  Using three-dimensional magnetohydrodynamics simulations, we investigate
general properties of a blast wave shock interacting with interstellar clouds.
The shock-cloud interaction generates a turbulent shell through the vorticity
generations. In the turbulent shell, the magnetic field is amplified as a
result of turbulent dynamo action. In the case of a young supernova remnant,
the corresponding strength of the magnetic field is approximately 1 mG. The
propagation speed of the shock wave is significantly stalled in the clouds
because of the high density, while the shock maintains a high velocity in the
diffuse surrounding. In addition, when the shock wave hits the clouds,
reflection shock waves are generated that propagate back into the shocked
shell. From these simulation results, many observational characteristics of a
young SNR RX J1713.7-3946 that is suggested to be interacting with molecular
clouds, can be explained as follows: The reflection shocks can accelerate
particles in the turbulent downstream region where the magnetic field strength
reaches 1mG, which causes short-time variability of synchrotron X-rays. Since
the shock velocity is stalled locally in the clouds, the temperature in the
shocked cloud is suppressed below 1 keV. Thus, thermal X-ray line emissions
would be faint even if the SNR is interacting with molecular clouds. We also
find that the photon index of the $\pi^0$-decay gamma rays generated by
cosmic-ray protons can be 1.5, because the penetration depth of high-energy
particles into the clumpy clouds depends on their energy, which is consistent
with the recent gamma-ray observation by Fermi space telescope.

High Energy Astrophysical Phenomena