The IR Section at PMOD/WRC have organised a number of instrument intercomparison campaigns more notably the International Pyrgeometer Comparisons (IPgC). These are held every 5 years in parallel with the International Pyrheliometer Comparisons (IPC) and the Filter Radiometer Comparisons (FRC) at PMOD/WRC. Further details can be found below.
The 3rd International Pyrgeometer Intercomparison (IPgC-III) was organised in conjunction with IPC-XIII and FRC-V at PMOD/WRC from 27 September to 15 October 2021.
WMO IOM Report No. 141 IPgC-III, WMO website →
WMO IOM Report No. 141 IPgC-III, PMOD/WRC website →
Proceedings of the Symposium on Radiation Measurement, WMO website →
The 2nd International Pyrgeometer Intercomparison (IPgC-II) was organised in conjunction with the IPC-XII and FRC-IV at PMOD/WRC from 26 September to 16 October, 2015 (Gröbner and Thomann, 2018). Thirty-three pyrgeometers from 23 institutions and 18 countries were calibrated relative to the World Infrared Standard Group of Pyrgeometers (WISG). In addition, four IRIS radiometers and two ACPs were also operated alongside the WISG.
Pyrgeometers which participated in IPgC-II were from three manufacturers: 19 K&Z CG4/CGR4, five Eppley PIR, and nine Hukseflux IR20. Even though most pyrgeometers were operated using the PMOD/WRC data acquisition system, several groups brought their own systems and collected the pyrgeometer signals themselves. Due to the large number of instruments, only a small sub-set were installed at shaded positions on a tracker. Nevertheless, all instruments were operated in ventilated units, most of them of PMOD/WRC type, while a few participants brought their own ventilation units.
The calibration against the WISG was obtained from atmospheric night-time measurements. Furthermore, all pyrgeometers were also placed in the PMOD/WRC blackbody cavity in order to determine their instrument parameters according to the standard PMOD/WRC equation and to derive a blackbody based calibration as well. Of the 33 pyrgeometers, 19 had been previously calibrated relative to the WISG, where their last calibration dates ranged from 2006 to 2013. The sensitivities of 15 of these pyrgeometers were within 1% or better compared to their previous calibration, as shown in Figure 2.
In view of establishing the WISG traceability to the international system of units, four IRIS radiometers and two ACP were operated during IPgC-II on the roof platform during clear nights. Measurements during the night on 29 and 30 September are shown in Figure 3. IRIS and ACP agree to within ± 2 W.m-2, while the WISG measured between 4 – 5 W.m-2 less. The close agreement between the IRIS, ACP and WISG during the first hours of the first night occurred during cloudy conditions when the net radiation was near zero, and the emission from the radiometer was the largest component of the downwelling long-wave irradiance. However, this is not the case during clear conditions. These measurements confirm previously published results (Gröbner et al., 2014). The results of IPgC-II were published in a WMO report (Gröbner and Thomann, 2018).
Figure 1. View of the PMOD/WRC roof-platform during IPgC-II.
Figure 2. Sensitivity changes relative to the WISG between the current calibration during IPgC-II and a previous calibration at PMOD/WRC. The year of the previous calibration is displayed at the top of the figure.
Figure 3. Measurements of downwelling long-wave irradiance from four IRIS, an ACP, and the WISG on the nights of 29 and 30 September 2015.