Calculation of global radiation (SumSW) from DIF and DIR
Global radiation (SumSW) can be calculated in the following way:
SumSW = DIF + DIR * cos(z)
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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.