  The IRIS Mg II k line serves as a very good tool to diagnose the heating
processes in solar flares. Recent studies have shown that apart from the usual
red asymmetries which are interpreted as the result of condensation downflows,
this line could also show a blue-wing enhancement. To investigate how such a
blue asymmetry is formed, we perform a grid of radiative hydrodynamic
simulations and calculate the corresponding line profiles. We find that such a
spectral feature is likely to originate from the upward plasma motion in the
upper chromosphere. However, the formation region that is responsible for the
blue-wing enhancement could be located in an evaporation region, in an upward
moving blob, and even an upward moving condensation region. We discuss how the
electron beam parameters affect these different dynamics of the atmosphere.
