Abstract
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 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. 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 retrieval of the cloud-free evolution of PAR and calculation of the cloud radiative effect, CRE, for downwelling PAR. The cloud-free monthly mean global PAR reaches 175 W m−2 in summer, while the diffuse PAR peaks at about 40 W m−2. The cloud radiative effect, CRE, on global and diffuse PAR is calculated as the difference between all-sky and cloud-free measurements. The annual average 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 in July, due to the high cloud-free condition 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. 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 deseasonalized to remove the influence of annual irradiance variations. The monthly mean normalized CRE for global PAR can be well represented by a multi-linear regression with respect to monthly cloud fraction, cloud top pressure, and cloud optical thickness, as determined from satellite MODIS observations. The behaviour of the normalized CRE for diffuse PAR can not be satisfactorily described by a simple multi-linear model with respect to the cloud properties, due to its non-linear dependency, in particular on the cloud optical depth. The analysis suggests that about 77 % of the global PAR interannual variability may be ascribed to cloud variability in winter.
Highlights
The solar radiation between 400 and 700 nm is defined as photosynthetically active radiation (PAR; McCree, 1972) because it is involved in primary production and photosynthetic processes
In a previous work (Trisolino et al, 2016), we developed a simple method to obtain calibrated global and diffuse PAR irradiances from measurements made with a multi-filter rotating shadowband radiometer, multifilter rotating shadowband radiometers (MFRSRs)
The instruments used for this study are Li-190 PAR sensors and MFRSR radiometers installed at the ENEA (Italian National Agency for New Technologies, Energy, and Sustainable Economic Development) Atmospheric Observatory on the island of Lampedusa (35.5◦ N, 12.6◦ E; http://www. lampedusa.enea.it/), in the central Mediterranean Sea
Summary
The solar radiation between 400 and 700 nm is defined as photosynthetically active radiation (PAR; McCree, 1972) because it is involved in primary production and photosynthetic processes. Gu et al, 2002; Mercado et al, 2009; Min, 2005) The determination of these two components is very important for the understanding and description of photosynthetic processes. Due to the complexity of the basin and of the occurring interactions, and the high anthropogenic pressure, long-term measurement programmes have been started in the Mediterranean with the aim of understanding the regional climate, underlying mechanisms, and impacts. Long-term observations play an important role within these experiments, for the understanding of variability and of the definition of conditions occurring during intensive measurement campaigns. We apply the method by Trisolino et al (2016) to MFRSR measurements made at Lampedusa and derive a long-term series of global and diffuse PAR.
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