Cloud and albedo enhancement impacts on solar irradiance using high-frequency measurements from thermopile and photodiode radiometers. Part 1: Impacts on global horizontal irradiance

•Various possible definitions of cloud enhancement effects are discussed and reviewed.•Physical explanations of cloud and albedo enhancements are proposed.•Frequency of enhancement episodes established from 1-min and 1-s measurements.•Frequency of enhancement episodes has large interannual variability.•Record global irradiance values are found: 1.9 suns at 1-s and 1.6 suns at 1-min. Using many years of high-quality measurements from a variety of radiometers at both 1-min and 1-s resolution, this study provides a detailed analysis of cloud enhancement (CE) and albedo enhancement (AE) effects on solar irradiance. This first part focuses on global horizontal irradiance. Various possible definitions of the CE phenomenon are extracted from the literature and discussed, in the context of PV applications most importantly. Based on 10years of 1-min measurements of all shortwave irradiance components at a high-elevation site (1829m) on the foothills of the Rocky Mountains in Colorado, a frequency analysis of extreme events triggered by enhancement effects is carried out, using three different criteria to delineate enhancement effects: global horizontal irradiance (GHI) above 1 sun, and clearness index (KT) above either 0.8 or 1.0. This analysis shows that the annual frequency of these extreme events is extremely variable, and also largely dependent on the type of instrumentation (thermopile vs. photodiode). Although the scattering of light off cumulus-type cloud edges is directly associated with CE effects, three different types of CE phenomenon are proposed, which depend on the relative mix of diffuse and direct irradiance prior and during an episode, and on the magnitude of the regional albedo. The maximum observed global irradiance varies between 1546 and 1891Wm−2 at this site, depending on type of instrument and temporal resolution. The latter value (≈1.9 suns), obtained with a photodiode sensor at 1-s resolution, corresponds to KT=1.62 and appears to constitute a new GHI world record. It results from the combination of CE and AE effects, the latter being caused by strong backscattering, itself triggered by a fresh snow cover over the region. If the magnitude and frequency of enhancement events are critical to detect rapid transients that can be harmful to PV installations, it is suggested to rely on photodiode sensors at 1-s resolution or better.