An all-sky imaging system has been deployed to derive data regarding cloud cover and cloud type, for the city of Thessaloniki, Greece. A 2-year (2006–2007) period is studied in order to estimate the effect of clouds on incident solar radiation. The method is based on irradiance measurements and modeled clear-sky irradiances, corresponding to the measurements’ conditions. The cloud effect is studied based on the cloud type and the percentage of visibility of the solar disk. Cloud radiative forcing shows a dependence on solar zenith angle with the highest values being found at the highest sun elevations, while it increases with total cloud cover. The effect is mainly cooling at the surface level, with the maximum instantaneous values reaching down to −900 Wm−2. Positive values are found, up to +200 Wm−2, caused by enhancements from broken clouds. The greatest cooling effects are caused by thick cumulus and cirrocumulus–altocumulus clouds, in cases where obstruction of the solar disk is present and by stratus, stratocumulus and cumulonimbus, when accompanied by high values of total cloud cover.
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E. Nikitidou, P. Tzoumanikas and A. Kazantzidis acknowledge the European Commission for funding the project DNICast (www.dnicast-project.net), grant agreement: 608623. The LibRadtran team is acknowledged for providing the model algorithm.
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