How can TRNSYS calculate direct and diffuse solar radiation given an average global radiation?

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TRNSYS has a radiation processor component that calculates direct/diffuse horizontal and tilted surface solar radiation. The component has different modes based on the input information that is available. There are two modes that use global horizontal as an input. One includes no other information and the other also uses the ambient temperature and relative humidity. The direct/diffuse split is determined based on the clearness index and solar altitude angle using relationships developed by Reindl (Reindl, D.T., Beckman, W.A. and Duffie, J.A., “Diffuse Fraction Correlations”, Solar Energy, Vol. 45, No. 1, (1990), pp.1-7.)