you can use the calculation method for this purpose

Dear Dr. Omer 

Do you have a reference explains this method ?

I did a small search on their site, and it appears that this info is proprietary, and you can get it by purchasing their "2017 ASHRAE Handbook—Fundamentals" handbook, online access to the handbook, or a DVD that is supposed to contain the data.

This is a phenomenological formula that apparently rolls in a whole bunch of observational data about cloud cover, season, etc. so I think you need a get access to this data if you want to solve your problem.

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BUT you might be able to make some guesses by noting that these equations are similar to Beer's Law, where the tau constants are related to the distance through the atmosphere.

So, let's say that you knew the values for these constants at one location on the earth, with similar weather to another location on the earth, and you wanted to isolate the effect of latitude.

In that case, the "distance" through the atmosphere at one location is like

D = T / cos(theta),

where D = the "distance", T = the "thickness" of the atmosphere, and theta is the angle between the sun and vertical at a given time of day, latitude, and longitude. You could the use the fact of "constant atmospheric thickness" to estimate that

tau_b * cos(theta) at some location =

new_tau_b * cos(new_theta) at a different location

and similarly for tau_d.

I would assume that m is related to the "thickness" of the atmosphere, which is likely to be fairly constant.

the ab and ad constants can be calculated from the listed formula, once you find your new tau_a and tau_b constants.

Does your problem have been solved? I have the similar problem, I want to get the hourly data of taob toad. However, I can't find the data or the calculation equations . Could you please explain the solution to me if you have the answers. Thank you.