Global and direct photosynthetically active radiation parameterizations for clear-sky conditions

Estimation of clear-sky photosynthetically active solar radiation (PAR) is an important first step in calculating this solar spectral band under all sky conditions. This procedure is commonly used to derive incoming solar radiation from remote sensing data or using digital elevation models. We introduce a new parameterization to calculate instantaneous values of direct normal and horizontal global PAR components. The main goal was to limit the number of input parameters for PAR predictions to Ångströmʹs turbidity coefficient, ground PAR-band albedo and atmospheric pressure (the latter two can be replaced by ground broadband albedo and site altitude). An important novel property of this parameterization is that it provides an analytical solution that provides integrated values along any time interval. Comparisons between calculated and measured values for different climatology sites demonstrate that the model can provide a good level of accuracy for instant, hourly and daily PAR estimates, with RMSE and MBE values of less than 4% and ±2%, respectively. The influence of turbidity parameters and ground albedo on model performance was also analyzed. The introduction of Ångströmʹs turbidity coefficient and ground albedo should allow the use of the increasing worldwide aerosol information available and to consider sites affected by snow cover in an easy and fast way.