  Early-time light curves/spectra of some hydrogen-rich supernovae (SNe) give
firm evidence on the existence of confined, dense circumstellar matter (CSM)
surrounding dying massive stars. We numerically and analytically study
radiative acceleration of CSM in such systems, where the radiation is mainly
powered by the interaction between the SN ejecta and the CSM. We find that the
acceleration of the unshocked dense CSM ahead of the shock is larger for
massive and compact CSM, with velocities reaching up to $\sim 10^3\ {\rm km\
s^{-1}}$ for a CSM of order $0.1\ M_\odot$ confined within $\sim 10^{15}$ cm.
We show that the dependence of the acceleration on the CSM density helps us
explain the diversity of the CSM velocity inferred from the early spectra of
some Type II SNe. For explosions in even denser CSM, radiative acceleration can
affect the dissipation of strong collisionless shocks formed after the shock
breakout, which would affect early non-thermal emission expected from particle
acceleration.
