Simplified atmospheric radiative transfer modelling for estimating incident PAR using MODIS atmosphere products

Photosynthetically active radiation (PAR) is an important parameter in the estimation of vegetation growth. A method is presented with which instantaneous PAR can be calculated with high accuracy from Moderate Resolution Imaging Spectroradiometer (MODIS) atmosphere data products. The method, PARcalc, is based on a simplification of the general radiative transfer equations, whereby the atmosphere is modelled in a few layers depending on the opacity of the atmosphere due to clouds and aerosol load. The four major processes of atmospheric attenuation of solar radiation, Rayleigh scattering, absorption by ozone and water vapour, and aerosol scattering, are treated individually, using calibrated data from MODIS imagery. Comparing the calculations to field measurements made in Costa Rica in 2002, absolute errors in the order of 2–3% are obtained, which should be sufficiently accurate for application of the PARcalc method in models of vegetation growth as a source of incident PAR.