Compatibility of different measurement techniques of global solar radiation and application for long-term observations at Izaña Observatory

A 1-year inter-comparison of classical and modern radiation and sunshine duration (SD) instruments has been performed at Izaña Atmospheric Observatory (IZO) located in Tenerife (Canary Islands, Spain) starting on 17 July 2014. We compare daily global solar radiation (GSR_<i>H</i>) records measured with a Kipp <i>&amp;</i> Zonen CM-21 pyranometer, taken in the framework of the Baseline Surface Radiation Network, with those measured with a multifilter rotating shadowband radiometer (MFRSR), a bimetallic pyranometer (PYR) and GSR_<i>H</i> estimated from sunshine duration performed by a Campbell–Stokes sunshine recorder (CS) and a Kipp <i>&amp;</i> Zonen sunshine duration sensor (CSD). Given that the BSRN GSR_<i>H</i> records passed strict quality controls (based on principles of physical limits and comparison with the LibRadtran model), they have been used as reference in the inter-comparison study. We obtain an overall root mean square error (RMSE) of ∼ 0.9 MJm⁻² (4 %) for PYR and MFRSR GSR_<i>H</i>, 1.9 (7 %) and 1.2 MJm⁻² (5 %) for CS and CSD GSR_<i>H</i>, respectively. Factors such as temperature, relative humidity (RH) and the solar zenith angle (SZA) have been shown to moderately affect the GSR_<i>H</i> observations. As an application of the methodology developed in this work, we have re-evaluated the GSR_<i>H</i> data time series obtained at IZO with two PYRs between 1977 and 1991. Their high consistency and temporal stability have been proved by comparing with GSR_<i>H</i> estimates obtained from SD observations. These results demonstrate that (1) the continuous-basis inter-comparison of different GSR_<i>H</i> techniques offers important diagnostics for identifying inconsistencies between GSR_<i>H</i> data records, and (2) the GSR_<i>H</i> measurements performed with classical and more simple instruments are consistent with more modern techniques and, thus, valid to recover GSR_<i>H</i> data time series and complete worldwide distributed GSR_<i>H</i> data. The inter-comparison and quality assessment of these different techniques have allowed us to obtain a complete and consistent long-term global solar radiation series (1977–2015) at Izaña