Analysis of cloud effects on long-term global and diffuse photosynthetically active radiation at a Mediterranean site

Abstract

This study addresses the analysis of the cloud effects on photosynthetically active radiation (PAR, 400–700 nm) for global (PARGlobal) and its diffuse component (PARDiffuse). To this aim, a 11-year database of measured PARGlobal and modeled PARDifuse recorded in a Mediterranean site was analyzed, for both clear-sky and all-sky scenarios. PARGlobal mean values for the entire period were estimated in (200 ± 50) Wm−2 and (240 ± 50) Wm−2 for all- and clear-sky scenarios, respectively, while the values obtained for PARDiffuse were (59 ± 6) Wm−2 for all-skies and (51 ± 5) Wm−2 for clear-skies. PARGlobal monthly averages show the typical annual pattern driven by the annual course of solar position and PARDiffuse presents a similar but less marked pattern. The observed seasonal behavior was explained in terms of cloud cover, cloud frequency, liquid and ice content for all-sky scenarios. Higher variability during spring was detected due to the more complex cloud features in this season. Cloud Radiative Effect (CRE) showed negative values associated with a decrease in PARGlobal (−36 ± 14) Wm−2 and positive values corresponding to an increase in PARDiffuse (+7 ± 5) Wm−2. A clear seasonal pattern was found for CREGlobal and CREDiffuse with higher values in spring and autumn, and lower values in summer and winter. Additionally, the PAR-to-Total ratio and diffuse fraction (kPAR) values were evaluated under different sky conditions. Monthly mean values of the PAR-to-Total ratio showed steady values around 0.44 and any dependence on clearness index (kt) nor total cloud cover (TCC) was found. However, kPAR seemed to increase with TCC, taking averages values of 0.45 for all-sky and 0.28 for clear-sky scenarios. For all-sky conditions a clear seasonal pattern was observed with higher values in colder months. A secondary maximum value for kPAR was found in summer for clear-sky scenarios, revealing the important effect of the Saharan dust intrusions in the Mediterranean region. Finally, a well-defined logistic relationship was found between kPAR and kt, leading to estimate kPAR from total solar irradiance measurements.