The effect of clouds on surface solar irradiance, based on data from an all-sky imaging system

•The cloud radiative effect on incident solar irradiance is studied.•An all-sky imaging system provides the cloud type, cloud cover and visible percentage of the solar disk.•The cloud radiative effect is estimated from surface measurements and modeled irradiances.•The effect is mainly one of cooling, but enhancements events are also present.•Strongest cooling effects are caused by stratocumulus, stratus-altostratus and cumulonimbus clouds.

An all-sky imaging system is deployed to estimate the effect of clouds on incident solar irradiance, for a 2-year period over the city of Thessaloniki, Greece. The minutely cloud radiative effect (CRE) is examined in relevance to the cloud cover and type as well as the percentage of the solar disk covered by clouds and the relative position of Sun and clouds in the sky. CRE increases with the cloud cover and decreases with the solar zenith angle (SZA). The minimum instantaneous values can reach −900 W m−2 while enhancement events are found to reach up to +200 W m−2. The greatest cooling effects are caused by thick cumulus clouds, in cases where obstruction of the solar disk is visible and by stratocumulus, stratus-altostratus and cumulonimbus-nimbostratus when accompanied by high values of cloud cover. The enhancement events are mostly found when the clouds are in the vicinity of the Sun and when the clouds are accumulated at the upper part of the sky but the Sun is in a lower position.