International Pyrheliometer Comparisons (IPC)

Solar RadiometryInternational Pyrheliometer Comparisons (IPC)

Introduction

The PMOD/WRC was mandated by the WMO to manage the World Standard Group (WSG) of solar irradiation instruments, and is responsible for the international dissemination of this standard. The WSG was defined by a group of Absolute Radiometers in 1977.

In order to disseminate this standard, reference instruments from throughout the world have been gathering in Davos every five years since 1959 for comparison measurements. These so-called International Pyrheliometer Comparisons (IPC) run for three weeks and were last conducted for the 13th time in 2021. The IPCs ensure the worldwide homogeneity of solar irradiance instruments, and are an important opportunity for scientists from various countries to meet and exchange ideas.

International Importance of the IPC

Solar irradiation is by far the most important energy source affecting the Earth which drives all climate and weather phenomena. The global temperature is a direct reflection of the global radiation budget. In order to understand the budget, solar irradiance measurements must be conducted using the same scale. The PMOD/WRC has been mandated by the WMO to define and disseminate this scale. The five-yearly IPCs allow this scale to be validated and ensure that it is correctly applied on a worldwide basis. The IPCs run for three weeks during which up to 100 participants from 40 countries conduct comparison measurements in order to determine and correct any deviations of their instruments with respect to the World Standard Group.

IPC-XIII 2021

The IPC-XIII, FRC-V and IPgC-III Comparisons were held between Monday 27 September and Friday 15 October 2021 at PMOD/WRC. The Comparisons were attended by WMO-affiliated (RRC, NRC) or WMO-related institutions, such as national hydrometeorological services. Manufacturers of radiometric equipment, calibration and research institutions, and other stakeholders were also present.

WMO IOM Report No. 140 IPC-XIII, WMO website →

WMO IOM Report No. 140 IPC-XIII, PMOD/WRC website →

Proceedings of the Symposium on Radiation Measurement, WMO website →

IPC-XIII website →

Images from IPC-XIII 2021

Image credits: A. Andreas, J. Balenzategui, P. Russo, and F. Vignali.

IPC-XII 2015

An increasing number of participants from the solar energy sector took part in IPC-XII (WMO, 2016) due to the growing demand for traceable solar irradiance data in this emerging field. IPC-XII was also held for the first time as a Euramet supplementary comparison (EURAMET, PR-S6) which allows calibration laboratories holding a current “Calibration and Measurement Capabilities” (CMC) for solar radiation to compare and validate their standards.

Weather conditions during IPC-XII allowed measurements on 11 days, although all data from 2 – 5 October were later rejected due to unstable atmospheric conditions. On the remaining 9 days, 578 valid WRR data points were acquired which were used to determine the new WRR factors for the six World Standard Group (WSG) and all participating radiometers. Throughout IPC-XII, an increasing nuisance was the growing fraction of high cirrus clouds induced by contrails. Although the air traffic controllers (Skyguide) were helpful by routing air traffic away from Davos whenever possible, some measurements were still adversely affected by contrails.

Unfortunately, two of six WSG instruments have not passed the stability test since IPC-XI in 2010, which therefore prevents them from being considered in the new WRR calculation. Accumulated dust in the cavities may have affected the sensitivity of the WSG instruments, HF18748 and MK67814, over the years. With help from the manufacturer and other experts, these instruments were thoroughly cleaned and put back in to service. It remains to be seen if the cleaning fixed the stability problems. On the other hand, CSAR/MITRA performed well, and work continues towards the eventual replacement of the conventional WSG standard with this new technology-based standard. The CSAR/MITRA data confirmed the previously detected WRR-to-SI scale difference of a few tenths of a percent. The CIMO Task Team on Radiation References is addressing this issue and will publish recommendations to further improve traceability of solar irradiance by harmonising both scales. Meanwhile the IPC-XII has confirmed the stability of the WRR over the past inter-IPC period. During bad weather the IPC-XII symposium was held, which consisted of over 40 talks and several posters. PMOD/WRC staff also helped to repair instruments which had suffered shipping damage or had otherwise failed during IPC-XII.
 

Publications and Media

WMO (2016), International Pyrheliometer Comparison IPC-XII, WMO IOM Report No. 124

MeteoSwiss and PMOD/WRC video showing excerpts from IPC-XII

 

Images from IPC-XII

For further information please contact: Dr. Wolfgang Finsterle