UV Index monitoring and forecasting in Cyprus: the effect of ozone, aerosols and clouds

Abstract. Surface ultraviolet (UV) radiation is strongly influenced by clouds, aerosols, and total column ozone, making accurate operational forecasting challenging in regions with highly variable atmospheric conditions. This study evaluates UV Index (UVI) variability and forecasting over Cyprus using more than one year of observations from a five-station ground-based monitoring network. Spectral measurements from a Bentham spectroradiometer were combined with broadband UV observations, all-sky imagery, AERONET aerosol measurements, and satellite ozone products to evaluate the regional Cyprus Erythemal Radiation Forecasting System (CERYFOS) and the Copernicus Atmosphere Monitoring Service (CAMS) corresponding product. Ground-based observations showed a strong seasonal cycle across Cyprus, together with regional differences associated with altitude and local atmospheric variability, particularly at the mountainous Kyperounda station. Both forecasting systems reproduced observed UVI well under typical atmospheric conditions. For the all sky conditions, CERYFOS and CAMS achieved RMSE values of 0.72 and 0.60 UVI, respectively, decreasing to 0.47 and 0.40 UVI under clear-sky cases. In contrast, performance deteriorated under cloudy conditions, with RMSE increasing to 1.65 and 1.12 UVI, respectively, identifying cloud representation as a major source of forecast uncertainty. Ozone-related uncertainties generally produced relatively small UVI deviations, whereas aerosol errors became increasingly important under enhanced aerosol loading, as particularly evident during the 23–24 July 2025 wildfire, underestimated aerosol loading resulted in forecast errors exceeding 4 UVI units. These results demonstrate the importance of accurate cloud and aerosol representation and coordinated ground-based observations for improving operational UVI forecasting in the Eastern Mediterranean.