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. Measurements of global and diffuse photosynthetically activeradiation (PAR) have been carried out on the island of Lampedusa, in thecentral Mediterranean Sea, since 2002. PAR is derived from observations madewith multi-filter rotating shadowband radiometers (MFRSRs) by comparison witha freshly calibrated PAR sensor and by relying on the on-site Langley plots.In this way, a long-term calibrated record covering the period 2002–2016 isobtained and is presented in this work. The monthly mean global PAR peaks in June, with about 160 W m −2 , whilethe diffuse PAR reaches 60 W m −2 in spring or summer. The global PARdisplays a clear annual cycle with a semi amplitude of about 52 W m −2 .The diffuse PAR annual cycle has a semi amplitude of about 12 W m −2 . A simple method to retrieve the cloud-free PAR global and diffuse irradiancesin days characterized by partly cloudy conditions has been implemented andapplied to the dataset. This method allows retrieval of the cloud-freeevolution of PAR and calculation of the cloud radiative effect, CRE, fordownwelling PAR. The cloud-free monthly mean global PAR reaches175 W m −2 in summer, while the diffuse PAR peaks at about40 W m −2 . The cloud radiative effect, CRE, on global and diffuse PAR is calculated asthe difference between all-sky and cloud-free measurements. The annualaverage CRE is about − 14.7 W m −2 for the global PAR and + 8.1 W m −2 for the diffuse PAR. The smallest CRE is observed inJuly, due to the high cloud-free condition frequency. Maxima (negative forthe global, and positive for the diffuse component) occur in March–April andin October, due to the combination of elevated PAR irradiances and highoccurrence of cloudy conditions. Summer clouds appear to be characterized bya low frequency of occurrence, low altitude, and low optical thickness,possibly linked to the peculiar marine boundary layer structure. Theseproperties also contribute to produce small radiative effects on PAR insummer. The cloud radiative effect has been deseasonalized to remove the influenceof annual irradiance variations. The monthly mean normalized CRE for globalPAR can be well represented by a multi-linear regression with respect tomonthly cloud fraction, cloud top pressure, and cloud optical thickness, asdetermined from satellite MODIS observations. The behaviour of the normalizedCRE for diffuse PAR can not be satisfactorily described by a simplemulti-linear model with respect to the cloud properties, due to its non-linear dependency, in particular on the cloud optical depth. The analysissuggests that about 77 % of the global PAR interannual variability may beascribed to cloud variability in winter.

A long-term time series of global and diffuse photosynthetically active radiation in the Mediterranean: interannual variability and cloud effects