  Few published ultraviolet (UV) spectra exist for stripped-envelope
supernovae, and none to date for broad-lined Type Ic supernovae (SN Ic-bl).
These objects have extremely high ejecta velocities and are the only supernova
type directly linked to gamma-ray bursts (GRBs). Here we present two epochs of
HST/STIS spectra of the SN Ic-bl 2014ad, the first UV spectra for this class.
We supplement this with 26 new epochs of ground-based optical spectra,
augmenting a rich spectral time series. The UV spectra do not show strong
features and are consistent with broadened versions of other SN Ic spectra
observed in the UV. We measure Fe II 5169 Angstrom velocities and show that SN
2014ad has even higher ejecta velocities than most SNe Ic both with and without
observed GRBs. We construct models of the SN 2014ad UV+optical spectra using
TARDIS, a 1D Monte-Carlo radiative-transfer spectral synthesis code. The models
fit the data well at multiple epochs in the optical but underestimate the flux
in the UV, likely due to simplifying assumptions. We find that high densities
at high velocities are needed to reproduce the spectra, with $\sim$3 M$_\odot$
of material at $v >$ 22,000 km s$^{-1}$, assuming spherical symmetry. Our
nebular line fits suggest a steep density profile at low velocities. Together,
these results imply a higher total ejecta mass than estimated from previous
light curve analysis and expected from theory. This may be reconciled by a
flattening of the density profile at low velocity and extra emission near the
center of the ejecta.
