of the 11 institutes were within about 2% of the median for all five PIR pyrgeometers. Among the six laboratories, the absolute d eviation around the median of the deviations of the five instruments i s less than 1%. This small scatter suggests that PII: pyrgeometers wer e stable at least during the two years of the experiment and that the six different calibration devices reproduce the responsivity C of PIR pyrgeometers consistently and within the precision required for climat e applications. The results also suggest that the responsivity C can b e determined without simultaneous determination of the dome correction factor k, if the temperature difference between pyrgeometer body and dome is negligible during calibration. For field measurements, however , k has to be precisely known. The calibration of the MRF pyrgeometer, although not performed by all institutes, also showed satisfactory re sults.