Analysis of cloud enhancement events in a 30-year record of global solar irradiance at Thessaloniki, Greece Athanasios N. Natsis * , Alkiviadis Bais , Charikleia Meleti Laboratory of Atmospheric Physics, Physics Department, Aristotle University of Thessaloniki, Thessaloniki 54124, Greece A R T I C L E INFO Keywords: Cloud enhancement Total solar radiation Global horizontal irradiance Over irradiance ABSTRACT In this study, we investigate the characteristics of global horizontal irradiance enhancement events induced by clouds over Thessaloniki for the period 1994–2023 using data recorded every one minute. We identified the cloud enhancement (CE) events by creating an appropriate cloud-free irradiance reference using a radiative transfer model and aerosol optical depth data from a collocated Cimel sun photometer and a Brewer spectro- photometer. We found a trend in CE events of + 112 ± 35 cases/year, and a trend in the corresponding irra- diation of + 329.9 ± 112.0 kJ/year. To our knowledge, such long-term changes in CE events have not been presented in the past. The peak of the CE events was observed during May and June. CE events with duration longer than 10 min are very rare (< 8%), with exceptions lasting over an hour and up to 140 min. Finally, we have detected enhancements above the total solar irradiance at the top of the atmosphere for the same solar zenith angle of up to 204 W/m 2 , with the 75 % of the cases below 40 W/m 2 . Most of these extreme events occur in spring – early summer, with a secondary peak in autumn. 1. Introduction The shortwave solar radiation, reaching Earth’s surface, is the main energy source for the atmosphere and the biosphere, and drives and governs the climate (Gray et al., 2010). It has direct practical application in industries related to energy and agricultural production. The vari- ability of its intensity can impose difficulties in predicting the yield and in designing the specifications of the appropriate equipment. A signifi- cant portion of the relevant research has been focused on predicting renewable energy production in a fine timescale and in near real-time (for a review see Inman et al., 2013; Graabak and Korpås, 2016). An important aspect of the variability of solar radiation is its inter- action with the clouds. In general, clouds can attenuate a fraction of solar irradiance, but under certain conditions, can lead to enhancement of the global horizontal irradiance (GHI) reaching the ground. This cloud enhancement (CE) effect can locally increase the observed GHI to levels even higher than the expected cloud-free irradiance [Cordero et al., 2023; Vamvakas et al., 2020; Castillejo-Cuberos and Escobar, 2020; and references therein]. Some of the proposed underlying mechanisms of those enhance- ments have been summarized by Gueymard (2017); the most important being the scattering of radiation on the edges of cumulus clouds. It has also been suggested that enhancement of GHI can be produced by thin cirrus clouds through refraction and scattering (Thuillier et al., 2013). Further investigation with radiative transfer modeling and observations pointed as the prevailing mechanism, the strong forward Mie scattering through clouds of low optical depth (Pecenak et al., 2016; Thuillier et al., 2013; Yordanov et al., 2013, 2015). Overall, the appearance of CE events depends on different interactive factors, which include cloud thickness, structure and type, and the relative position of the sun and the clouds (Gueymard, 2017; Veerman et al., 2022). On multiple sites, cloud enhancements have been reported that exceed momentarily the solar constant, resulting in clearness indices above unit. A summary of extreme cloud enhancement (ECE) cases has been compiled by Martins et al. (2022). Cloud enhancements can also have some practical implications. The intensity and duration of en- hancements can affect the efficiency and stability of photovoltaic power production (Lappalainen and Kleissl, 2020; J¨ arvel¨ a and Valkealahti, 2020), while ECEs have the potential to compromise the integrity of photovoltaic plants infrastructure (Do Nascimento et al., 2019). It has also been demonstrated that these events can interfere in the compari- son of ground-based and satellite observations of radiation (Damiani et al., 2018). Global warming has likely affected cloud coverage in the last few decades. Liu et al. (2023) reported increases in cloud cover over * Corresponding author. E-mail addresses: natsisa@auth.gr (A.N. Natsis), abais@auth.gr (A. Bais), meleti@auth.gr (C. Meleti). Contents lists available at ScienceDirect Atmospheric Research journal homepage: www.elsevier.com/locate/atmosres https://doi.org/10.1016/j.atmosres.2024.107731 Received 10 July 2024; Received in revised form 12 October 2024; Accepted 12 October 2024 Atmospheric Research 312 (2024) 107731 Available online 16 October 2024 0169-8095/© 2024 Elsevier B.V. All rights are reserved, including those for text and data mining, AI training, and similar technologies.