  Direct-imaging techniques of exoplanets have made significant progress
recently, and will eventually enable to monitor photometric and spectroscopic
signals of earth-like habitable planets in the future. The presence of clouds,
however, would remain as one of the most uncertain components in deciphering
such direct-imaged signals of planets. We attempt to examine how the planetary
obliquity produce different cloud patterns by performing a series of GCM
(General Circulation Model) simulation runs using a set of parameters relevant
for our Earth. Then we use the simulated photometric lightcurves to compute
their frequency modulation due to the planetary spin-orbit coupling over an
entire orbital period, and attempt to see to what extent one can estimate the
obliquity of an Earth-twin. We find that it is possible to estimate the
obliquity of an Earth-twin within the uncertainty of several degrees with a
dedicated 4 m space telescope at 10 pc away from the system if the stellar flux
is completely blocked. While our conclusion is based on several idealized
assumptions, a frequency modulation of a directly-imaged earth-like planet
offers a unique methodology to determine its obliquity.
