How to develop an Epw file about solar-air temperature

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Sol-air temperature is meant to be used as a predictive variable for cooling load. It is a combination of several variables into one for use in simple statistical models of cooling load. EPW (energyplus weather) files include temperature and solar data separately, and the simulation uses each of these data explicitly. There is no sol-air temperature in an EPW file.

If you are trying to mimic an EPW file, you could use the explicit calculation of solar air temperature to back-calculate the irradiance values to use in the EPW file.

I think a much more appropriate thing to do would be to use an existing weather file and then fit the parameters in your harmonic function to match to the outdoor air temperature and solar-air temperature calculated from data in the EPW file.