A long-term time series of global and diffuse photosynthetically active radiation in the Mediterranean: interannual variability and cloud effects
Authors: Trisolino Pamela , di Sarra Alcide , Anello Fabrizio , Bommarito Carlo , Di Iorio Tatiana , Meloni Daniela , Monteleone Francesco , Pace Giandomenico , Piacentino Salvatore , Sferlazzo Damiano 
Published: December 2017

Abstract. Measurements of global and diffuse photosynthetically active radiation (PAR) have been carried out on the island of Lampedusa, in the central Mediterranean Sea, since 2002. PAR is derived from observations made with multi filter rotating shadowband radiometers (MFRSRs) by comparison with a 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 is obtained and is presented in this work. The monthly mean global PAR peaks in June, with about 160 W m−2, while the diffuse PAR reaches 60 W m−2 in some cases in spring or summer. The global PAR displays 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 (about 23 % of the annual mean value). The diffuse PAR is about 39 % of the global, with a marked seasonal variation, between about 25–30 % in winter and about 50 % in summer. A simple method to retrieve the cloud-free PAR global and diffuse irradiances in days characterized by partly cloudy conditions has been implemented and applied to the dataset. This method allows to retrieve the cloud-free evolution of PAR, and to calculate the cloud radiative effect. CRE, for downwelling PAR. The cloud-free monthly mean global PAR reaches 175 W m−2 in summer, and the diffuse PAR about 40 W m−2. The annual semi amplitudes are similar for all-sky and cloud-free data. The diffuse PAR for the cloud-free cases is about 24 % of the global. The cloud radiative effect, CRE, on global and diffuse PAR is calculated as the difference between all-sky and cloud-free measurements. The average CRE is about −14.7 W m−2 for the global, and +8.1 W m−2 for the diffuse PAR. The smallest CRE is observed in July, due to the high cloud-free conditions frequency. Maxima (negative for the global, and positive for the diffuse component) occur in March–April and in October, due to the combination of elevated PAR irradiances and high occurrence of cloudy conditions. Largest monthly mean values of CRE are at about −31 W m−2 for the global (April 2007), and +18 W m−2 for the diffuse component (April 2010). Summer clouds appear to be characterized by a low frequency of occurrence, low altitude, and low optical thickness, possibly linked to the peculiar marine boundary layer structure. These properties also contribute to produce small radiative effects on PAR in summer. The cloud radiative effect has been de-seasonalized to remove the influence of annual irradiance variations. The monthly mean normalized CRE for global PAR shows a statistically significant high correlation with monthly cloud fraction, cloud top pressure, and cloud optical thickness, as determined from satellite MODIS observations. The normalized CRE for diffuse PAR show lower correlations, although still statistically significant, with cloud fraction and cloud top pressure, while displays a limited correlation with cloud optical thickness. Cloud fraction seems to be the most relevant parameter driving the cloud radiative effects. Normalized CRE data have been de-seasonalized and related with variations of the de-seasonalized PAR. A statistically significant correlation is found between the de-seasonalized PAR and the de-seasonalized normalized CRE. This correlation is seasonally dependent, and suggests that about 77 % of the global PAR interannual variability may be ascribed to clouds variability in winter.