Dear Railung,
I agree that general Rayleigh damping includes both stiffness- and mass-proportional terms. For a given alpha and beta, constant for all modes, the end result is that the alpha (mass-proportional) term has more effect at low frequencies and the beta (stiffness-proportional) term has more effect at high frequencies.
However, the tower damping in FAST is not identical to Rayleigh damping. The tower damping in FAST is only based on the stiffness-proportional term and is specified independently for each mode.
Best regards,