Pyranometer intercomparison at the BSRN site in Cabauw, the Netherlands

A pyranometer intercomparison study has been performed with the objective of assessing the suitability for the KNMI observation network and the impact of replacement of the Kipp & Zonen CM-11 pyranometer by the Kipp & Zonen SMP-11. The intercomparison has been executed at the Cabauw BSRN site, where quality controlled, high-end instruments are available as reference for the measurement of global radiation. Other instruments from other manufacturers have been placed at the site in order to enable comparison with other instruments that are commercially available. Values for global radiation and the derived quantity of sunshine duration have been studied in this work.

A key finding of this study is that an overestimation of 1% of the currently operational KNMI reference could be determined. Previous studies were inconclusive on this overestimation, but through the intercomparison the presence and degree of overestimation could be confirmed. With the introduction of a new instrument, a new reference pyranometer will also be introduced at KNMI, which will not contain the 1% overestimation of its sensitivity. An overestimation of sensitivity results in an underestimation of global radiation. Therefore, with the introduction of the new pyranometers, it is expected that global radiation measurements will increase approximately 1% ± 1%.

Requirements as set by the WMO can only be fully met by the proposed Kipp & Zonen SMP-11 if the instrument is conditioned such that the impact of dew and rime events is minimized.

The change in estimated sunshine duration was found to be proportional to the difference in measured global radiation. Therefore also a 1% ± 1% increase in sunshine duration is expected. On the basis of the study it was concluded that the Kipp & Zonen SMP-11 is a suitable pyranometer instrument for the KNMI network .