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

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.

Photosynthetically Active Radiation Separation Model for High-Latitude Regions in Agrivoltaic Systems Modelling

“…For the investigated 374 sites, the trend is similar for all the years with a slight alteration in 2018 due to decreased amount of clouds, 375 which brought overall lower values of PAR .01123-. The annual mean PAR .01123-value for the locations 376 studied was 46.65 W/m 2 , marginally lower (59 W/m 2 ) than the one reported by Lozano et al (2022) in 377 Granada (Spain) 2008-2018 and higher (35 W/m 2 ) than the one reported by Trisolino et al (2018) The analysis demonstrates that the seasonality variation of PAR components and the relationship with cloudiness in high latitudes is similar to mid-latitudes in the northern hemisphere. However, the magnitude of the PAR components decreases as the location moves further north.…”

European Radiometry Buoy and Infrastructure (EURYBIA): A Contribution to the Design of the European Copernicus Infrastructure for Ocean Colour System Vicarious Calibration

“…The method is capable to identify cloud-free cases and has been tested against all-sky imager data. In the paper by [59] (Figure 4 of the cited reference) the frequency of occurrence of cloud-free conditions during the central part of the day (UTC time between 09:00 and 14:00), as derived from 15 years of MFRSR measurements, is shown. Cloud free conditions occur in more than 60% of the cases at Lampedusa during June, July, and August, with peaks of 80%.…”

Functional Seasonality of Free-Living and Particle-Associated Prokaryotic Communities in the Coastal Adriatic Sea

“…No ASVs were found enriched in spring AW and MS, indicating freshly generated and hence, recently populated MS, or a strong effect of environmental parameters on both AW and MS communities. The slight increase in seawater temperature and photosynthetically active radiation (Trisolino et al, 2018) triggered an increase in primary production as indicated by the increasing chlorophyll a concentrations (Supplementary Figure S5) favoring copiotrophic opportunistic prokaryotes such as Rhodobacterales (ASV4), Flavobacteriales (ASV5, 11 and 140) and Opitutales (ASV9; Wagner-Döbler and Biebl, 2006;Chafee et al, 2018;Jain et al, 2020). In summer, 50 ASVs were enriched in the AW as compared to MS and 68 ASVs in the MS over AW.…”