Correcting Daytime Thermal Offset in Unventilated Pyranometers

Abstract

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.

An intercomparison of the thermal offset for different pyranometers

“…On the contrary, a positive trend is observed in daytime thermal offset with respect to relative humidity, Net IR, and wind speed for all pyranometer. These findings agree with results reported by other authors such as Dutton et al [] and Serrano et al [] and confirm that thermal offset increases under cloud‐free conditions, low relative humidity, low wind speed, and high temperature. The least squares regression analysis has been conducted to assess the significance of the relationships.…”

“…All environmental and radiative magnitudes showed high variations during this time period of increasing absolute thermal offset, except the wind speed which remained fairly constant. Increasing ambient temperatures, global and diffuse irradiances, simultaneously to decreasing net IR irradiance and relative humidity, favor the increase in the absolute value of the thermal offset [ Sanchez et al , ; Serrano et al , ]. The opposite situation occurs around sunset, explaining the decrease in the absolute thermal offset observed from around 17:00 UTC onward.…”

“…This widely used methodology has the advantage of providing realistic values of the thermal offset independently of other reference instruments without requiring installing thermometers inside the pyranometers. For these reasons measurements obtained by means of capping events have been used as reference to validate other methodologies [ Bush et al , ; Dutton et al , ; Haeffelin et al , ; Michalsky et al , , ] or as the main procedure to get thermal offset values [ Carlund , ; Sanchez et al , ; Serrano et al , ].…”

“…These two summer days were selected due to their clear‐sky conditions and their high temperature and irradiance values. Several studies point out that the higher thermal offset values are expected under these conditions [ Bush et al , ; Dutton et al , ; Sanchez et al , ; Serrano et al , ].…”

“…The decrease in thermal offset under cloudy conditions is mainly due to the enhancement of downward IR by clouds, which reduces the dome‐detector temperature difference. Additionally, a direct relationship between thermal offset and ambient temperature has been suggested by several authors [ Haeffelin et al , ; Dutton et al , ; Ji and Tsay , ; Sanchez et al , ; Serrano et al , ]. The uncertainty caused by the thermal offset can be very large compared with trends observed in mean global irradiance at the Earth's surface and therefore must be accounted for in radiative forcing studies.…”