  Using CLARA (Code for Lyman Alpha Radiation Analysis) we constrain the escape
fraction of Lyman-Alpha radiation in galaxies in the redshift range 5<z<7,
based on the MareNostrum High-z Universe, a SPH cosmological simulation with
more than 2 billion particles. We approximate Lyman-Alpha Emitters (LAEs) as
dusty gaseous slabs with Lyman-Alpha radiation sources homogeneously mixed in
the gas. Escape fractions for such a configuration and for different gas and
dust contents are calculated using our newly developed radiative transfer code
CLARA. The results are applied to the MareNostrum High-z Universe numerical
galaxies. The model shows a weak redshift evolution and good agreement with
estimations of the escape fraction as a function of reddening from observations
at z \sim 2.2 and z \sim 3. We extend the slab model by including additional
dust in a clumpy component in order to reproduce the UV con- tinuum luminosity
function and UV colours at redshifts z>~5. The LAE Luminosity Function (LF)
based on the extended clumpy model reproduces broadly the bright end of the LF
derived from observations at z \sim 5 and z \sim 6. At z \sim 7 our model
over-predicts the LF by roughly a factor of four, presumably because the
effects of the neutral intergalactic medium are not taken into account. The
remaining tension between the observed and simulated faint end of the LF, both
in the UV-continuum and Lyman-Alpha at redshifts z \sim 5 and z \sim 6 points
towards an overabundance of simulated LAEs hosted in haloes of masses
1.0x10^10h-1Msol < Mh < 4.0x10^10h-1Msol. Given the difficulties in explaining
the observed overabundance by dust absorption, a probable origin of the
mismatch are the high star formation rates in the simulated haloes around the
quoted mass range. A more efficient supernova feedback should be able to
regulate the star formation process in the shallow potential wells of these
haloes.
