Deriving Photosynthetically Active Radiation at ground level in cloud-free conditions from Copernicus Atmospheric Monitoring Service (CAMS) products

Abstract. A method is described that estimates the photosynthetically active radiation (PAR) at ground level in cloud-free conditions. It uses a fast approximation of the libRadtran radiative transfer numerical model, known as the k-distribution method and the correlated-k approximation of Kato et al. (1999). LibRadtran provides irradiances aggregated over several fixed spectral bands and a spectral resampling is proposed followed by an aggregation in the range [400, 700] nm. The Copernicus Atmosphere Monitoring Service (CAMS) produces daily estimates of the aerosol properties, and total column contents in water vapor and ozone that are input to the method. A comparison of the results is performed against instantaneous measurements of global Photosynthetic Photon Flux Density (PPFD) on a horizontal plane made in cloud-free conditions at seven sites of the Surface Radiation network (SURFRAD) in the USA in various climates. The bias ranges between −12 µmol m⁻² s⁻¹ (−1 % of the mean value at Desert Rock) and +61 µmol m⁻² s⁻¹ (+5 % at Penn. State Univ). The root mean square error ranges from 37 µmol m⁻² s⁻¹ (3 %) to 82 µmol m⁻² s⁻¹ (6 %). The coefficient of determination R² ranges between 0.97 and 0.99. This work demonstrates the quality of the proposed method combined with the CAMS products.