The dependence of global and diffuse PAR radiation components on sky conditions at Athens, Greece

• Published in: Agricultural and forest meteorology Vol. 143; no. 3; pp. 277 - 287
• Main Authors: Jacovides, C.P., Tymvios, F.S., Assimakopoulos, V.D., Kaltsounides, N.A.
• Format: Journal Article
• Language: English
• Published: Amsterdam Elsevier B.V 10.04.2007; Oxford Elsevier; New York, NY
• Subjects: Agricultural and forest climatology and meteorology. Irrigation. Drainage; Agronomy. Soil science and plant productions; Athens; Biological and medical sciences; Broadband solar irradiance components; clouds; diurnal variation; Fundamental and applied biological sciences. Psychology; General agronomy. Plant production; Global-diffuse photosynthetic photon flux densities; meteorological data; photosynthetically active radiation; seasonal variation; Sky conditions; solar radiation; temporal variation; weather; Greece; Southern Europe; Balkan Peninsula; Europe; Sky conditions; Athens; Global-diffuse photosynthetic photon flux densities; Broadband solar irradiance components; Biometeorology; Sky; Atmospheric condition; Global radiation; Photosynthetically active radiation; Photosynthetic photon flux density; Environmental factor; Physical environment; Irradiance; Scattered radiation; Temperate zone; Mediterranean region; Component; Solar radiation
• ISSN: 0168-1923; 1873-2240
• DOI: 10.1016/j.agrformet.2007.01.004

The interrelationship between broadband solar components, global ( R S) and diffuse ( R d) and the photosynthetic photon flux density components, global ( Q P) and diffuse ( Q Pd), is investigated using a 2-year data archive obtained at Athens, Greece. These data are used to determine temporal variability of the ratios ( Q P/ R S) and ( Q Pd/ R d) and their dependence on sky conditions. The seasonal variation of the ratios obtained from hourly data ranges from 2.006 ± 0.032 (winter) to 1.989 ± 0.028 μmol J −1 (summer) with an annual mean of 1.995 ± 0.032 μmol J −1 for ( Q P/ R S) and from 2.298 ± 0.27 (winter) to 2.538 ± 0.22 μmol J −1 (summer) with an annual mean of 2.434 ± 0.26 μmol J −1 for ( Q Pd/ R d). The hourly ratio ( Q P/ R S) increased from 1.982 ± 0.025 to 2.106 ± 0.029 μmol J −1 as sky conditions changed from clear to overcast; whereas the ratio ( Q Pd/ R d) decreased from 2.556 ± 0.27 to 2.422 ± 0.24 μmol J −1 as sky conditions changed from clear to overcast skies. The seasonal daily ratio patterns are presented and discussed through clearness index k t (the ratio of the global-to-extraterrestrial solar radiation). Finally, the relationship between the spectral diffuse fraction k dP (the ratio of diffuse PAR-to-global PAR radiation) and the clearness index k tP (the ratio of global PAR-to-extraterrestrial solar PAR) is established for the Athens atmosphere.