ESTIMATING PHOTOSYNTHETICALLY ACTIVE RADIATION (PAR) AT THE EARTHS SURFACE FROM SATELLITE-OBSERVATIONS

Current satellite algorithms to estimate photosynthetically active radiation (PAR) at the Earth's surface are reviewed, and selected results are presented. PAR can be obtained directly from top-of-atmosphere solar radiance, which is used to determine the transmissivity of the atmosphere. Since clouds do not absorb significantly at PAR wavelengths, the radiative transfer modeling is generally simplified compared to that for total insolation. The accuracies reported, about 10% and 6% on daily and monthly time scales, respectively, are useful for modeling oceanic and terrestrial primary productivity. The large short-term variability in the ratio of PAR and insolation, essentially due to clouds, is reduced at those time scales, suggesting that reasonably accurate PAR climatologies may be obtained from available insolation climatologies (satellite or other).