Update of Radiation References: Considerations from the WMO SC-MINT Expert Team on Radiation References (ET-RR)
by Dr. Laurent Vuilleumier

Abstract

New primary references are proposed for solar and terrestrial (infrared) radiation. In both cases, they would result in better traceability to SI and reduced uncertainty, but they also would result in scale changes about the size of the uncertainty of the current references. The scale change is relatively small for solar radiation (0.3%), but for terrestrial radiation, it is more consequential (2-7 Wm-2 for clear-sky situations). The main task of ET-RR is to evaluate the consequences of replacing the current references with the proposed ones and issuing recommendations for mitigating potential adverse consequences. The most important ones are anticipated to result from the scale changes. The anticipated scale change for terrestrial radiation should impact weather and climate model validation. The CMIP6 multimodel mean is now 2 Wm-2 higher than the best estimate inferred from BSRN data for all-sky data and 4 Wm-2 for clear-sky (comparable to scale change and would improve agreement). However, step artefacts due to measurements related to the different scales may introduce significant error in trend analysis. For solar radiation, the scale change is small compared to the uncertainties in the most recent model estimates. It may still have some impact since a primary reference change applies worldwide. For the satellite community, the impact can be significant for terrestrial radiation product validation (surface observations are used). The expected scale change for clear-sky situations is larger than the mean bias of satellite monthly mean downward longwave irradiance. The monitoring community will progressively replace its instruments with those traceable to the new references, possibly introducing inhomogeneity in time series. Metadata documenting the traceability must be recorded and the community must contribute to harmonisation efforts (next paragraph). Similarly, instrument manufacturers will provide radiometers traceable to the new references and must make their customers are aware of it. ET-RR considers the best strategy to mitigate the consequences of the scale change is performing a harmonisation so that historical data can be traced back to the new references. ET-RR believes the centres responsible for the international radiation databases should perform this task by central processing, but the feasibility of this strategy should be assessed. The centres would require supplementary resources for this task particularly challenging in the case of terrestrial radiation.