A main source of error in solar radiation measurements is the thermal offset inherent to pyranometers. Despite acknowledgment of its importance, its correction has been widely ignored for several decades. This neglect may have caused a generalized underestimation in solar radiation measurements. This study focuses on the correction of this error in solar irradiance measurements. For this aim a plethora of correction models built as a linear combination of several environmental variables related to the ambient temperature and to the incoming radiation were proposed. The models are fitted to experimental measurements obtained during capping events and, finally, their performance is evaluated and compared. The main results indicate that models with only one independent variable moderately correct the thermal offset error. These simple models are useful when no additional instrumentation other than the pyranometer is available. On the other hand, the more complex models show the best performance, with a coefficient of determination R2 over 0.8, an RMSE under 2 W m−2, and an absolute value of mean bias error (MBE) under 0.5 W m−2. Additionally, these models are used to study the differences between nighttime and daytime correction, revealing the unsuitability of using nighttime-fitted models to correct the daytime thermal offset. The general validity of the models is tested by their application to two different pyranometers. Results indicate that, whereas the factors involved in the best-performing models are the same, the values of the loading coefficients differ and therefore must be specifically calculated for each pyranometer.

The reliable estimation of the radiative forcing and trends in radiation requires very accurate measurements of global and diffuse solar irradiance at the earth's surface. To improve measurement accuracy, error sources such as the pyranometer thermal offset should be thoroughly evaluated. This study focuses on the measurement and analysis of this effect in a widely used type of pyranometer. For this aim, a methodology based on capping the pyranometer has been used and different criteria for determining the thermal offset have been applied and compared. The thermal offset of unventilated pyranometers for global and diffuse irradiance has been measured under a wide range of cloud, ambient temperature, wind speed, and radiation conditions. Significant differences in absolute values and variability have been observed between daytime and nighttime, advising against correcting the thermal offset effect based only on nighttime values. Notable differences in the thermal offset between cloudy and cloud-free conditions have been also observed. The main results show that the ambient temperature, the radiation, and its direct/diffuse partitioning are the variables more related to the daytime thermal offset.

[1] Reliable and accurate measurements of diffuse solar irradiance are needed in order to partition global irradiance into its direct and diffuse components. Diffuse irradiance is commonly measured using sun tracking systems or shadow rings. Data obtained using a shadow ring must be corrected for the portion of diffuse irradiance blocked by the ring. In this paper we have examined and evaluated six of the most widely used correction models. Approaches that account for radiation anisotropy perform notably better than those using only geometric corrections. Our results also argue for the need to adjust empirical models to local conditions. Empirical approaches developed by LeBaron et al. (1990) and by Batlles et al. (1995) perform best when compared with the more theoretical models.Citation: Sánchez, G., A. Serrano, M. L. Cancillo, and J. A. García (2012), Comparison of shadow-ring correction models for diffuse solar irradiance,

Abstract. Despite its important role on the human health and numerous biological processes, the diffuse component of the erythemal ultraviolet irradiance (UVER) is scarcely measured at standard radiometric stations and therefore needs to be estimated. This study proposes and compares 10 empirical models to estimate the UVER diffuse fraction. These models are inspired from mathematical expressions originally used to estimate total diffuse fraction, but, in this study, they are applied to the UVER case and tested against experimental measurements. In addition to adapting to the UVER range the various independent variables involved in these models, the total ozone column has been added in order to account for its strong impact on the attenuation of ultraviolet radiation. The proposed models are fitted to experimental measurements and validated against an independent subset. The bestperforming model (RAU3) is based on a model proposed by Ruiz-Arias et al. (2010) and shows values of r 2 equal to 0.91 and relative root-mean-square error (rRMSE) equal to 6.1 %. The performance achieved by this entirely empirical model is better than those obtained by previous semi-empirical approaches and therefore needs no additional information from other physically based models. This study expands on previous research to the ultraviolet range and provides reliable empirical models to accurately estimate the UVER diffuse fraction.

An unprecedented intensive intercomparison campaign focused on the experimental measurement of the thermal offset of pyranometers has been conducted at Badajoz (Spain) with the participation of three main manufacturers. The purpose of this study is to compare the thermal offset of six commercially available pyranometers, being some of them widely used and others recently commercialized. In this campaign, the capping methodology has been used to experimentally measure the daytime thermal offset of the pyranometers. Thus, a short but intense campaign has been conducted in two selected summer days under clear‐sky conditions, covering a large range of solar zenith angle, irradiance, and temperature. Along the campaign, a total of 305 capping events have been performed, 61 for each pyranometer. The daytime thermal offset obtained for different pyranometers ranges between 0 and −16.8 W/m2 depending on the environmental conditions, being sometimes notably higher than values estimated indoors by manufacturers. The thermal offset absolute value of all instruments shows a diurnal cycle, increasing from sunrise to central hours of the day and decreasing from midafternoon to sunset. The analysis demonstrates that thermal offset is notably higher and more variable during daytime than during nighttime, requiring specific daytime measurements. Main results emphasize the key role played by wind speed in modulating the thermal offset.