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

Current satellite algorithms to estimate photosynthetically active rad iation (PAR) at the Earth's surface are reviewed, and selected results are presented. PAR can be obtained directly from top-of-atmosphere so lar radiance, which is used to determine the transmissivity of the atm osphere. Since clouds do not absorb significantly at PAR wavelengths, the radiative transfer modeling is generally simplified compared to th at for total insolation. The accuracies reported, about 10% and 6% on daily and monthly time scales, respectively, are useful for modeling o ceanic and terrestrial primary productivity. The large short-term vari ability 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 climatolog ies (satellite or other).