A two-band model for the calculation of clear sky solar irradiance, illuminance, and photosynthetically active radiation at the earth's surface

A two-band radiation modelling technique is proposed for the clear sky case. The solar spectrum is divided into a UV/Visible band (0.29–0.7 μm) and an infra-red band (0.7–2.7 μm). In each band, the transmittance of each extinction layer (ozone, water vapor, mixed gases, molecules and aerosols) is parameterized using preliminary integrations of state-of-the-art spectral transmittance functions. The beam and diffuse radiation components are obtained as functions of these layer transmittances. The photosynthetically active radiation (0.4–0.7 μm) and illuminance (0.39–0.76 μm) components may be easily derived from the UV/Visible band irradiances. In the case of typical atmospheric conditions, the model predictions are generally in good agreement with results from three accepted rigorous spectral codes. The effect of changing the solar elevation, the Angström turbidity coefficient and the site altitude on the beam, diffuse and global components (or their ratio) is further discussed.