Well as I explained I was not sure it worked and to be honest I do not think it should be an option given what the definition of atmospheric attenuation is.
Just set the options to not set the icon or sunshine hours until sometime after sunrise that way you will not get spurious sunny conditions, or at least it will reduce.
I am not going to comment further, you have the advice and an explanation of why it happens. This is NOT fixable using this method of calculating sunshine hours, only reducible as I have explained.
Stuart