Journal cover Journal topic
Atmospheric Chemistry and Physics An interactive open-access journal of the European Geosciences Union
Journal topic

Journal metrics

Journal metrics

  • IF value: 5.509 IF 5.509
  • IF 5-year value: 5.689 IF 5-year 5.689
  • CiteScore value: 5.44 CiteScore 5.44
  • SNIP value: 1.519 SNIP 1.519
  • SJR value: 3.032 SJR 3.032
  • IPP value: 5.37 IPP 5.37
  • h5-index value: 86 h5-index 86
  • Scimago H index value: 161 Scimago H index 161
Volume 18, issue 11 | Copyright

Special issue: CHemistry and AeRosols Mediterranean EXperiments (ChArMEx)...

Atmos. Chem. Phys., 18, 7985-8000, 2018
https://doi.org/10.5194/acp-18-7985-2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 07 Jun 2018

Research article | 07 Jun 2018

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

Pamela Trisolino1,2, Alcide di Sarra1, Fabrizio Anello3, Carlo Bommarito3, Tatiana Di Iorio1, Daniela Meloni1, Francesco Monteleone3, Giandomenico Pace1, Salvatore Piacentino3, and Damiano Sferlazzo4 Pamela Trisolino et al.
  • 1ENEA, Laboratory for Observations and Analyses of Earth and Climate, Via Anguillarese 301, Rome, 00123, Italy
  • 2Department of Ecological and Biological Sciences, University of Tuscia, Largo dell'Università snc, Viterbo, 01100, Italy
  • 3ENEA, Laboratory for Observations and Analyses of Earth and Climate, Via Principe di Granatelli 24, Palermo, 90139, Italy
  • 4ENEA, Laboratory for Observations and Analyses of Earth and Climate, Contrada Capo Grecale, Lampedusa, 92010, Italy

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 160Wm−2, while the diffuse PAR reaches 60Wm−2 in spring or summer. The global PAR displays a clear annual cycle with a semi amplitude of about 52Wm−2. The diffuse PAR annual cycle has a semi amplitude of about 12Wm−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 175Wm−2 in summer, while the diffuse PAR peaks at about 40Wm−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 −14.7Wm−2 for the global PAR and +8.1Wm−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.

Download & links
Publications Copernicus
Special issue
Download
Short summary
The long-term (2002–2016) variability of global and diffuse PAR over the central Mediterranean is investigated based on measurements from Lampedusa. PAR modulates biological processes and this study provides useful insight into its variability. Seasonal and interannual variability of global and diffuse PAR is characterized and the effects of clouds are quantified. The analysis suggests that 77 % of the global PAR interannual variability may be ascribed to clouds.
The long-term (2002–2016) variability of global and diffuse PAR over the central Mediterranean...
Citation
Share