  Spectroscopic observations of solar flares provide critical diagnostics of
the physical conditions in the flaring atmosphere. Some key features in
observed spectra have not yet been accounted for in existing flare models. Here
we report a data-driven simulation of the well-observed X1.0 flare on 2014
March 29 that can reconcile some well-known spectral discrepancies. We analyzed
spectra of the flaring region from the Interface Region Imaging Spectrograph
(IRIS) in MgII h&k, the Interferometric BIdimensional Spectropolarimeter at the
Dunn Solar Telescope (DST/IBIS) in H$\alpha$ 6563 \AA\ and CaII 8542 \AA, and
the Reuven Ramaty High Energy Solar Spectroscope Imager (RHESSI) in hard
X-rays. We constructed a multi-threaded flare loop model and used the electron
flux inferred from RHESSI data as the input to the radiative hydrodynamic code
RADYN to simulate the atmospheric response. We then synthesized various
chromospheric emission lines and compared them with the IRIS and IBIS
observations. In general, the synthetic intensities agree with the observed
ones, especially near the northern footpoint of the flare. The simulated MgII
line profile has narrower wings than the observed one. This discrepancy can be
reduced by using a higher microturbulent velocity (27 km/s) in a narrow
chromospheric layer. In addition, we found that an increase of electron density
in the upper chromosphere within a narrow height range of $\approx$800 km below
the transition region can turn the simulated MgII line core into emission and
thus reproduce the single peaked profile, which is a common feature in all IRIS
flares.
