  Radio relics are diffuse radio sources in galaxy clusters that are associated
with merger shock waves. Detailed observations of radio relics in total
intensity and in polarisation show complex structures on kiloparsec scales. The
relation between the observed features and the underlying morphology of the
magnetic field is not clear. Using three-dimensional magneto-hydrodynamical
simulations, we study the polarised emission produced by a shock wave that
propagates through a turbulent medium that resembles the intracluster medium.
We model the polarised synchrotron emission on the basis of diffusive shock
acceleration of cosmic-ray electrons. We find that the synchrotron emission
produced in a shocked turbulent medium can reproduce some of the observed
features in radio relics. Shock compression can give rise to a high
polarisation fraction at the shock front and a partial alignment of the
polarisation $E$-vectors with the shock normal. Our work confirms that radio
relics can also be formed in an environment with a tangled magnetic field. We
also discuss the effect of Faraday Rotation intrinsic to the source, and how
our results depend on the angular resolution of observations.
