| Article ID | Journal | Published Year | Pages | File Type |
|---|---|---|---|---|
| 82938 | Agricultural and Forest Meteorology | 2007 | 11 Pages |
The interrelationship between broadband solar components, global (RS) and diffuse (Rd) and the photosynthetic photon flux density components, global (QP) and diffuse (QPd), is investigated using a 2-year data archive obtained at Athens, Greece. These data are used to determine temporal variability of the ratios (QP/RS) and (QPd/Rd) 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 (QP/RS) 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 (QPd/Rd). The hourly ratio (QP/RS) 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 (QPd/Rd) 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 kt (the ratio of the global-to-extraterrestrial solar radiation). Finally, the relationship between the spectral diffuse fraction kdP (the ratio of diffuse PAR-to-global PAR radiation) and the clearness index ktP (the ratio of global PAR-to-extraterrestrial solar PAR) is established for the Athens atmosphere.
