Analysis of cloud enhancement events in a 30-year record of global solar irradiance at Thessaloniki, Greece

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 spectrophotometer. We found a trend in CE events of $+112\pm 35\text{cases}/\text{year}$, and a trend in the corresponding irradiation of $+329.9\pm 112.0\mathrm{kJ}/\text{year}$. 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 $204W/m^2$, with the 75 % of the cases below $40W/m^2$. Most of these extreme events occur in spring – early summer, with a secondary peak in autumn.