Calculation of global radiation (SumSW) from DIF and DIR

Global radiation (SumSW) can be calculated in the following way: SumSW = DIF + DIR * cos(z) SumSW = calculated global radiation DIF = diffuse sky radiation DIR = direct normal radiation z = solar zenith angle The solar zenith angle can be calculated for any given time and location. Within the BSRN Toolbox (https://wiki.pangaea.de/wiki/BSRN_Toolbox) several options are offered. How to quickly compare SumSW with SWD is described here. Results can be found at: http://bsrn.awi.de/products/quality-code/comparisons/